What is the pythonic way to avoid default parameters that are empty lists?

def my_func(working_list=None):
    if working_list is None: 
        working_list = []

    # alternative:
    # working_list = [] if working_list is None else working_list

    working_list.append("a")
    print(working_list)

The docs say you should use None as the default and explicitly test for it in the body of the function.

Other answers have already already provided the direct solutions as asked for, however, since this is a very common pitfall for new Python programmers, it's worth adding the explanation of why Python behaves this way, which is nicely summarized in The Hitchhikers Guide to Python under Mutable Default Arguments:

Python's default arguments are evaluated once when the function is defined, not each time the function is called (like it is in say, Ruby). This means that if you use a mutable default argument and mutate it, you will and have mutated that object for all future calls to the function as well.