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functools

The functools library provides higher-order functions that act on or return other functions, compatible with Python’s functools module. Reach for it when reducing a list to a single value or pre-filling some of a function’s arguments.

Available Functions

Function Description
reduce(function, iterable[, initializer]) Apply function(acc, item) left to right to reduce a list to one value; raises on an empty list with no initializer.
partial(func, *args, **kwargs) New callable with positional and/or keyword arguments pre-filled; call-time arguments are appended.

Example

import functools

def add(x, y):
    return x + y

print(functools.reduce(add, [1, 2, 3, 4, 5]))   # 15  (((1+2)+3)+4)+5
print(functools.reduce(add, [1, 2, 3], 10))     # 16  (starts from the initializer)

def merge(acc, pair):
    acc[pair[0]] = pair[1]
    return acc

merged = functools.reduce(merge, [["a", 1], ["b", 2]], {})
print(merged["a"], merged["b"])                 # 1 2

add_five = functools.partial(add, 5)
print(add_five(3))                              # 8

def greet(name, greeting="Hello"):
    return greeting + ", " + name + "!"

say_hi = functools.partial(greet, greeting="Hi")
print(say_hi("Alice"))                          # Hi, Alice!

Python Compatibility

This library implements a subset of Python’s functools module. reduce() requires a list: wrap tuples, ranges and other iterables with list(...).

Function Supported
reduce Yes
partial Yes
partialmethod No
lru_cache No
cache No
cached_property No
wraps No
total_ordering No
cmp_to_key No

See Also

  • itertools: iteration and combinatorics utilities, including accumulate() for running totals.
  • contextlib: context manager utilities.