How Can I Preset Arguments for a Python Function Using `functools.partial`?

How to Bind Arguments to a Python Function

In Python, one can encounter situations where binding arguments to a function is necessary for later execution without the need for additional arguments. Consider the example:

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

add_5 = magic_function(add, 5)
assert add_5(3) == 8

In this scenario, we need to determine the magic_function that can bind the argument 5 to the add function.

The solution lies in using the functools.partial module:

import functools

add_5 = functools.partial(add, 5)  # Bind '5' as the first argument to 'add'
assert add_5(3) == 8  # Call 'add_5' with '3' as the second argument

functools.partial effectively creates a callable object that wraps the original function with a preset argument. This allows us to execute the function later on without providing the fixed argument explicitly.

The following code demonstrates an equivalent approach using a lambda expression:

print_hello = lambda *a, **kw: sys.stdout.write("Hello world\n", *a, **kw)

Here, the arguments to print_hello are treated as if they were given to sys.stdout.write, allowing for flexible argument passing during execution.

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