4.5. Filter¶

4.5.1. Rationale¶

Select elements from sequence
Generator (lazy evaluated)
Built-in

Syntax:

filter(callable, *iterables)

required callable - Function

required iterables - 1 or many sequence or iterator objects

>>> def even(x):
...     return x % 2 == 0
>>>
>>> result = (x for x in range(0,5) if even(x))
>>> result = filter(even, range(0,5))

>>> result = (x for x in range(0,5) if x%2==0)
>>> result = filter(lambda x: x%2==0, range(0,5))

4.5.2. Problem¶

Plain code:

>>> def even(x):
...     return x % 2 == 0
>>>
>>>
>>> DATA = [1, 2, 3, 4, 5, 6]
>>> result = []
>>>
>>> for x in DATA:
...     if even(x):
...         result.append(x)
>>>
>>> print(result)
[2, 4, 6]

Comprehension:

>>> def even(x):
...     return x % 2 == 0
>>>
>>>
>>> DATA = [1, 2, 3, 4, 5, 6]
>>> result = [x for x in DATA if even(x)]
>>>
>>> print(result)
[2, 4, 6]

4.5.3. Solution¶

>>> def even(x):
...     return x % 2 == 0
>>>
>>>
>>> DATA = [1, 2, 3, 4, 5, 6]
>>> result = filter(even, DATA)
>>>
>>> list(result)
[2, 4, 6]

4.5.4. Lazy Evaluation¶

>>> def even(x):
...     return x % 2 == 0
>>>
>>>
>>> DATA = [1, 2, 3, 4, 5, 6]
>>> result = filter(even, DATA)
>>>
>>> next(result)
2
>>> next(result)
4
>>> next(result)
6
>>> next(result)
Traceback (most recent call last):
StopIteration

4.5.5. Use Cases¶

>>> people = [
...     {'age': 21, 'name': 'Jan Twardowski'},
...     {'age': 25, 'name': 'Mark Watney'},
...     {'age': 18, 'name': 'Melissa Lewis'}]
>>>
>>>
>>> def adult(person):
...     return person['age'] >= 21
>>>
>>>
>>> result = filter(adult, people)
>>> list(result)  
[{'age': 21, 'name': 'Jan Twardowski'},
 {'age': 25, 'name': 'Mark Watney'}]

>>> people = [
...     {'is_astronaut': False, 'name': 'Jan Twardowski'},
...     {'is_astronaut': True, 'name': 'Mark Watney'},
...     {'is_astronaut': True, 'name': 'Melissa Lewis'}]
>>>
>>>
>>> def astronaut(person):
...     return person['is_astronaut']
>>>
>>>
>>> result = filter(astronaut, people)
>>> list(result)  
[{'is_astronaut': True, 'name': 'Mark Watney'},
 {'is_astronaut': True, 'name': 'Melissa Lewis'}]

>>> astronauts = ['Mark Watney', 'Melissa Lewis']
>>>
>>> people = ['Jan Twardowski', 'Mark Watney',
...           'Melissa Lewis', 'Jimenez']
>>>
>>>
>>> def is_astronaut(person):
...     return person in astronauts
>>>
>>>
>>> result = filter(is_astronaut, people)
>>> list(result)
['Mark Watney', 'Melissa Lewis']

4.5.6. Performance¶

>>> # %%timeit -r 10 -n 100_000
>>> # result = (x for x in range(0,5) if x%2==0)
>>> # 490 ns ± 44 ns per loop (mean ± std. dev. of 10 runs, 100000 loops each)

>>> # %%timeit -r 10 -n 100_000
>>> # result = filter(lambda x: x%2==0, range(0,5))
>>> # 384 ns ± 34.2 ns per loop (mean ± std. dev. of 10 runs, 100000 loops each)

4.5.7. Assignments¶

Code 4.65. Solution¶

"""
* Assignment: Idioms Filter Chain
* Complexity: easy
* Lines of code: 5 lines
* Time: 8 min

English:
    1. Use generator expression to create `numbers`
    2. In generator use `range()` to get numbers from 1 to 33
       (inclusive) divisible by 3
    3. Use `filter()` to get odd numbers from `numbers`
    4. Use `map()` to cube all numbers in `numbers`
    5. Create `result: float` with arithmetic mean of `numbers`
    6. Do not use `lambda` expressions
    7. Run doctests - all must succeed

Polish:
    1. Użyj wyrażenia generatorowego do stworzenia `numbers`
    2. W generatorze użyj `range()` aby otrzymać liczby od 1 do 33
       (włącznie) podzielne przez 3
    3. Użyj `filter()` aby otrzymać liczby nieparzyste z `numbers`
    4. Użyj `map()` aby podnieść wszystkie liczby w `numbers` do sześcianu
    5. Stwórz `result: float` ze średnią arytmetyczną z `numbers`
    6. Nie używaj wyrażeń lambda
    7. Uruchom doctesty - wszystkie muszą się powieść

Hints:
    * type cast to `list()` to expand generator before calculating mean
    * `mean = sum(...) / len(...)`
    * TypeError: object of type 'map' has no len()
    * ZeroDivisionError: division by zero

Tests:
    >>> import sys; sys.tracebacklimit = 0
    >>> from inspect import isfunction

    >>> isfunction(odd)
    True
    >>> isfunction(cube)
    True
    >>> type(result) is float
    True
    >>> result
    11502.0
"""

def odd(x):
    return x % 2


def cube(x):
    return x ** 3


# float: generator expr with numbers from 1 to 33 (inclusive) divisible by 3
#        filter out even numbers; cube result; calculate mean
result: float