8.3. Functional 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))
>>>
>>> result = (x for x in range(0,5) if x%2==0)
>>> result = filter(lambda x: x%2==0, range(0,5))
>>> from inspect import isgeneratorfunction, isgenerator
>>>
>>>
>>> def even(x):
... return x % 2 == 0
>>>
>>>
>>> isgeneratorfunction(filter)
False
>>>
>>> result = filter(even, [1,2,3])
>>> isgenerator(result)
False
8.3.1. 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]
8.3.2. Solution¶
>>> def even(x):
... return x % 2 == 0
>>>
>>>
>>> DATA = [1, 2, 3, 4, 5, 6]
>>> result = filter(even, DATA)
>>>
>>> list(result)
[2, 4, 6]
8.3.3. 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
8.3.4. 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)
8.3.5. 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(person):
... return person['age'] >= 40
>>>
>>> def under40(person):
... return person['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'}]
8.3.6. Use Case - 0x02¶
>>> users = [
... {'is_staff': True, 'username': 'mwatney'},
... {'is_staff': True, 'username': 'mlewis'},
... {'is_staff': True, 'username': 'rmartinez'},
... {'is_staff': False, 'username': 'avogel'},
... {'is_staff': True, 'username': 'bjohanssen'},
... {'is_staff': True, 'username': 'cbeck'},
... ]
>>>
>>>
>>> def can_login(user):
... return user['is_staff']
>>>
>>>
>>> staff = filter(can_login, users)
>>> list(staff)
[{'is_staff': True, 'username': 'mwatney'},
{'is_staff': True, 'username': 'mlewis'},
{'is_staff': True, 'username': 'rmartinez'},
{'is_staff': True, 'username': 'bjohanssen'},
{'is_staff': True, 'username': 'cbeck'}]
8.3.7. Use Case - 0x03¶
>>> users = [
... 'mwatney',
... 'mlewis',
... 'rmartinez',
... 'avogel',
... 'bjohanssen',
... 'cbeck',
... ]
>>>
>>> staff = [
... 'mwatney',
... 'mlewis',
... 'ptwardowski',
... 'jjimenez',
... ]
>>>
>>>
>>> def can_login(user):
... return user in staff
>>>
>>>
>>> result = filter(can_login, users)
>>> list(result)
['mwatney', 'mlewis']