What are mutable default parameters in Python?

Can you explain what mutable default parameters are in Python?

Sure! Mutable default parameters are default values for function arguments that can be changed. For example, using a list as a default value.

Can you give me an example of how that works?

Of course! Here's a simple example:

def append_to_list(value, my_list=[]):
    my_list.append(value)
    return my_list

In this case, my_list is mutable and retains changes across function calls.

What happens if I call that function multiple times?

If you call append_to_list(1), then append_to_list(2), the second call will modify the same list from the first call, resulting in [1, 2].

So how can I avoid that issue?

To avoid this, you can use None as a default value and create a new list inside the function:

def append_to_list(value, my_list=None):
    if my_list is None:
        my_list = []
    my_list.append(value)
    return my_list