Mutable Default Arguments - Why they surprise you

This example shows why using a mutable object as a default argument causes shared state across calls. We include a step-by-step trace table and a production-safe fix.

def append_to(element, target=[]):
    target.append(element)
    return target

print(append_to(1))
print(append_to(2))

Trace walkthrough

1. At function definition time, Python creates the default list object: list_ref_1 = []
2. First call: append_to(1) uses target = list_ref_1, appends 1, returns [1]
3. Second call: append_to(2) again uses target = list_ref_1 (same object), appends 2, returns [1,2]

Correct pattern

def append_to(element, target=None):
    if target is None:
        target = []
    target.append(element)
    return target

Use `None` as the default and create the mutable inside the function to avoid accidental shared state. This is the recommended production pattern.