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Question: What will be the output of the following Python code?

def append_to_list(value, target_list=[]):
    target_list.append(value)
    return target_list

print(append_to_list(1))
print(append_to_list(2))

Options:

    
• A) [1]
[2]
    
• B) [1]
[1, 2]
    
• C) [1]
[2, 1]
    
• D) TypeError: default parameter cannot be modified

Correct Answer: B) [1] and [1, 2]

Explanation:

In Python, default arguments are evaluated only once at the time the function is defined, not each time the function is invoked. Because a list is a mutable object, changes made to the default list persist across subsequent calls.

    
• First invocation: append_to_list(1) uses the default list, appends 1, and outputs [1].
    
• Second invocation: append_to_list(2) uses the exact same list instance in memory, which already contains [1]. Appending 2 results in [1, 2].

Best Practice: To prevent this behavior, use None as the default value and initialize a new list inside the function body if no argument is passed.