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

Question

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.

Not that it matters in this case, but you can use object identity to test for None:

if working_list is None: working_list = []

You could also take advantage of how the boolean operator or is defined in python:

working_list = working_list or []

Though this will behave unexpectedly if the caller gives you an empty list (which counts as false) as working_list and expects your function to modify the list he gave it.

If the intent of the function is to modify the parameter passed as working_list, see HenryR's answer (=None, check for None inside).

But if you didn't intend to mutate the argument, just use it as starting point for a list, you can simply copy it:

def myFunc(starting_list = []):
    starting_list = list(starting_list)
    starting_list.append("a")
    print starting_list

(or in this simple case just print starting_list + ["a"] but I guess that was just a toy example)

In general, mutating your arguments is bad style in Python. The only functions that are fully expected to mutate an object are methods of the object. It's even rarer to mutate an optional argument — is a side effect that happens only in some calls really the best interface?

• If you do it from the C habit of "output arguments", that's completely unnecessary - you can always return multiple values as a tuple.

• If you do this to efficiently build a long list of results without building intermediate lists, consider writing it as a generator and using result_list.extend(myFunc()) when you are calling it. This way your calling conventions remains very clean.

One pattern where mutating an optional arg is frequently done is a hidden "memo" arg in recursive functions:

def depth_first_walk_graph(graph, node, _visited=None):
    if _visited is None:
        _visited = set()  # create memo once in top-level call

    if node in _visited:
        return
    _visited.add(node)
    for neighbour in graph[node]:
        depth_first_walk_graph(graph, neighbour, _visited)