Python: Beware of mutable default arguments

While mutable default arguments don’t directly cause memory leaks in Python, they can lead to unexpected behaviour. Here’s an example that demonstrates the issue:

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

print(append_to_list(1)) # Output:  [1]
print(append_to_list(2)) # Output:  [1, 2]
print(append_to_list(3)) # Output:  [1, 2, 3]

Explanation:

• The default value for my_list is evaluated only once when the function is defined, not each time it's called.
• This means that the same list is used across all calls to the function if you don’t provide a different list as an argument.
• As a result, the list keeps growing with each call.

How to avoid this:

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The standard way to avoid this unexpected behaviour is to use None as the default value and create a new list inside the function if needed:

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

print(append_to_list(1)) # Output:  [1]
print(append_to_list(2)) # Output:  [2]
print(append_to_list(3)) # Output:  [3]