4.9. Idiom Filter¶
filter(callable, *iterables)Select elements from sequence
Generator (lazy evaluated)
required
callable- Functionrequired
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))
4.9.1. Not-a-Generator¶
>>> from inspect import isgeneratorfunction, isgenerator
>>>
>>>
>>> def even(x):
... return x % 2 == 0
>>>
>>>
>>> isgeneratorfunction(filter)
False
>>>
>>> result = filter(even, [1,2,3])
>>> isgenerator(result)
False
4.9.2. 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]
4.9.3. Solution¶
>>> def even(x):
... return x % 2 == 0
>>>
>>>
>>> DATA = [1, 2, 3, 4, 5, 6]
>>> result = filter(even, DATA)
>>>
>>> list(result)
[2, 4, 6]
4.9.4. 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
4.9.5. Performance¶
>>> def even(x):
... return x % 2 == 0
>>>
>>>
>>> data = [1, 2, 3, 4, 5, 6]
>>>
... %%timeit -r 1000 -n 1000
... result = [x for x in data if even(x)]
1.11 µs ± 139 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
>>>
... %%timeit -r 1000 -n 1000
... result = list(filter(even, data))
921 ns ± 112 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
4.9.6. Use Case - 0x01¶
>>> users = [
... {'age': 41, 'username': 'mwatney'},
... {'age': 40, 'username': 'mlewis'},
... {'age': 39, 'username': 'rmartinez'},
... {'age': 40, 'username': 'avogel'},
... {'age': 29, 'username': 'bjohanssen'},
... {'age': 36, 'username': 'cbeck'},
... ]
>>> def above40(user):
... return user['age'] >= 40
>>>
>>> def under40(user):
... return user['age'] < 40
>>> result = filter(above40, users)
>>> list(result)
[{'age': 41, 'username': 'mwatney'},
{'age': 40, 'username': 'mlewis'},
{'age': 40, 'username': 'avogel'}]
>>> result = filter(under40, users)
>>> list(result)
[{'age': 39, 'username': 'rmartinez'},
{'age': 29, 'username': 'bjohanssen'},
{'age': 36, 'username': 'cbeck'}]
4.9.7. Use Case - 0x02¶
>>> users = [
... {'is_admin': False, 'name': 'Mark Watney'},
... {'is_admin': True, 'name': 'Melissa Lewis'},
... {'is_admin': False, 'name': 'Rick Martinez'},
... {'is_admin': False, 'name': 'Alex Vogel'},
... {'is_admin': True, 'name': 'Beth Johanssen'},
... {'is_admin': False, 'name': 'Chris Beck'},
... ]
>>>
>>>
>>> def admin(user):
... return user['is_admin'] is True
>>>
>>>
>>> result = filter(admin, users)
>>> list(result)
[{'is_admin': True, 'name': 'Melissa Lewis'},
{'is_admin': True, 'name': 'Beth Johanssen'}]
4.9.8. Use Case - 0x03¶
>>> users = [
... 'mwatney',
... 'mlewis',
... 'rmartinez',
... 'avogel',
... 'bjohanssen',
... 'cbeck',
... ]
>>>
>>> admins = [
... 'mlewis',
... 'bjohanssen',
... ]
>>>
>>>
>>> def is_admin(user):
... return user in admins
>>>
>>>
>>> result = filter(is_admin, users)
>>> list(result)
['mlewis', 'bjohanssen']
4.9.9. Use Case - 0x04¶
>>> class User:
... firstname: str
... lastname: str
... groups: list[str]
...
... def __init__(self, firstname, lastname, groups):
... self.firstname = firstname
... self.lastname = lastname
... self.groups = groups
...
... def __repr__(self):
... return f'{self.firstname}'
...
>>> DATABASE = [
... User('Mark', 'Watney', groups=['user', 'staff']),
... User('Melissa', 'Lewis', groups=['user', 'staff', 'admin']),
... User('Rick', 'Martinez', groups=['user', 'staff']),
... User('Alex', 'Vogel', groups=['user']),
... User('Beth', 'Johanssen', groups=['user', 'staff', 'admin']),
... User('Chris', 'Beck', groups=['user', 'staff']),
... ]
>>> def is_user(user: User) -> bool:
... return 'user' in user.groups
>>>
>>> def is_staff(user: User) -> bool:
... return 'staff' in user.groups
>>>
>>> def is_admin(user: User) -> bool:
... return 'admin' in user.groups
>>> users = filter(is_user, DATABASE)
>>> staff = filter(is_staff, DATABASE)
>>> admins = filter(is_admin, DATABASE)
>>> list(users)
[Mark, Melissa, Rick, Alex, Beth, Chris]
>>>
>>> list(staff)
[Mark, Melissa, Rick, Beth, Chris]
>>>
>>> list(admins)
[Melissa, Beth]
4.9.10. Assignments¶
"""
* Assignment: Idiom Filter Apply
* Required: yes
* Complexity: easy
* Lines of code: 3 lines
* Time: 3 min
English:
1. Define function `odd()`:
a. takes one argument
b. returns True if argument is odd
c. returns False if argument is even
2. Use `filter()` to apply function `odd()` to DATA
3. Define `result: filter` with result
4. Run doctests - all must succeed
Polish:
1. Zdefiniuj funckję `odd()`:
a. przyjmuje jeden argument
b. zwraca True jeżeli argument jest nieparzysty
c. zwraca False jeżeli argument jest parzysty
2. Użyj `filter()` zaaplikować funkcję `odd()` do DATA
3. Zdefiniuj `result: filter` z wynikiem
4. Uruchom doctesty - wszystkie muszą się powieść
Hints:
* filter()
Tests:
>>> import sys; sys.tracebacklimit = 0
>>> from inspect import isfunction
>>> assert isfunction(odd), \
'Object `odd` must be a function'
>>> assert result is not Ellipsis, \
'Assign result to variable: `result`'
>>> assert type(result) is filter, \
'Variable `result` has invalid type, should be filter'
>>> result = list(result)
>>> assert type(result) is list, \
'Evaluated `result` has invalid type, should be list'
>>> assert all(type(x) is int for x in result), \
'All rows in `result` should be int'
>>> result
[1, 3, 5, 7, 9]
"""
DATA = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# Returns if number is odd (modulo divisible by 2 without reminder)
# type: Callable[[int], int]
def odd(x):
...
# Cube numbers in DATA
# type: filter
result = ...