How Do * and Operators Unpack Arguments in Python Function Calls?

Unpacking in Function Calls Using and Operators*

Python's and * operators play crucial roles in function calls, allowing for convenient unpacking of sequences, collections, and dictionaries.

Unpacking Sequences and Collections with * (Single Star)

The * (single star) operator unpacks a sequence or collection into positional arguments. For example, consider the following code:

def add(a, b):
    return a + b

values = (1, 2)
s = add(*values)  # unpacks values into individual arguments

This is equivalent to writing:

s = add(1, 2)

Unpacking Dictionaries with (Double Star)**

The ** (double star) operator performs a similar operation for dictionaries, extracting named arguments. For instance, given:

values = {'a': 1, 'b': 2}
s = add(**values)  # unpacks values as keyword arguments

This is equivalent to:

s = add(a=1, b=2)

Combining Operators for Function Call Unpacking

Both and * operators can be used together in a function call. For instance, given:

def sum(a, b, c, d):
    return a + b + c + d

values1 = (1, 2)
values2 = {'c': 10, 'd': 15}
s = add(*values1, **values2)  # combines sequence and dictionary unpacking

This is equivalent to:

s = sum(1, 2, c=10, d=15)

Performance Implications

Unpacking operations with and * incur some overhead due to tuple and dictionary creation. However, for small data sets, the performance impact is generally negligible. For larger data sets, consider alternative methods for efficiency, such as using tuple and dictionary comprehension.

Additional Uses in Function Parameters

• and can also be used to accept variable-length arguments in function parameters. Refer to the complementary question "What does (double star/asterisk) and * (star/asterisk) do for parameters?" for more information.

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