Partial Functions ​

The functools module in Python provides many useful features, one of which is partial functions. Note that the term "partial function" here is different from its mathematical meaning.

When discussing function parameters, we mentioned that setting default parameter values can simplify function calls. Partial functions can also achieve this. Here’s an example:

The int() function can convert a string to an integer. When only the string is passed in, the int() function converts it to a decimal integer by default:

>>> int('12345')
12345

However, the int() function also provides an additional base parameter, which defaults to 10. If the base parameter is passed in, it can perform conversions for other number systems:

>>> int('12345', base=8)
5349
>>> int('12345', 16)
74565

Suppose we need to convert a large number of binary strings; passing int(x, base=2) each time can be cumbersome. So, we might think about defining an int2() function that defaults base to 2:

def int2(x, base=2):
    return int(x, base)

This makes converting binary numbers much more convenient:

>>> int2('1000000')
64
>>> int2('1010101')
85

functools.partial helps us create a partial function, eliminating the need to define int2() ourselves. We can directly use the following code to create a new function int2:

>>> import functools
>>> int2 = functools.partial(int, base=2)
>>> int2('1000000')
64
>>> int2('1010101')
85

To summarize, the role of functools.partial is to fix some parameters of a function (i.e., set default values) and return a new function, making it easier to call.

Note that the new int2 function above simply resets the default value of the base parameter to 2, but other values can still be passed in during the function call:

>>> int2('1000000', base=10)
1000000

Finally, when creating a partial function, it can actually accept three parameters: a function object, *args, and **kw. When passing in:

int2 = functools.partial(int, base=2)

It essentially fixes the keyword argument base of the int() function, equivalent to:

int2('10010')

which is equivalent to:

kw = { 'base': 2 }
int('10010', **kw)

When passing in:

max2 = functools.partial(max, 10)

It will automatically add 10 as part of *args to the left, equivalent to:

max2(5, 6, 7)

which is equivalent to:

args = (10, 5, 6, 7)
max(*args)

The result will be 10.

Summary ​

When a function has too many parameters and needs to be simplified, using functools.partial can create a new function that fixes some of the original function's parameters, making the function calls simpler.