Partial Functions in Python

Partial functions allow us to fix a certain number of arguments of a function and generate a new function. In this article, we will try to understand the concept of Partial Functions with different examples in Python.

What are Partial functions and the use of partial functions in Python?

Partial functions in Python is a function that is created by fixing a certain number of arguments of another function. Python provides a built-in module called functools that includes a function called partial that can be used to create partial functions. The partial function takes a callable (usually another function) and a series of arguments to be pre-filled in the new partial function. This feature is similar to bind in C++.

How do you implement a partial function in Python?

Partial functions support both positional and keyword arguments to be used as fixed arguments.

Example 1

In this example, we use default values to implement the partial function. The default values start replacing variables from the left. In the example, we have pre-filled our function with some constant values of a, b, and c. And g() just takes a single argument i.e. the variable x.

Python3

from functools import partial

# A normal function
def f(a, b, c, x):
    return 1000*a + 100*b + 10*c + x

# A partial function that calls f with
# a as 3, b as 1 and c as 4.
g = partial(f, 3, 1, 4)

# Calling g()
print(g(5))

Output:

3145

Example 2

In the example, we have used pre-defined value constant values in which we have assigned the values of c and b and add_part() takes a single argument i.e. the variable a.

Python3

from functools import *

# A normal function
def add(a, b, c):
    return 100 * a + 10 * b + c

# A partial function with b = 1 and c = 2
add_part = partial(add, c = 2, b = 1)

# Calling partial function
print(add_part(3))

Output:

312

Uses of partial functions

• Integration with Libraries: Partial functions can be used to customize the behavior of third-party functions or methods by providing partial arguments and can be used to integrate it with other libraries.
• Simplifying Callbacks: Partial functions can be used to create specialized callback handlers by fixing some callback-specific parameters and providing a cleaner interface for the rest of the code.
• Parameter Fixing: : Partial functions can be very useful when we have a function with multiple parameters and we frequently want to use it with some parameters fixed. Instead of repeatedly passing those fixed parameters, we can create a partial function and call it with the remaining arguments.
• Reducing Duplication: If we are using the same arguments for a function in various places, creating a partial function with those fixed arguments can help to reduce code duplication and maintenance efforts.
• Default Arguments: Python’s built-in functools.partial can be used to set default values for function arguments.
• Reusability of code: Partial functions can be used to derive specialized functions from general functions and therefore help us to reuse our code.