Mutable Default Arguments are a Trap!

In Python, using mutable objects (like lists or dictionaries) as default argument values can lead to unexpected behavior. This happens because the default value is created once when the function is defined — not each time it’s called.

The Problem

Here’s an example of what not to do:

def append_to_list(val, lst=[]):  # Bad!
    lst.append(val)
    return lst

print(append_to_list(1))  # [1]
print(append_to_list(2))  # [1, 2] - oops, shared list!

Because lst is defined once, all calls share the same list — which is usually not what you want.

The Right Way

Use None as the default, then create a new list inside the function if needed:

def append_to_list(val, lst=None):  # Good!
    if lst is None:
        lst = []
    lst.append(val)
    return lst

print(append_to_list(1))  # [1]
print(append_to_list(2))  # [2] - separate list each time

Key Takeaways

• Default argument values are evaluated once, at function definition time.
• Using mutable defaults can cause data to be shared between calls.
• Instead, use None and create a new object inside the function.

Remember: Immutable defaults are safe (like numbers, strings, or tuples), but mutable defaults can be a subtle source of bugs.