Clean Code in Python

Mutable default arguments

Simply put, don't use mutable objects as the default arguments of functions. If you use mutable objects as default arguments, you will get results that are not the expected ones.

Consider the following erroneous function definition:

def wrong_user_display(user_metadata: dict = {"name": "John", "age": 30}):    name = user_metadata.pop("name")    age = user_metadata.pop("age")    return f"{name} ({age})"

This has two problems, actually. Besides the default mutable argument, the body of the function is mutating a mutable object, hence creating a side effect. But the main problem is the default argument for user_medatada.

This will actually only work the first time it is called without arguments. For the second time, we call ...