Making an N-Argument Callable Work As a Callable with Fewer Arguments

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

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Problem

You have a callable that you would like to use with some other Python code, possibly as a callback function or handler, but it takes too many arguments and causes an exception when called.

Solution

If you need to reduce the number of arguments to a function, you should use functools.partial(). The partial() function allows you to assign fixed values to one or more of the arguments, thus reducing the number of arguments that need to be supplied to subsequent calls. To illustrate, suppose you have this function:

def spam(a, b, c, d):
    print(a, b, c, d)

Now consider the use of partial() to fix certain argument values:

>>> from functools import partial
>>> s1 = partial(spam, 1)       # a = 1
>>> s1(2, 3, 4)
1 2 3 4
>>> s1(4, 5, 6)
1 4 5 6
>>> s2 = partial(spam, d=42)    # d = 42
>>> s2(1, 2, 3)
1 2 3 42
>>> s2(4, 5, 5)
4 5 5 42
>>> s3 = partial(spam, 1, 2, d=42) # a = 1, b = 2, d = 42
>>> s3(3)
1 2 3 42
>>> s3(4)
1 2 4 42
>>> s3(5)
1 2 5 42
>>>

Observe that partial() fixes the values for certain arguments and returns a new callable as a result. This new callable accepts the still unassigned arguments, combines them with the arguments given to partial(), and passes everything to the original function.

Discussion

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