8.20. Functional Stdlib Functools

  • import functools

  • functools.partial()

  • functools.partialmethod()

  • functools.reduce()

  • functools.singledispatch()

  • functools.singledispatchmethod()

>>> import functools
>>>
>>> [method for method in dir(functools)
...         if not method.startswith('_')
...         and callable(getattr(functools, method))]   
['GenericAlias', 'RLock', 'cache', 'cached_property', 'cmp_to_key',
 'get_cache_token', 'lru_cache', 'namedtuple', 'partial', 'partialmethod',
 'recursive_repr', 'reduce', 'singledispatch', 'singledispatchmethod',
 'total_ordering', 'update_wrapper', 'wraps']

8.20.1. Partial

  • Create alias function and its arguments

  • Useful when you need to pass function with arguments to for example map or filter

>>> from functools import partial
>>>
>>>
>>> basetwo = partial(int, base=2)
>>> basetwo.__doc__ = 'Convert base 2 string to an int.'
>>> basetwo('10010')
18

8.20.2. Partialmethod

>>> from functools import partialmethod
>>>
>>>
>>> class Cell(object):
...     def __init__(self):
...         self._alive = False
...
...     @property
...     def alive(self):
...         return self._alive
...
...     def set_state(self, state):
...         self._alive = bool(state)
...
...     set_alive = partialmethod(set_state, True)
...     set_dead = partialmethod(set_state, False)
>>>
>>>
>>> c = Cell()
>>>
>>> c.alive
False
>>>
>>> c.set_alive()
>>> c.alive
True

8.20.3. Reduce

  • split-apply-combine strategy

Apply function of two arguments cumulatively to the items of iterable, from left to right, so as to reduce the iterable to a single value. For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). The left argument, x, is the accumulated value and the right argument, y, is the update value from the iterable. If the optional initializer is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty. If initializer is not given and iterable contains only one item, the first item is returned.

Roughly equivalent to:

>>> def reduce(function, iterable, initializer=None):
...     it = iter(iterable)
...     if initializer is None:
...         value = next(it)
...     else:
...         value = initializer
...     for element in it:
...         value = function(value, element)
...     return value

SetUp:

>>> from functools import reduce
>>>
>>> DATA = [1, 2, 3, 4, 5]

Usage:

>>> def add(x, y):
...     return (x + y)
>>>
>>> reduce(add, DATA)
15

>>> reduce(lambda x, y: x + y, DATA)
15

8.20.4. Singledispatch

  • Since Python 3.4

  • Overload a method

  • Python will choose function to run based on argument type

>>> from functools import singledispatch
>>>
>>>
>>> @singledispatch
... def celsius_to_kelvin(arg):
...     raise NotImplementedError('Argument must be int or list')
>>>
>>> @celsius_to_kelvin.register
... def _(degree: int):
...     return degree + 273.15
>>>
>>> @celsius_to_kelvin.register
... def _(degrees: list):
...     return [d+273.15 for d in degrees]
>>>
>>>
>>> celsius_to_kelvin(1)
274.15
>>>
>>> celsius_to_kelvin([1,2])
[274.15, 275.15]
>>>
>>> celsius_to_kelvin((1,2))
Traceback (most recent call last):
NotImplementedError: Argument must be int or list

>>> from functools import singledispatch
>>>
>>>
>>> @singledispatch
... def km_to_m(km):
...     raise NotImplementedError('...')
>>>
>>>
>>> @km_to_m.register
... def _(km: int):
...     return km * 1000
>>>
>>>
>>> @km_to_m.register
... def _(km: float):
...     return km * 1000.0
>>>
>>>
>>> @km_to_m.register
... def _(km: list):
...     return [x*1000 for x in km]

8.20.5. Singledispatchmethod

  • Since Python 3.8

  • Overload a method

  • Python will choose method to run based on argument type

>>> from functools import singledispatchmethod
>>>
>>>
>>> class Converter:
...
...     @singledispatchmethod
...     def celsius_to_kelvin(*args):
...         raise NotImplementedError('Argument must be int or list')
...
...     @celsius_to_kelvin.register
...     def _(self, degree: int):
...         return degree + 273.15
...
...     @celsius_to_kelvin.register
...     def _(self, degrees: list):
...         return [d+273.15 for d in degrees]
>>>
>>>
>>> conv = Converter()
>>>
>>> conv.celsius_to_kelvin(1)
274.15
>>>
>>> conv.celsius_to_kelvin([1,2])
[274.15, 275.15]
>>>
>>> conv.celsius_to_kelvin((1,2))
Traceback (most recent call last):
NotImplementedError: Argument must be int or list

8.20.6. Use Case - 0x01

>>> def square(x):
...     return x ** 2
>>>
>>> def cube(x):
...     return x ** 3
>>>
>>> def apply(data, fn):
...     return map(fn, data)
>>>
>>> def add(x, y):
...     return x + y

>>> data = [1,2,3,4]
>>> transformations = [square, cube]

>>> result = reduce(apply, transformations, data)
>>> list(result)
[1, 64, 729, 4096]

>>> result = reduce(apply, transformations, data)
>>> reduce(add, result)
4890

8.20.7. References