# Python Snippets

## Search for keywords in files reachable from current directory.

{% embed url="<https://gist.github.com/bgoonz/4070fe43eadfcf574c5711f4ed5ad0a9>" %}

#### Calculates the date of `n` days from the given date.

* Use `datetime.timedelta` and the `+` operator to calculate the new `datetime.datetime` value after adding `n` days to `d`.
* Omit the second argument, `d`, to use a default value of `datetime.today()`.

```python
from datetime import datetime, timedelta

def add_days(n, d = datetime.today()):
  return d + timedelta(n)
```

```python
from datetime import date

add_days(5, date(2020, 10, 25)) # date(2020, 10, 30)
add_days(-5, date(2020, 10, 25)) # date(2020, 10, 20)
```

#### Checks if all elements in a list are equal.

* Use `set()` to eliminate duplicate elements and then use `len()` to check if length is `1`.

```python
def all_equal(lst):
  return len(set(lst)) == 1
```

```python
all_equal([1, 2, 3, 4, 5, 6]) # False
all_equal([1, 1, 1, 1]) # True
```

#### Checks if all the values in a list are unique.

* Use `set()` on the given list to keep only unique occurrences.
* Use `len()` to compare the length of the unique values to the original list.

```python
def all_unique(lst):
  return len(lst) == len(set(lst))
```

```python
x = [1, 2, 3, 4, 5, 6]
y = [1, 2, 2, 3, 4, 5]
all_unique(x) # True
all_unique(y) # False
```

#### Generates a list of numbers in the arithmetic progression starting with the given positive integer and up to the specified limit.

* Use `range()` and `list()` with the appropriate start, step and end values.

```python
def arithmetic_progression(n, lim):
  return list(range(n, lim + 1, n))
```

```python
arithmetic_progression(5, 25) # [5, 10, 15, 20, 25]
```

#### Calculates the average of two or more numbers.

* Use `sum()` to sum all of the `args` provided, divide by `len()`.

```python
def average(*args):
  return sum(args, 0.0) / len(args)
```

```python
average(*[1, 2, 3]) # 2.0
average(1, 2, 3) # 2.0
```

#### Calculates the average of a list, after mapping each element to a value using the provided function.

* Use `map()` to map each element to the value returned by `fn`.
* Use `sum()` to sum all of the mapped values, divide by `len(lst)`.
* Omit the last argument, `fn`, to use the default identity function.

```python
def average_by(lst, fn = lambda x: x):
  return sum(map(fn, lst), 0.0) / len(lst)
```

```python
average_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda x: x['n'])
# 5.0
```

#### Splits values into two groups, based on the result of the given `filter` list.

* Use a list comprehension and `zip()` to add elements to groups, based on `filter`.
* If `filter` has a truthy value for any element, add it to the first group, otherwise add it to the second group.

```python
def bifurcate(lst, filter):
  return [
    [x for x, flag in zip(lst, filter) if flag],
    [x for x, flag in zip(lst, filter) if not flag]
  ]
```

```python
bifurcate(['beep', 'boop', 'foo', 'bar'], [True, True, False, True])
# [ ['beep', 'boop', 'bar'], ['foo'] ]
```

#### Splits values into two groups, based on the result of the given filtering function.

* Use a list comprehension to add elements to groups, based on the value returned by `fn` for each element.
* If `fn` returns a truthy value for any element, add it to the first group, otherwise add it to the second group.

```python
def bifurcate_by(lst, fn):
  return [
    [x for x in lst if fn(x)],
    [x for x in lst if not fn(x)]
  ]
```

```python
bifurcate_by(['beep', 'boop', 'foo', 'bar'], lambda x: x[0] == 'b')
# [ ['beep', 'boop', 'bar'], ['foo'] ]
```

#### Calculates the number of ways to choose `k` items from `n` items without repetition and without order.

* Use `math.comb()` to calculate the binomial coefficient.

```python
from math import comb

def binomial_coefficient(n, k):
  return comb(n, k)
```

```python
binomial_coefficient(8, 2) # 28
```

#### Returns the length of a string in bytes.

* Use `str.encode('utf-8')` to encode the given string and return its length.

```python
def byte_size(s):
  return len(s.encode('utf-8'))
```

```python
byte_size('😀') # 4
byte_size('Hello World') # 11
```

#### Converts a string to camelcase.

* Use `re.sub()` to replace any `-` or `_` with a space, using the regexp `r"(_|-)+"`.
* Use `str.title()` to capitalize the first letter of each word and convert the rest to lowercase.
* Finally, use `str.replace()` to remove spaces between words.

```python
from re import sub

def camel(s):
  s = sub(r"(_|-)+", " ", s).title().replace(" ", "")
  return ''.join([s[0].lower(), s[1:]])
```

```python
camel('some_database_field_name') # 'someDatabaseFieldName'
camel('Some label that needs to be camelized')
# 'someLabelThatNeedsToBeCamelized'
camel('some-javascript-property') # 'someJavascriptProperty'
camel('some-mixed_string with spaces_underscores-and-hyphens')
# 'someMixedStringWithSpacesUnderscoresAndHyphens'
```

#### Capitalizes the first letter of a string.

* Use list slicing and `str.upper()` to capitalize the first letter of the string.
* Use `str.join()` to combine the capitalized first letter with the rest of the characters.
* Omit the `lower_rest` parameter to keep the rest of the string intact, or set it to `True` to convert to lowercase.

```python
def capitalize(s, lower_rest = False):
  return ''.join([s[:1].upper(), (s[1:].lower() if lower_rest else s[1:])])
```

```python
capitalize('fooBar') # 'FooBar'
capitalize('fooBar', True) # 'Foobar'
```

#### Capitalizes the first letter of every word in a string.

* Use `str.title()` to capitalize the first letter of every word in the string.

```python
def capitalize_every_word(s):
  return s.title()
```

```python
capitalize_every_word('hello world!') # 'Hello World!'
```

#### Casts the provided value as a list if it's not one.

* Use `isinstance()` to check if the given value is enumerable.
* Return it by using `list()` or encapsulated in a list accordingly.

```python
def cast_list(val):
  return list(val) if isinstance(val, (tuple, list, set, dict)) else [val]
```

```python
cast_list('foo') # ['foo']
cast_list([1]) # [1]
cast_list(('foo', 'bar')) # ['foo', 'bar']
```

### unlisted: true

Converts Celsius to Fahrenheit.

* Follow the conversion formula `F = 1.8 * C + 32`.

```python
def celsius_to_fahrenheit(degrees):
  return ((degrees * 1.8) + 32)
```

```python
celsius_to_fahrenheit(180) # 356.0
```

#### Creates a function that will invoke a predicate function for the specified property on a given object.

* Return a `lambda` function that takes an object and applies the predicate function, `fn` to the specified property.

```python
def check_prop(fn, prop):
  return lambda obj: fn(obj[prop])
```

```python
check_age = check_prop(lambda x: x >= 18, 'age')
user = {'name': 'Mark', 'age': 18}
check_age(user) # True
```

#### Chunks a list into smaller lists of a specified size.

* Use `list()` and `range()` to create a list of the desired `size`.
* Use `map()` on the list and fill it with splices of the given list.
* Finally, return the created list.

```python
from math import ceil

def chunk(lst, size):
  return list(
    map(lambda x: lst[x * size:x * size + size],
      list(range(ceil(len(lst) / size)))))
```

```python
chunk([1, 2, 3, 4, 5], 2) # [[1, 2], [3, 4], [5]]
```

#### Chunks a list into `n` smaller lists.

* Use `math.ceil()` and `len()` to get the size of each chunk.
* Use `list()` and `range()` to create a new list of size `n`.
* Use `map()` to map each element of the new list to a chunk the length of `size`.
* If the original list can't be split evenly, the final chunk will contain the remaining elements.

```python
from math import ceil

def chunk_into_n(lst, n):
  size = ceil(len(lst) / n)
  return list(
    map(lambda x: lst[x * size:x * size + size],
    list(range(n)))
  )
```

```python
chunk_into_n([1, 2, 3, 4, 5, 6, 7], 4) # [[1, 2], [3, 4], [5, 6], [7]]
```

#### Clamps `num` within the inclusive range specified by the boundary values.

* If `num` falls within the range (`a`, `b`), return `num`.
* Otherwise, return the nearest number in the range.

```python
def clamp_number(num, a, b):
  return max(min(num, max(a, b)), min(a, b))
```

```python
clamp_number(2, 3, 5) # 3
clamp_number(1, -1, -5) # -1
```

#### Inverts a dictionary with non-unique hashable values.

* Create a `collections.defaultdict` with `list` as the default value for each key.
* Use `dictionary.items()` in combination with a loop to map the values of the dictionary to keys using `dict.append()`.
* Use `dict()` to convert the `collections.defaultdict` to a regular dictionary.

```python
from collections import defaultdict

def collect_dictionary(obj):
  inv_obj = defaultdict(list)
  for key, value in obj.items():
    inv_obj[value].append(key)
  return dict(inv_obj)
```

```python
ages = {
  'Peter': 10,
  'Isabel': 10,
  'Anna': 9,
}
collect_dictionary(ages) # { 10: ['Peter', 'Isabel'], 9: ['Anna'] }
```

#### Combines two or more dictionaries, creating a list of values for each key.

* Create a new `collections.defaultdict` with `list` as the default value for each key and loop over `dicts`.
* Use `dict.append()` to map the values of the dictionary to keys.
* Use `dict()` to convert the `collections.defaultdict` to a regular dictionary.

```python
from collections import defaultdict

def combine_values(*dicts):
  res = defaultdict(list)
  for d in dicts:
    for key in d:
      res[key].append(d[key])
  return dict(res)
```

```python
d1 = {'a': 1, 'b': 'foo', 'c': 400}
d2 = {'a': 3, 'b': 200, 'd': 400}

combine_values(d1, d2) # {'a': [1, 3], 'b': ['foo', 200], 'c': [400], 'd': [400]}
```

#### Removes falsy values from a list.

* Use `filter()` to filter out falsy values (`False`, `None`, `0`, and `""`).

```python
def compact(lst):
  return list(filter(None, lst))
```

```python
compact([0, 1, False, 2, '', 3, 'a', 's', 34]) # [ 1, 2, 3, 'a', 's', 34 ]
```

#### Performs right-to-left function composition.

* Use `functools.reduce()` to perform right-to-left function composition.
* The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.

```python
from functools import reduce

def compose(*fns):
  return reduce(lambda f, g: lambda *args: f(g(*args)), fns)
```

```python
add5 = lambda x: x + 5
multiply = lambda x, y: x * y
multiply_and_add_5 = compose(add5, multiply)
multiply_and_add_5(5, 2) # 15
```

#### Performs left-to-right function composition.

* Use `functools.reduce()` to perform left-to-right function composition.
* The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.

```python
from functools import reduce

def compose_right(*fns):
  return reduce(lambda f, g: lambda *args: g(f(*args)), fns)
```

```python
add = lambda x, y: x + y
square = lambda x: x * x
add_and_square = compose_right(add, square)
add_and_square(1, 2) # 9
```

#### Groups the elements of a list based on the given function and returns the count of elements in each group.

* Use `collections.defaultdict` to initialize a dictionary.
* Use `map()` to map the values of the given list using the given function.
* Iterate over the map and increase the element count each time it occurs.

```python
from collections import defaultdict

def count_by(lst, fn = lambda x: x):
  count = defaultdict(int)
  for val in map(fn, lst):
    count[val] += 1
  return dict(count)
```

```python
from math import floor

count_by([6.1, 4.2, 6.3], floor) # {6: 2, 4: 1}
count_by(['one', 'two', 'three'], len) # {3: 2, 5: 1}
```

#### Counts the occurrences of a value in a list.

* Use `list.count()` to count the number of occurrences of `val` in `lst`.

```python
def count_occurrences(lst, val):
  return lst.count(val)
```

```python
count_occurrences([1, 1, 2, 1, 2, 3], 1) # 3
```

#### Creates a list of partial sums.

* Use `itertools.accumulate()` to create the accumulated sum for each element.
* Use `list()` to convert the result into a list.

```python
from itertools import accumulate

def cumsum(lst):
  return list(accumulate(lst))
```

```python
cumsum(range(0, 15, 3)) # [0, 3, 9, 18, 30]
```

#### Curries a function.

* Use `functools.partial()` to return a new partial object which behaves like `fn` with the given arguments, `args`, partially applied.

```python
from functools import partial

def curry(fn, *args):
  return partial(fn, *args)
```

```python
add = lambda x, y: x + y
add10 = curry(add, 10)
add10(20) # 30
```

#### Creates a list of dates between `start` (inclusive) and `end` (not inclusive).

* Use `datetime.timedelta.days` to get the days between `start` and `end`.
* Use `int()` to convert the result to an integer and `range()` to iterate over each day.
* Use a list comprehension and `datetime.timedelta()` to create a list of `datetime.date` objects.

```python
from datetime import timedelta, date

def daterange(start, end):
  return [start + timedelta(n) for n in range(int((end - start).days))]
```

```python
from datetime import date

daterange(date(2020, 10, 1), date(2020, 10, 5))
# [date(2020, 10, 1), date(2020, 10, 2), date(2020, 10, 3), date(2020, 10, 4)]
```

#### Calculates the date of `n` days ago from today.

* Use `datetime.date.today()` to get the current day.
* Use `datetime.timedelta` to subtract `n` days from today's date.

```python
from datetime import timedelta, date

def days_ago(n):
  return date.today() - timedelta(n)
```

```python
days_ago(5) # date(2020, 10, 23)
```

#### Calculates the day difference between two dates.

* Subtract `start` from `end` and use `datetime.timedelta.days` to get the day difference.

```python
def days_diff(start, end):
  return (end - start).days
```

```python
from datetime import date

days_diff(date(2020, 10, 25), date(2020, 10, 28)) # 3
```

#### Calculates the date of `n` days from today.

* Use `datetime.date.today()` to get the current day.
* Use `datetime.timedelta` to add `n` days from today's date.

```python
from datetime import timedelta, date

def days_from_now(n):
  return date.today() + timedelta(n)
```

```python
days_from_now(5) # date(2020, 11, 02)
```

#### Decapitalizes the first letter of a string.

* Use list slicing and `str.lower()` to decapitalize the first letter of the string.
* Use `str.join()` to combine the lowercase first letter with the rest of the characters.
* Omit the `upper_rest` parameter to keep the rest of the string intact, or set it to `True` to convert to uppercase.

```python
def decapitalize(s, upper_rest = False):
  return ''.join([s[:1].lower(), (s[1:].upper() if upper_rest else s[1:])])
```

```python
decapitalize('FooBar') # 'fooBar'
decapitalize('FooBar', True) # 'fOOBAR'
```

#### Deep flattens a list.

* Use recursion.
* Use `isinstance()` with `collections.abc.Iterable` to check if an element is iterable.
* If it is iterable, apply `deep_flatten()` recursively, otherwise return `[lst]`.

```python
from collections.abc import Iterable

def deep_flatten(lst):
  return ([a for i in lst for a in
          deep_flatten(i)] if isinstance(lst, Iterable) else [lst])
```

```python
deep_flatten([1, [2], [[3], 4], 5]) # [1, 2, 3, 4, 5]
```

#### Converts an angle from degrees to radians.

* Use `math.pi` and the degrees to radians formula to convert the angle from degrees to radians.

```python
from math import pi

def degrees_to_rads(deg):
  return (deg * pi) / 180.0
```

```python
degrees_to_rads(180) # ~3.1416
```

#### Invokes the provided function after `ms` milliseconds.

* Use `time.sleep()` to delay the execution of `fn` by `ms / 1000` seconds.

```python
from time import sleep

def delay(fn, ms, *args):
  sleep(ms / 1000)
  return fn(*args)
```

```python
delay(lambda x: print(x), 1000, 'later') # prints 'later' after one second
```

#### Converts a dictionary to a list of tuples.

* Use `dict.items()` and `list()` to get a list of tuples from the given dictionary.

```python
def dict_to_list(d):
  return list(d.items())
```

```python
d = {'one': 1, 'three': 3, 'five': 5, 'two': 2, 'four': 4}
dict_to_list(d)
# [('one', 1), ('three', 3), ('five', 5), ('two', 2), ('four', 4)]
```

#### Calculates the difference between two iterables, without filtering duplicate values.

* Create a `set` from `b`.
* Use a list comprehension on `a` to only keep values not contained in the previously created set, `_b`.

```python
def difference(a, b):
  _b = set(b)
  return [item for item in a if item not in _b]
```

```python
difference([1, 2, 3], [1, 2, 4]) # [3]
```

#### Returns the difference between two lists, after applying the provided function to each list element of both.

* Create a `set`, using `map()` to apply `fn` to each element in `b`.
* Use a list comprehension in combination with `fn` on `a` to only keep values not contained in the previously created set, `_b`.

```python
def difference_by(a, b, fn):
  _b = set(map(fn, b))
  return [item for item in a if fn(item) not in _b]
```

```python
from math import floor

difference_by([2.1, 1.2], [2.3, 3.4], floor) # [1.2]
difference_by([{ 'x': 2 }, { 'x': 1 }], [{ 'x': 1 }], lambda v : v['x'])
# [ { x: 2 } ]
```

#### Converts a number to a list of digits.

* Use `map()` combined with `int` on the string representation of `n` and return a list from the result.

```python
def digitize(n):
  return list(map(int, str(n)))
```

```python
digitize(123) # [1, 2, 3]
```

#### Returns a list with `n` elements removed from the left.

* Use slice notation to remove the specified number of elements from the left.
* Omit the last argument, `n`, to use a default value of `1`.

```python
def drop(a, n = 1):
  return a[n:]
```

```python
drop([1, 2, 3]) # [2, 3]
drop([1, 2, 3], 2) # [3]
drop([1, 2, 3], 42) # []
```

#### Returns a list with `n` elements removed from the right.

* Use slice notation to remove the specified number of elements from the right.
* Omit the last argument, `n`, to use a default value of `1`.

```python
def drop_right(a, n = 1):
  return a[:-n]
```

```python
drop_right([1, 2, 3]) # [1, 2]
drop_right([1, 2, 3], 2) # [1]
drop_right([1, 2, 3], 42) # []
```

#### Checks if the provided function returns `True` for every element in the list.

* Use `all()` in combination with `map()` and `fn` to check if `fn` returns `True` for all elements in the list.

```python
def every(lst, fn = lambda x: x):
  return all(map(fn, lst))
```

```python
every([4, 2, 3], lambda x: x > 1) # True
every([1, 2, 3]) # True
```

#### Returns every `nth` element in a list.

* Use slice notation to create a new list that contains every `nth` element of the given list.

```python
def every_nth(lst, nth):
  return lst[nth - 1::nth]
```

```python
every_nth([1, 2, 3, 4, 5, 6], 2) # [ 2, 4, 6 ]
```

#### Calculates the factorial of a number.

* Use recursion.
* If `num` is less than or equal to `1`, return `1`.
* Otherwise, return the product of `num` and the factorial of `num - 1`.
* Throws an exception if `num` is a negative or a floating point number.

```python
def factorial(num):
  if not ((num >= 0) and (num % 1 == 0)):
    raise Exception("Number can't be floating point or negative.")
  return 1 if num == 0 else num * factorial(num - 1)
```

```python
factorial(6) # 720
```

### unlisted: true

Converts Fahrenheit to Celsius.

* Follow the conversion formula `C = (F - 32) * 5/9`.

```python
def fahrenheit_to_celsius(degrees):
  return ((degrees - 32) * 5/9)
```

```python
fahrenheit_to_celsius(77) # 25.0
```

#### Generates a list, containing the Fibonacci sequence, up until the nth term.

* Starting with `0` and `1`, use `list.append()` to add the sum of the last two numbers of the list to the end of the list, until the length of the list reaches `n`.
* If `n` is less or equal to `0`, return a list containing `0`.

```python
def fibonacci(n):
  if n <= 0:
    return [0]
  sequence = [0, 1]
  while len(sequence) <= n:
    next_value = sequence[len(sequence) - 1] + sequence[len(sequence) - 2]
    sequence.append(next_value)
  return sequence
```

```python
fibonacci(7) # [0, 1, 1, 2, 3, 5, 8, 13]
```

#### Creates a list with the non-unique values filtered out.

* Use `collections.Counter` to get the count of each value in the list.
* Use a list comprehension to create a list containing only the unique values.

```python
from collections import Counter

def filter_non_unique(lst):
  return [item for item, count in Counter(lst).items() if count == 1]
```

```python
filter_non_unique([1, 2, 2, 3, 4, 4, 5]) # [1, 3, 5]
```

#### Creates a list with the unique values filtered out.

* Use `collections.Counter` to get the count of each value in the list.
* Use a list comprehension to create a list containing only the non-unique values.

```python
from collections import Counter

def filter_unique(lst):
  return [item for item, count in Counter(lst).items() if count > 1]
```

```python
filter_unique([1, 2, 2, 3, 4, 4, 5]) # [2, 4]
```

#### Finds the value of the first element in the given list that satisfies the provided testing function.

* Use a list comprehension and `next()` to return the first element in `lst` for which `fn` returns `True`.

```python
def find(lst, fn):
  return next(x for x in lst if fn(x))
```

```python
find([1, 2, 3, 4], lambda n: n % 2 == 1) # 1
```

#### Finds the index of the first element in the given list that satisfies the provided testing function.

* Use a list comprehension, `enumerate()` and `next()` to return the index of the first element in `lst` for which `fn` returns `True`.

```python
def find_index(lst, fn):
  return next(i for i, x in enumerate(lst) if fn(x))
```

```python
find_index([1, 2, 3, 4], lambda n: n % 2 == 1) # 0
```

#### Finds the indexes of all elements in the given list that satisfy the provided testing function.

* Use `enumerate()` and a list comprehension to return the indexes of the all element in `lst` for which `fn` returns `True`.

```python
def find_index_of_all(lst, fn):
  return [i for i, x in enumerate(lst) if fn(x)]
```

```python
find_index_of_all([1, 2, 3, 4], lambda n: n % 2 == 1) # [0, 2]
```

#### Finds the first key in the provided dictionary that has the given value.

* Use `dictionary.items()` and `next()` to return the first key that has a value equal to `val`.

```python
def find_key(dict, val):
  return next(key for key, value in dict.items() if value == val)
```

```python
ages = {
  'Peter': 10,
  'Isabel': 11,
  'Anna': 9,
}
find_key(ages, 11) # 'Isabel'
```

#### Finds all keys in the provided dictionary that have the given value.

* Use `dictionary.items()`, a generator and `list()` to return all keys that have a value equal to `val`.

```python
def find_keys(dict, val):
  return list(key for key, value in dict.items() if value == val)
```

```python
ages = {
  'Peter': 10,
  'Isabel': 11,
  'Anna': 10,
}
find_keys(ages, 10) # [ 'Peter', 'Anna' ]
```

#### Finds the value of the last element in the given list that satisfies the provided testing function.

* Use a list comprehension and `next()` to return the last element in `lst` for which `fn` returns `True`.

```python
def find_last(lst, fn):
  return next(x for x in lst[::-1] if fn(x))
```

```python
find_last([1, 2, 3, 4], lambda n: n % 2 == 1) # 3
```

#### Finds the index of the last element in the given list that satisfies the provided testing function.

* Use a list comprehension, `enumerate()` and `next()` to return the index of the last element in `lst` for which `fn` returns `True`.

```python
def find_last_index(lst, fn):
  return len(lst) - 1 - next(i for i, x in enumerate(lst[::-1]) if fn(x))
```

```python
find_last_index([1, 2, 3, 4], lambda n: n % 2 == 1) # 2
```

#### Finds the items that are parity outliers in a given list.

* Use `collections.Counter` with a list comprehension to count even and odd values in the list.
* Use `collections.Counter.most_common()` to get the most common parity.
* Use a list comprehension to find all elements that do not match the most common parity.

```python
from collections import Counter

def find_parity_outliers(nums):
  return [
    x for x in nums
    if x % 2 != Counter([n % 2 for n in nums]).most_common()[0][0]
  ]
```

```python
find_parity_outliers([1, 2, 3, 4, 6]) # [1, 3]
```

#### Flattens a list of lists once.

* Use a list comprehension to extract each value from sub-lists in order.

```python
def flatten(lst):
  return [x for y in lst for x in y]
```

```python
flatten([[1, 2, 3, 4], [5, 6, 7, 8]]) # [1, 2, 3, 4, 5, 6, 7, 8]
```

#### Executes the provided function once for each list element.

* Use a `for` loop to execute `fn` for each element in `itr`.

```python
def for_each(itr, fn):
  for el in itr:
    fn(el)
```

```python
for_each([1, 2, 3], print) # 1 2 3
```

#### Executes the provided function once for each list element, starting from the list's last element.

* Use a `for` loop in combination with slice notation to execute `fn` for each element in `itr`, starting from the last one.

```python
def for_each_right(itr, fn):
  for el in itr[::-1]:
    fn(el)
```

```python
for_each_right([1, 2, 3], print) # 3 2 1
```

#### Creates a dictionary with the unique values of a list as keys and their frequencies as the values.

* Use `collections.defaultdict()` to store the frequencies of each unique element.
* Use `dict()` to return a dictionary with the unique elements of the list as keys and their frequencies as the values.

```python
from collections import defaultdict

def frequencies(lst):
  freq = defaultdict(int)
  for val in lst:
    freq[val] += 1
  return dict(freq)
```

```python
frequencies(['a', 'b', 'a', 'c', 'a', 'a', 'b']) # { 'a': 4, 'b': 2, 'c': 1 }
```

#### Converts a date from its ISO-8601 representation.

* Use `datetime.datetime.fromisoformat()` to convert the given ISO-8601 date to a `datetime.datetime` object.

```python
from datetime import datetime

def from_iso_date(d):
  return datetime.fromisoformat(d)
```

```python
from_iso_date('2020-10-28T12:30:59.000000') # 2020-10-28 12:30:59
```

#### Calculates the greatest common divisor of a list of numbers.

* Use `functools.reduce()` and `math.gcd()` over the given list.

```python
from functools import reduce
from math import gcd as _gcd

def gcd(numbers):
  return reduce(_gcd, numbers)
```

```python
gcd([8, 36, 28]) # 4
```

#### Initializes a list containing the numbers in the specified range where `start` and `end` are inclusive and the ratio between two terms is `step`.

Returns an error if `step` equals `1`.

* Use `range()`, `math.log()` and `math.floor()` and a list comprehension to create a list of the appropriate length, applying the step for each element.
* Omit the second argument, `start`, to use a default value of `1`.
* Omit the third argument, `step`, to use a default value of `2`.

```python
from math import floor, log

def geometric_progression(end, start=1, step=2):
  return [start * step ** i for i in range(floor(log(end / start)
          / log(step)) + 1)]
```

```python
geometric_progression(256) # [1, 2, 4, 8, 16, 32, 64, 128, 256]
geometric_progression(256, 3) # [3, 6, 12, 24, 48, 96, 192]
geometric_progression(256, 1, 4) # [1, 4, 16, 64, 256]
```

#### Retrieves the value of the nested key indicated by the given selector list from a dictionary or list.

* Use `functools.reduce()` to iterate over the `selectors` list.
* Apply `operator.getitem()` for each key in `selectors`, retrieving the value to be used as the iteratee for the next iteration.

```python
from functools import reduce
from operator import getitem

def get(d, selectors):
  return reduce(getitem, selectors, d)
```

```python
users = {
  'freddy': {
    'name': {
      'first': 'fred',
      'last': 'smith'
    },
    'postIds': [1, 2, 3]
  }
}
get(users, ['freddy', 'name', 'last']) # 'smith'
get(users, ['freddy', 'postIds', 1]) # 2
```

#### Groups the elements of a list based on the given function.

* Use `collections.defaultdict` to initialize a dictionary.
* Use `fn` in combination with a `for` loop and `dict.append()` to populate the dictionary.
* Use `dict()` to convert it to a regular dictionary.

```python
from collections import defaultdict

def group_by(lst, fn):
  d = defaultdict(list)
  for el in lst:
    d[fn(el)].append(el)
  return dict(d)
```

```python
from math import floor

group_by([6.1, 4.2, 6.3], floor) # {4: [4.2], 6: [6.1, 6.3]}
group_by(['one', 'two', 'three'], len) # {3: ['one', 'two'], 5: ['three']}
```

#### Calculates the Hamming distance between two values.

* Use the XOR operator (`^`) to find the bit difference between the two numbers.
* Use `bin()` to convert the result to a binary string.
* Convert the string to a list and use `count()` of `str` class to count and return the number of `1`s in it.

```python
def hamming_distance(a, b):
  return bin(a ^ b).count('1')
```

```python
hamming_distance(2, 3) # 1
```

#### Checks if there are duplicate values in a flat list.

* Use `set()` on the given list to remove duplicates, compare its length with the length of the list.

```python
def has_duplicates(lst):
  return len(lst) != len(set(lst))
```

```python
x = [1, 2, 3, 4, 5, 5]
y = [1, 2, 3, 4, 5]
has_duplicates(x) # True
has_duplicates(y) # False
```

#### Checks if two lists contain the same elements regardless of order.

* Use `set()` on the combination of both lists to find the unique values.
* Iterate over them with a `for` loop comparing the `count()` of each unique value in each list.
* Return `False` if the counts do not match for any element, `True` otherwise.

```python
def have_same_contents(a, b):
  for v in set(a + b):
    if a.count(v) != b.count(v):
      return False
  return True
```

```python
have_same_contents([1, 2, 4], [2, 4, 1]) # True
```

#### Returns the head of a list.

* Use `lst[0]` to return the first element of the passed list.

```python
def head(lst):
  return lst[0]
```

```python
head([1, 2, 3]) # 1
```

#### Converts a hexadecimal color code to a tuple of integers corresponding to its RGB components.

* Use a list comprehension in combination with `int()` and list slice notation to get the RGB components from the hexadecimal string.
* Use `tuple()` to convert the resulting list to a tuple.

```python
def hex_to_rgb(hex):
  return tuple(int(hex[i:i+2], 16) for i in (0, 2, 4))
```

```python
hex_to_rgb('FFA501') # (255, 165, 1)
```

#### Checks if the given number falls within the given range.

* Use arithmetic comparison to check if the given number is in the specified range.
* If the second parameter, `end`, is not specified, the range is considered to be from `0` to `start`.

```python
def in_range(n, start, end = 0):
  return start <= n <= end if end >= start else end <= n <= start
```

```python
in_range(3, 2, 5) # True
in_range(3, 4) # True
in_range(2, 3, 5) # False
in_range(3, 2) # False
```

#### Checks if all the elements in `values` are included in `lst`.

* Check if every value in `values` is contained in `lst` using a `for` loop.
* Return `False` if any one value is not found, `True` otherwise.

```python
def includes_all(lst, values):
  for v in values:
    if v not in lst:
      return False
  return True
```

```python
includes_all([1, 2, 3, 4], [1, 4]) # True
includes_all([1, 2, 3, 4], [1, 5]) # False
```

#### Checks if any element in `values` is included in `lst`.

* Check if any value in `values` is contained in `lst` using a `for` loop.
* Return `True` if any one value is found, `False` otherwise.

```python
def includes_any(lst, values):
  for v in values:
    if v in lst:
      return True
  return False
```

```python
includes_any([1, 2, 3, 4], [2, 9]) # True
includes_any([1, 2, 3, 4], [8, 9]) # False
```

#### Returns a list of indexes of all the occurrences of an element in a list.

* Use `enumerate()` and a list comprehension to check each element for equality with `value` and adding `i` to the result.

```python
def index_of_all(lst, value):
  return [i for i, x in enumerate(lst) if x == value]
```

```python
index_of_all([1, 2, 1, 4, 5, 1], 1) # [0, 2, 5]
index_of_all([1, 2, 3, 4], 6) # []
```

#### Returns all the elements of a list except the last one.

* Use `lst[:-1]` to return all but the last element of the list.

```python
def initial(lst):
  return lst[:-1]
```

```python
initial([1, 2, 3]) # [1, 2]
```

#### Initializes a 2D list of given width and height and value.

* Use a list comprehension and `range()` to generate `h` rows where each is a list with length `h`, initialized with `val`.
* Omit the last argument, `val`, to set the default value to `None`.

```python
def initialize_2d_list(w, h, val = None):
  return [[val for x in range(w)] for y in range(h)]
```

```python
initialize_2d_list(2, 2, 0) # [[0, 0], [0, 0]]
```

#### Initializes a list containing the numbers in the specified range where `start` and `end` are inclusive with their common difference `step`.

* Use `list()` and `range()` to generate a list of the appropriate length, filled with the desired values in the given range.
* Omit `start` to use the default value of `0`.
* Omit `step` to use the default value of `1`.

```python
def initialize_list_with_range(end, start = 0, step = 1):
  return list(range(start, end + 1, step))
```

```python
initialize_list_with_range(5) # [0, 1, 2, 3, 4, 5]
initialize_list_with_range(7, 3) # [3, 4, 5, 6, 7]
initialize_list_with_range(9, 0, 2) # [0, 2, 4, 6, 8]
```

#### Initializes and fills a list with the specified value.

* Use a list comprehension and `range()` to generate a list of length equal to `n`, filled with the desired values.
* Omit `val` to use the default value of `0`.

```python
def initialize_list_with_values(n, val = 0):
  return [val for x in range(n)]
```

```python
initialize_list_with_values(5, 2) # [2, 2, 2, 2, 2]
```

#### Returns a list of elements that exist in both lists.

* Create a `set` from `a` and `b`.
* Use the built-in set operator `&` to only keep values contained in both sets, then transform the `set` back into a `list`.

```python
def intersection(a, b):
  _a, _b = set(a), set(b)
  return list(_a & _b)
```

```python
intersection([1, 2, 3], [4, 3, 2]) # [2, 3]
```

#### Returns a list of elements that exist in both lists, after applying the provided function to each list element of both.

* Create a `set`, using `map()` to apply `fn` to each element in `b`.
* Use a list comprehension in combination with `fn` on `a` to only keep values contained in both lists.

```python
def intersection_by(a, b, fn):
  _b = set(map(fn, b))
  return [item for item in a if fn(item) in _b]
```

```python
from math import floor

intersection_by([2.1, 1.2], [2.3, 3.4], floor) # [2.1]
```

#### Inverts a dictionary with unique hashable values.

* Use `dictionary.items()` in combination with a list comprehension to create a new dictionary with the values and keys inverted.

```python
def invert_dictionary(obj):
  return { value: key for key, value in obj.items() }
```

```python
ages = {
  'Peter': 10,
  'Isabel': 11,
  'Anna': 9,
}
invert_dictionary(ages) # { 10: 'Peter', 11: 'Isabel', 9: 'Anna' }
```

#### Checks if a string is an anagram of another string (case-insensitive, ignores spaces, punctuation and special characters).

* Use `str.isalnum()` to filter out non-alphanumeric characters, `str.lower()` to transform each character to lowercase.
* Use `collections.Counter` to count the resulting characters for each string and compare the results.

```python
from collections import Counter

def is_anagram(s1, s2):
  return Counter(
    c.lower() for c in s1 if c.isalnum()
  ) == Counter(
    c.lower() for c in s2 if c.isalnum()
  )
```

```python
is_anagram('#anagram', 'Nag a ram!')  # True
```

#### Checks if the elements of the first list are contained in the second one regardless of order.

* Use `count()` to check if any value in `a` has more occurrences than it has in `b`.
* Return `False` if any such value is found, `True` otherwise.

```python
def is_contained_in(a, b):
  for v in set(a):
    if a.count(v) > b.count(v):
      return False
  return True
```

```python
is_contained_in([1, 4], [2, 4, 1]) # True
```

### unlisted: true

Checks if the first numeric argument is divisible by the second one.

* Use the modulo operator (`%`) to check if the remainder is equal to `0`.

```python
def is_divisible(dividend, divisor):
  return dividend % divisor == 0
```

```python
is_divisible(6, 3) # True
```

### unlisted: true

Checks if the given number is even.

* Check whether a number is odd or even using the modulo (`%`) operator.
* Return `True` if the number is even, `False` if the number is odd.

```python
def is_even(num):
  return num % 2 == 0
```

```python
is_even(3) # False
```

### unlisted: true

Checks if the given number is odd.

* Checks whether a number is even or odd using the modulo (`%`) operator.
* Returns `True` if the number is odd, `False` if the number is even.

```python
def is_odd(num):
  return num % 2 != 0
```

```python
is_odd(3) # True
```

#### Checks if the provided integer is a prime number.

* Return `False` if the number is `0`, `1`, a negative number or a multiple of `2`.
* Use `all()` and `range()` to check numbers from `3` to the square root of the given number.
* Return `True` if none divides the given number, `False` otherwise.

```python
from math import sqrt

def is_prime(n):
  if n <= 1 or (n % 2 == 0 and n > 2):
    return False
  return all(n % i for i in range(3, int(sqrt(n)) + 1, 2))
```

```python
is_prime(11) # True
```

#### Checks if the given date is a weekday.

* Use `datetime.datetime.weekday()` to get the day of the week as an integer.
* Check if the day of the week is less than or equal to `4`.
* Omit the second argument, `d`, to use a default value of `datetime.today()`.

```python
from datetime import datetime

def is_weekday(d = datetime.today()):
  return d.weekday() <= 4
```

```python
from datetime import date

is_weekday(date(2020, 10, 25)) # False
is_weekday(date(2020, 10, 28)) # True
```

#### Checks if the given date is a weekend.

* Use `datetime.datetime.weekday()` to get the day of the week as an integer.
* Check if the day of the week is greater than `4`.
* Omit the second argument, `d`, to use a default value of `datetime.today()`.

```python
from datetime import datetime

def is_weekend(d = datetime.today()):
  return d.weekday() > 4
```

```python
from datetime import date

is_weekend(date(2020, 10, 25)) # True
is_weekend(date(2020, 10, 28)) # False
```

#### Converts a string to kebab case.

* Use `re.sub()` to replace any `-` or `_` with a space, using the regexp `r"(_|-)+"`.
* Use `re.sub()` to match all words in the string, `str.lower()` to lowercase them.
* Finally, use `str.join()` to combine all word using `-` as the separator.

```python
from re import sub

def kebab(s):
  return '-'.join(
    sub(r"(\s|_|-)+"," ",
    sub(r"[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+",
    lambda mo: ' ' + mo.group(0).lower(), s)).split())
```

```python
kebab('camelCase') # 'camel-case'
kebab('some text') # 'some-text'
kebab('some-mixed_string With spaces_underscores-and-hyphens')
# 'some-mixed-string-with-spaces-underscores-and-hyphens'
kebab('AllThe-small Things') # 'all-the-small-things'
```

#### Checks if the given key exists in a dictionary.

* Use the `in` operator to check if `d` contains `key`.

```python
def key_in_dict(d, key):
  return (key in d)
```

```python
d = {'one': 1, 'three': 3, 'five': 5, 'two': 2, 'four': 4}
key_in_dict(d, 'three') # True
```

#### Finds the key of the maximum value in a dictionary.

* Use `max()` with the `key` parameter set to `dict.get()` to find and return the key of the maximum value in the given dictionary.

```python
def key_of_max(d):
  return max(d, key = d.get)
```

```python
key_of_max({'a':4, 'b':0, 'c':13}) # c
```

#### Finds the key of the minimum value in a dictionary.

* Use `min()` with the `key` parameter set to `dict.get()` to find and return the key of the minimum value in the given dictionary.

```python
def key_of_min(d):
  return min(d, key = d.get)
```

```python
key_of_min({'a':4, 'b':0, 'c':13}) # b
```

#### Creates a flat list of all the keys in a flat dictionary.

* Use `dict.keys()` to return the keys in the given dictionary.
* Return a `list()` of the previous result.

```python
def keys_only(flat_dict):
  return list(flat_dict.keys())
```

```python
ages = {
  'Peter': 10,
  'Isabel': 11,
  'Anna': 9,
}
keys_only(ages) # ['Peter', 'Isabel', 'Anna']
```

### unlisted: true

Converts kilometers to miles.

* Follows the conversion formula `mi = km * 0.621371`.

```python
def km_to_miles(km):
  return km * 0.621371
```

```python
km_to_miles(8.1) # 5.0331051
```

#### Returns the last element in a list.

* Use `lst[-1]` to return the last element of the passed list.

```python
def last(lst):
  return lst[-1]
```

```python
last([1, 2, 3]) # 3
```

#### Returns the least common multiple of a list of numbers.

* Use `functools.reduce()`, `math.gcd()` and `lcm(x,y) = x * y / gcd(x,y)` over the given list.

```python
from functools import reduce
from math import gcd

def lcm(numbers):
  return reduce((lambda x, y: int(x * y / gcd(x, y))), numbers)
```

```python
lcm([12, 7]) # 84
lcm([1, 3, 4, 5]) # 60
```

#### Takes any number of iterable objects or objects with a length property and returns the longest one.

* Use `max()` with `len()` as the `key` to return the item with the greatest length.
* If multiple objects have the same length, the first one will be returned.

```python
def longest_item(*args):
  return max(args, key = len)
```

```python
longest_item('this', 'is', 'a', 'testcase') # 'testcase'
longest_item([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]) # [1, 2, 3, 4, 5]
longest_item([1, 2, 3], 'foobar') # 'foobar'
```

#### Maps the values of a list to a dictionary using a function, where the key-value pairs consist of the original value as the key and the result of the function as the value.

* Use `map()` to apply `fn` to each value of the list.
* Use `zip()` to pair original values to the values produced by `fn`.
* Use `dict()` to return an appropriate dictionary.

```python
def map_dictionary(itr, fn):
  return dict(zip(itr, map(fn, itr)))
```

```python
map_dictionary([1, 2, 3], lambda x: x * x) # { 1: 1, 2: 4, 3: 9 }
```

#### Creates a dictionary with the same keys as the provided dictionary and values generated by running the provided function for each value.

* Use `dict.items()` to iterate over the dictionary, assigning the values produced by `fn` to each key of a new dictionary.

```python
def map_values(obj, fn):
  return dict((k, fn(v)) for k, v in obj.items())
```

```python
users = {
  'fred': { 'user': 'fred', 'age': 40 },
  'pebbles': { 'user': 'pebbles', 'age': 1 }
}
map_values(users, lambda u : u['age']) # {'fred': 40, 'pebbles': 1}
```

#### Returns the maximum value of a list, after mapping each element to a value using the provided function.

* Use `map()` with `fn` to map each element to a value using the provided function.
* Use `max()` to return the maximum value.

```python
def max_by(lst, fn):
  return max(map(fn, lst))
```

```python
max_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) # 8
```

#### Returns the index of the element with the maximum value in a list.

* Use `max()` and `list.index()` to get the maximum value in the list and return its index.

```python
def max_element_index(arr):
  return arr.index(max(arr))
```

```python
max_element_index([5, 8, 9, 7, 10, 3, 0]) # 4
```

#### Returns the `n` maximum elements from the provided list.

* Use `sorted()` to sort the list.
* Use slice notation to get the specified number of elements.
* Omit the second argument, `n`, to get a one-element list.
* If `n` is greater than or equal to the provided list's length, then return the original list (sorted in descending order).

```python
def max_n(lst, n = 1):
  return sorted(lst, reverse = True)[:n]
```

```python
max_n([1, 2, 3]) # [3]
max_n([1, 2, 3], 2) # [3, 2]
```

#### Finds the median of a list of numbers.

* Sort the numbers of the list using `list.sort()`.
* Find the median, which is either the middle element of the list if the list length is odd or the average of the two middle elements if the list length is even.
* [`statistics.median()`](https://docs.python.org/3/library/statistics.html#statistics.median) provides similar functionality to this snippet.

```python
def median(list):
  list.sort()
  list_length = len(list)
  if list_length % 2 == 0:
    return (list[int(list_length / 2) - 1] + list[int(list_length / 2)]) / 2
  return float(list[int(list_length / 2)])
```

```python
median([1, 2, 3]) # 2.0
median([1, 2, 3, 4]) # 2.5
```

#### Merges two or more lists into a list of lists, combining elements from each of the input lists based on their positions.

* Use `max()` combined with a list comprehension to get the length of the longest list in the arguments.
* Use `range()` in combination with the `max_length` variable to loop as many times as there are elements in the longest list.
* If a list is shorter than `max_length`, use `fill_value` for the remaining items (defaults to `None`).
* [`zip()`](https://docs.python.org/3/library/functions.html#zip) and [`itertools.zip_longest()`](https://docs.python.org/3/library/itertools.html#itertools.zip_longest) provide similar functionality to this snippet.

```python
def merge(*args, fill_value = None):
  max_length = max([len(lst) for lst in args])
  result = []
  for i in range(max_length):
    result.append([
      args[k][i] if i < len(args[k]) else fill_value for k in range(len(args))
    ])
  return result
```

```python
merge(['a', 'b'], [1, 2], [True, False]) # [['a', 1, True], ['b', 2, False]]
merge(['a'], [1, 2], [True, False]) # [['a', 1, True], [None, 2, False]]
merge(['a'], [1, 2], [True, False], fill_value = '_')
# [['a', 1, True], ['_', 2, False]]
```

#### Merges two or more dictionaries.

* Create a new `dict` and loop over `dicts`, using `dictionary.update()` to add the key-value pairs from each one to the result.

```python
def merge_dictionaries(*dicts):
  res = dict()
  for d in dicts:
    res.update(d)
  return res
```

```python
ages_one = {
  'Peter': 10,
  'Isabel': 11,
}
ages_two = {
  'Anna': 9
}
merge_dictionaries(ages_one, ages_two)
# { 'Peter': 10, 'Isabel': 11, 'Anna': 9 }
```

### unlisted: true

Converts miles to kilometers.

* Follows the conversion formula `km = mi * 1.609344`.

```python
def miles_to_km(miles):
  return miles * 1.609344
```

```python
miles_to_km(5.03) # 8.09500032
```

#### Returns the minimum value of a list, after mapping each element to a value using the provided function.

* Use `map()` with `fn` to map each element to a value using the provided function.
* Use `min()` to return the minimum value.

```python
def min_by(lst, fn):
  return min(map(fn, lst))
```

```python
min_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) # 2
```

#### Returns the index of the element with the minimum value in a list.

* Use `min()` and `list.index()` to obtain the minimum value in the list and then return its index.

```python
def min_element_index(arr):
  return arr.index(min(arr))
```

```python
min_element_index([3, 5, 2, 6, 10, 7, 9]) # 2
```

#### Returns the `n` minimum elements from the provided list.

* Use `sorted()` to sort the list.
* Use slice notation to get the specified number of elements.
* Omit the second argument, `n`, to get a one-element list.
* If `n` is greater than or equal to the provided list's length, then return the original list (sorted in ascending order).

```python
def min_n(lst, n = 1):
  return sorted(lst, reverse = False)[:n]
```

```python
min_n([1, 2, 3]) # [1]
min_n([1, 2, 3], 2) # [1, 2]
```

#### Calculates the month difference between two dates.

* Subtract `start` from `end` and use `datetime.timedelta.days` to get the day difference.
* Divide by `30` and use `math.ceil()` to get the difference in months (rounded up).

```python
from math import ceil

def months_diff(start, end):
  return ceil((end - start).days / 30)
```

```python
from datetime import date

months_diff(date(2020, 10, 28), date(2020, 11, 25)) # 1
```

#### Returns the most frequent element in a list.

* Use `set()` to get the unique values in `lst`.
* Use `max()` to find the element that has the most appearances.

```python
def most_frequent(lst):
  return max(set(lst), key = lst.count)
```

```python
most_frequent([1, 2, 1, 2, 3, 2, 1, 4, 2]) #2
```

#### Generates a string with the given string value repeated `n` number of times.

* Repeat the string `n` times, using the `*` operator.

```python
def n_times_string(s, n):
  return (s * n)
```

```python
n_times_string('py', 4) #'pypypypy'
```

#### Checks if the provided function returns `True` for at least one element in the list.

* Use `all()` and `fn` to check if `fn` returns `False` for all the elements in the list.

```python
def none(lst, fn = lambda x: x):
  return all(not fn(x) for x in lst)
```

```python
none([0, 1, 2, 0], lambda x: x >= 2 ) # False
none([0, 0, 0]) # True
```

#### Maps a number from one range to another range.

* Return `num` mapped between `outMin`-`outMax` from `inMin`-`inMax`.

```python
def num_to_range(num, inMin, inMax, outMin, outMax):
  return outMin + (float(num - inMin) / float(inMax - inMin) * (outMax
                  - outMin))
```

```python
num_to_range(5, 0, 10, 0, 100) # 50.0
```

#### Moves the specified amount of elements to the end of the list.

* Use slice notation to get the two slices of the list and combine them before returning.

```python
def offset(lst, offset):
  return lst[offset:] + lst[:offset]
```

```python
offset([1, 2, 3, 4, 5], 2) # [3, 4, 5, 1, 2]
offset([1, 2, 3, 4, 5], -2) # [4, 5, 1, 2, 3]
```

#### Pads a string on both sides with the specified character, if it's shorter than the specified length.

* Use `str.ljust()` and `str.rjust()` to pad both sides of the given string.
* Omit the third argument, `char`, to use the whitespace character as the default padding character.

```python
from math import floor

def pad(s, length, char = ' '):
  return s.rjust(floor((len(s) + length)/2), char).ljust(length, char)
```

```python
pad('cat', 8) # '  cat   '
pad('42', 6, '0') # '004200'
pad('foobar', 3) # 'foobar'
```

#### Pads a given number to the specified length.

* Use `str.zfill()` to pad the number to the specified length, after converting it to a string.

```python
def pad_number(n, l):
  return str(n).zfill(l)
```

```python
pad_number(1234, 6); # '001234'
```

#### Checks if the given string is a palindrome.

* Use `str.lower()` and `re.sub()` to convert to lowercase and remove non-alphanumeric characters from the given string.
* Then, compare the new string with its reverse, using slice notation.

```python
from re import sub

def palindrome(s):
  s = sub('[\W_]', '', s.lower())
  return s == s[::-1]
```

```python
palindrome('taco cat') # True
```

#### Converts a list of dictionaries into a list of values corresponding to the specified `key`.

* Use a list comprehension and `dict.get()` to get the value of `key` for each dictionary in `lst`.

```python
def pluck(lst, key):
  return [x.get(key) for x in lst]
```

```python
simpsons = [
  { 'name': 'lisa', 'age': 8 },
  { 'name': 'homer', 'age': 36 },
  { 'name': 'marge', 'age': 34 },
  { 'name': 'bart', 'age': 10 }
]
pluck(simpsons, 'age') # [8, 36, 34, 10]
```

#### Returns the powerset of a given iterable.

* Use `list()` to convert the given value to a list.
* Use `range()` and `itertools.combinations()` to create a generator that returns all subsets.
* Use `itertools.chain.from_iterable()` and `list()` to consume the generator and return a list.

```python
from itertools import chain, combinations

def powerset(iterable):
  s = list(iterable)
  return list(chain.from_iterable(combinations(s, r) for r in range(len(s)+1)))
```

```python
powerset([1, 2]) # [(), (1,), (2,), (1, 2)]
```

#### Converts an angle from radians to degrees.

* Use `math.pi` and the radian to degree formula to convert the angle from radians to degrees.

```python
from math import pi

def rads_to_degrees(rad):
  return (rad * 180.0) / pi
```

```python
from math import pi

rads_to_degrees(pi / 2) # 90.0
```

#### Reverses a list or a string.

* Use slice notation to reverse the list or string.

```python
def reverse(itr):
  return itr[::-1]
```

```python
reverse([1, 2, 3]) # [3, 2, 1]
reverse('snippet') # 'teppins'
```

#### Reverses a number.

* Use `str()` to convert the number to a string, slice notation to reverse it and `str.replace()` to remove the sign.
* Use `float()` to convert the result to a number and `math.copysign()` to copy the original sign.

```python
from math import copysign

def reverse_number(n):
  return copysign(float(str(n)[::-1].replace('-', '')), n)
```

```python
reverse_number(981) # 189
reverse_number(-500) # -5
reverse_number(73.6) # 6.37
reverse_number(-5.23) # -32.5
```

#### Converts the values of RGB components to a hexadecimal color code.

* Create a placeholder for a zero-padded hexadecimal value using `'{:02X}'` and copy it three times.
* Use `str.format()` on the resulting string to replace the placeholders with the given values.

```python
def rgb_to_hex(r, g, b):
  return ('{:02X}' * 3).format(r, g, b)
```

```python
rgb_to_hex(255, 165, 1) # 'FFA501'
```

#### Moves the specified amount of elements to the start of the list.

* Use slice notation to get the two slices of the list and combine them before returning.

```python
def roll(lst, offset):
  return lst[-offset:] + lst[:-offset]
```

```python
roll([1, 2, 3, 4, 5], 2) # [4, 5, 1, 2, 3]
roll([1, 2, 3, 4, 5], -2) # [3, 4, 5, 1, 2]
```

#### Returns a random element from a list.

* Use `random.choice()` to get a random element from `lst`.

```python
from random import choice

def sample(lst):
  return choice(lst)
```

```python
sample([3, 7, 9, 11]) # 9
```

#### Randomizes the order of the values of an list, returning a new list.

* Uses the [Fisher-Yates algorithm](https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle) to reorder the elements of the list.
* [`random.shuffle`](https://docs.python.org/3/library/random.html#random.shuffle) provides similar functionality to this snippet.

```python
from copy import deepcopy
from random import randint

def shuffle(lst):
  temp_lst = deepcopy(lst)
  m = len(temp_lst)
  while (m):
    m -= 1
    i = randint(0, m)
    temp_lst[m], temp_lst[i] = temp_lst[i], temp_lst[m]
  return temp_lst
```

```python
foo = [1, 2, 3]
shuffle(foo) # [2, 3, 1], foo = [1, 2, 3]
```

#### Returns a list of elements that exist in both lists.

* Use a list comprehension on `a` to only keep values contained in both lists.

```python
def similarity(a, b):
  return [item for item in a if item in b]
```

```python
similarity([1, 2, 3], [1, 2, 4]) # [1, 2]
```

#### Converts a string to a URL-friendly slug.

* Use `str.lower()` and `str.strip()` to normalize the input string.
* Use `re.sub()` to to replace spaces, dashes and underscores with `-` and remove special characters.

```python
import re

def slugify(s):
  s = s.lower().strip()
  s = re.sub(r'[^\w\s-]', '', s)
  s = re.sub(r'[\s_-]+', '-', s)
  s = re.sub(r'^-+|-+$', '', s)
  return s
```

```python
slugify('Hello World!') # 'hello-world'
```

#### Converts a string to snake case.

* Use `re.sub()` to match all words in the string, `str.lower()` to lowercase them.
* Use `re.sub()` to replace any `-` characters with spaces.
* Finally, use `str.join()` to combine all words using `-` as the separator.

```python
from re import sub

def snake(s):
  return '_'.join(
    sub('([A-Z][a-z]+)', r' \1',
    sub('([A-Z]+)', r' \1',
    s.replace('-', ' '))).split()).lower()
```

```python
snake('camelCase') # 'camel_case'
snake('some text') # 'some_text'
snake('some-mixed_string With spaces_underscores-and-hyphens')
# 'some_mixed_string_with_spaces_underscores_and_hyphens'
snake('AllThe-small Things') # 'all_the_small_things'
```

#### Checks if the provided function returns `True` for at least one element in the list.

* Use `any()` in combination with `map()` to check if `fn` returns `True` for any element in the list.

```python
def some(lst, fn = lambda x: x):
  return any(map(fn, lst))
```

```python
some([0, 1, 2, 0], lambda x: x >= 2 ) # True
some([0, 0, 1, 0]) # True
```

#### Sorts one list based on another list containing the desired indexes.

* Use `zip()` and `sorted()` to combine and sort the two lists, based on the values of `indexes`.
* Use a list comprehension to get the first element of each pair from the result.
* Use the `reverse` parameter in `sorted()` to sort the dictionary in reverse order, based on the third argument.

```python
def sort_by_indexes(lst, indexes, reverse=False):
  return [val for (_, val) in sorted(zip(indexes, lst), key=lambda x: \
          x[0], reverse=reverse)]
```

```python
a = ['eggs', 'bread', 'oranges', 'jam', 'apples', 'milk']
b = [3, 2, 6, 4, 1, 5]
sort_by_indexes(a, b) # ['apples', 'bread', 'eggs', 'jam', 'milk', 'oranges']
sort_by_indexes(a, b, True)
# ['oranges', 'milk', 'jam', 'eggs', 'bread', 'apples']
```

#### Sorts the given dictionary by key.

* Use `dict.items()` to get a list of tuple pairs from `d` and sort it using `sorted()`.
* Use `dict()` to convert the sorted list back to a dictionary.
* Use the `reverse` parameter in `sorted()` to sort the dictionary in reverse order, based on the second argument.

```python
def sort_dict_by_key(d, reverse = False):
  return dict(sorted(d.items(), reverse = reverse))
```

```python
d = {'one': 1, 'three': 3, 'five': 5, 'two': 2, 'four': 4}
sort_dict_by_key(d) # {'five': 5, 'four': 4, 'one': 1, 'three': 3, 'two': 2}
sort_dict_by_key(d, True)
# {'two': 2, 'three': 3, 'one': 1, 'four': 4, 'five': 5}
```

#### Sorts the given dictionary by value.

* Use `dict.items()` to get a list of tuple pairs from `d` and sort it using a lambda function and `sorted()`.
* Use `dict()` to convert the sorted list back to a dictionary.
* Use the `reverse` parameter in `sorted()` to sort the dictionary in reverse order, based on the second argument.
* **⚠️ NOTICE:** Dictionary values must be of the same type.

```python
def sort_dict_by_value(d, reverse = False):
  return dict(sorted(d.items(), key = lambda x: x[1], reverse = reverse))
```

```python
d = {'one': 1, 'three': 3, 'five': 5, 'two': 2, 'four': 4}
sort_dict_by_value(d) # {'one': 1, 'two': 2, 'three': 3, 'four': 4, 'five': 5}
sort_dict_by_value(d, True)
# {'five': 5, 'four': 4, 'three': 3, 'two': 2, 'one': 1}
```

#### Splits a multiline string into a list of lines.

* Use `str.split()` and `'\n'` to match line breaks and create a list.
* [`str.splitlines()`](https://docs.python.org/3/library/stdtypes.html#str.splitlines) provides similar functionality to this snippet.

```python
def split_lines(s):
  return s.split('\n')
```

```python
split_lines('This\nis a\nmultiline\nstring.\n')
# ['This', 'is a', 'multiline', 'string.' , '']
```

#### Flattens a list, by spreading its elements into a new list.

* Loop over elements, use `list.extend()` if the element is a list, `list.append()` otherwise.

```python
def spread(arg):
  ret = []
  for i in arg:
    ret.extend(i) if isinstance(i, list) else ret.append(i)
  return ret
```

```python
spread([1, 2, 3, [4, 5, 6], [7], 8, 9]) # [1, 2, 3, 4, 5, 6, 7, 8, 9]
```

#### Calculates the sum of a list, after mapping each element to a value using the provided function.

* Use `map()` with `fn` to map each element to a value using the provided function.
* Use `sum()` to return the sum of the values.

```python
def sum_by(lst, fn):
  return sum(map(fn, lst))
```

```python
sum_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) # 20
```

#### Returns the sum of the powers of all the numbers from `start` to `end` (both inclusive).

* Use `range()` in combination with a list comprehension to create a list of elements in the desired range raised to the given `power`.
* Use `sum()` to add the values together.
* Omit the second argument, `power`, to use a default power of `2`.
* Omit the third argument, `start`, to use a default starting value of `1`.

```python
def sum_of_powers(end, power = 2, start = 1):
  return sum([(i) ** power for i in range(start, end + 1)])
```

```python
sum_of_powers(10) # 385
sum_of_powers(10, 3) # 3025
sum_of_powers(10, 3, 5) # 2925
```

#### Returns the symmetric difference between two iterables, without filtering out duplicate values.

* Create a `set` from each list.
* Use a list comprehension on each of them to only keep values not contained in the previously created set of the other.

```python
def symmetric_difference(a, b):
  (_a, _b) = (set(a), set(b))
  return [item for item in a if item not in _b] + [item for item in b
          if item not in _a]
```

```python
symmetric_difference([1, 2, 3], [1, 2, 4]) # [3, 4]
```

#### Returns the symmetric difference between two lists, after applying the provided function to each list element of both.

* Create a `set` by applying `fn` to each element in every list.
* Use a list comprehension in combination with `fn` on each of them to only keep values not contained in the previously created set of the other.

```python
def symmetric_difference_by(a, b, fn):
  (_a, _b) = (set(map(fn, a)), set(map(fn, b)))
  return [item for item in a if fn(item) not in _b] + [item
          for item in b if fn(item) not in _a]
```

```python
from math import floor

symmetric_difference_by([2.1, 1.2], [2.3, 3.4], floor) # [1.2, 3.4]
```

#### Returns all elements in a list except for the first one.

* Use slice notation to return the last element if the list's length is more than `1`.
* Otherwise, return the whole list.

```python
def tail(lst):
  return lst[1:] if len(lst) > 1 else lst
```

```python
tail([1, 2, 3]) # [2, 3]
tail([1]) # [1]
```

#### Returns a list with `n` elements removed from the beginning.

* Use slice notation to create a slice of the list with `n` elements taken from the beginning.

```python
def take(itr, n = 1):
  return itr[:n]
```

```python
take([1, 2, 3], 5) # [1, 2, 3]
take([1, 2, 3], 0) # []
```

#### Returns a list with `n` elements removed from the end.

* Use slice notation to create a slice of the list with `n` elements taken from the end.

```python
def take_right(itr, n = 1):
  return itr[-n:]
```

```python
take_right([1, 2, 3], 2) # [2, 3]
take_right([1, 2, 3]) # [3]
```

#### Returns the binary representation of the given number.

* Use `bin()` to convert a given decimal number into its binary equivalent.

```python
def to_binary(n):
  return bin(n)
```

```python
to_binary(100) # 0b1100100
```

#### Combines two lists into a dictionary, where the elements of the first one serve as the keys and the elements of the second one serve as the values.

The values of the first list need to be unique and hashable.

* Use `zip()` in combination with `dict()` to combine the values of the two lists into a dictionary.

```python
def to_dictionary(keys, values):
  return dict(zip(keys, values))
```

```python
to_dictionary(['a', 'b'], [1, 2]) # { a: 1, b: 2 }
```

#### Returns the hexadecimal representation of the given number.

* Use `hex()` to convert a given decimal number into its hexadecimal equivalent.

```python
def to_hex(dec):
  return hex(dec)
```

```python
to_hex(41) # 0x29
to_hex(332) # 0x14c
```

#### Converts a date to its ISO-8601 representation.

* Use `datetime.datetime.isoformat()` to convert the given `datetime.datetime` object to an ISO-8601 date.

```python
from datetime import datetime

def to_iso_date(d):
  return d.isoformat()
```

```python
from datetime import datetime

to_iso_date(datetime(2020, 10, 25)) # 2020-10-25T00:00:00
```

#### Converts an integer to its roman numeral representation.

Accepts value between `1` and `3999` (both inclusive).

* Create a lookup list containing tuples in the form of (roman value, integer).
* Use a `for` loop to iterate over the values in `lookup`.
* Use `divmod()` to update `num` with the remainder, adding the roman numeral representation to the result.

```python
def to_roman_numeral(num):
  lookup = [
    (1000, 'M'),
    (900, 'CM'),
    (500, 'D'),
    (400, 'CD'),
    (100, 'C'),
    (90, 'XC'),
    (50, 'L'),
    (40, 'XL'),
    (10, 'X'),
    (9, 'IX'),
    (5, 'V'),
    (4, 'IV'),
    (1, 'I'),
  ]
  res = ''
  for (n, roman) in lookup:
    (d, num) = divmod(num, n)
    res += roman * d
  return res
```

```python
to_roman_numeral(3) # 'III'
to_roman_numeral(11) # 'XI'
to_roman_numeral(1998) # 'MCMXCVIII'
```

#### Transposes a two-dimensional list.

* Use `*lst` to get the provided list as tuples.
* Use `zip()` in combination with `list()` to create the transpose of the given two-dimensional list.

```python
def transpose(lst):
  return list(zip(*lst))
```

```python
transpose([[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]])
# [(1, 4, 7, 10), (2, 5, 8, 11), (3, 6, 9, 12)]
```

#### Builds a list, using an iterator function and an initial seed value.

* The iterator function accepts one argument (`seed`) and must always return a list with two elements (\[`value`, `nextSeed`]) or `False` to terminate.
* Use a generator function, `fn_generator`, that uses a `while` loop to call the iterator function and `yield` the `value` until it returns `False`.
* Use a list comprehension to return the list that is produced by the generator, using the iterator function.

```python
def unfold(fn, seed):
  def fn_generator(val):
    while True:
      val = fn(val[1])
      if val == False: break
      yield val[0]
  return [i for i in fn_generator([None, seed])]
```

```python
f = lambda n: False if n > 50 else [-n, n + 10]
unfold(f, 10) # [-10, -20, -30, -40, -50]
```

#### Returns every element that exists in any of the two lists once.

* Create a `set` with all values of `a` and `b` and convert to a `list`.

```python
def union(a, b):
  return list(set(a + b))
```

```python
union([1, 2, 3], [4, 3, 2]) # [1, 2, 3, 4]
```

#### Returns every element that exists in any of the two lists once, after applying the provided function to each element of both.

* Create a `set` by applying `fn` to each element in `a`.
* Use a list comprehension in combination with `fn` on `b` to only keep values not contained in the previously created set, `_a`.
* Finally, create a `set` from the previous result and `a` and transform it into a `list`

```python
def union_by(a, b, fn):
  _a = set(map(fn, a))
  return list(set(a + [item for item in b if fn(item) not in _a]))
```

```python
from math import floor

union_by([2.1], [1.2, 2.3], floor) # [2.1, 1.2]
```

#### Returns the unique elements in a given list.

* Create a `set` from the list to discard duplicated values, then return a `list` from it.

```python
def unique_elements(li):
  return list(set(li))
```

```python
unique_elements([1, 2, 2, 3, 4, 3]) # [1, 2, 3, 4]
```

#### Returns a flat list of all the values in a flat dictionary.

* Use `dict.values()` to return the values in the given dictionary.
* Return a `list()` of the previous result.

```python
def values_only(flat_dict):
  return list(flat_dict.values())
```

```python
ages = {
  'Peter': 10,
  'Isabel': 11,
  'Anna': 9,
}
values_only(ages) # [10, 11, 9]
```

#### Returns the weighted average of two or more numbers.

* Use `sum()` to sum the products of the numbers by their weight and to sum the weights.
* Use `zip()` and a list comprehension to iterate over the pairs of values and weights.

```python
def weighted_average(nums, weights):
  return sum(x * y for x, y in zip(nums, weights)) / sum(weights)
```

```python
weighted_average([1, 2, 3], [0.6, 0.2, 0.3]) # 1.72727
```

#### Tests a value, `x`, against a testing function, conditionally applying a function.

* Check if the value of `predicate(x)` is `True` and if so return `when_true(x)`, otherwise return `x`.

```python
def when(predicate, when_true):
  return lambda x: when_true(x) if predicate(x) else x
```

```python
double_even_numbers = when(lambda x: x % 2 == 0, lambda x : x * 2)
double_even_numbers(2) # 4
double_even_numbers(1) # 1
```

#### Converts a given string into a list of words.

* Use `re.findall()` with the supplied `pattern` to find all matching substrings.
* Omit the second argument to use the default regexp, which matches alphanumeric and hyphens.

```python
import re

def words(s, pattern = '[a-zA-Z-]+'):
  return re.findall(pattern, s)
```

```python
words('I love Python!!') # ['I', 'love', 'Python']
words('python, javaScript & coffee') # ['python', 'javaScript', 'coffee']
words('build -q --out one-item', r'\b[a-zA-Z-]+\b')
# ['build', 'q', 'out', 'one-item']
```