mutable_default_argument_demo.py

from typing import List, Optional

def get_array_from_dict(d: dict, key: str, default: List[object] = list()) -> List[object]:
    """Gets an array from a dict. Casts the type to List. Allows defaults to be set - no errors thrown."""
    return d.get(key, default)

foo = get_array_from_dict({}, "foo")
bar = get_array_from_dict({}, "bar")

foo.append(1)

print("get_array_from_dict")
print(foo, bar)

# but there's a way around this...

def get_list_from_dict(d: dict, key: str, default: Optional[List[object]] = None) -> List[object]:
    """Gets an array from a dict. Casts the type to List. Allows defaults to be set - no errors thrown."""
    result = d.get(key)
    if result is None:
        result = []

    return result

foo = get_list_from_dict({}, "foo")
bar = get_list_from_dict({}, "bar")

foo.append(1)

print("\nget_list_from_dict")
print(foo, bar)

"""
output:

get_array_from_dict
[1] [1]

get_list_from_dict
[1] []
"""