Python's functools.partial

functools.partial creates a callable with some arguments already supplied. This is called partial application: specialize an existing function, then provide the remaining arguments later. Creating the partial does not execute the original function. Python's functional programming HOWTO

Pre-fill an argument

Suppose a function raises a number to a power. We can make a reusable version that defaults to squaring:

from functools import partial

def power(base, exponent):
    return base ** exponent

square = partial(power, exponent=2)

print(square(5))              # 25
print(square(3))              # 9
print(square(5, exponent=3))  # 125

square(5) calls power(5, exponent=2). Stored keyword arguments can be overridden when calling the partial, so the last call uses exponent 3. The original power function remains unchanged. partial documentation

Argument order matters

Without placeholders, stored positional arguments come first; new positional arguments follow them:

two_to_power = partial(power, 2)
print(two_to_power(5))  # 32: power(2, 5)

This fixes the base, whereas partial(power, exponent=2) supplies the exponent. Also, square(5, 3) raises TypeError: it supplies exponent both positionally and through the stored keyword. Override it by name instead.

Where it helps

Use partial when an API expects a callable but you want to configure some inputs beforehand: callbacks, reusable settings, or functions passed to map. Here, map supplies each base while square supplies the exponent:

print(list(map(square, [2, 3, 4])))  # [4, 9, 16]

A lambda such as lambda base: power(base, 2) expresses the same calculation. As a practical rule, use partial for argument binding and a regular def when the wrapper needs validation or additional logic. See the HOWTO's discussion of small functions.

Leaving positional gaps: Python 3.14+

Placeholder reserves a positional slot for a later argument. This lets us bind the second position while leaving the first open:

from functools import Placeholder

cube = partial(power, Placeholder, 3)
print(cube(4))  # 64: power(4, 3)

Every placeholder must receive a positional argument when the partial is called. The earlier examples also work on Python versions before 3.14.