What is the scope of a defaulted parameter in Python?

Question

When you define a function in Python with an array parameter, what is the scope of that parameter?

This example is taken from the Python tutorial:

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Answer 1

You have to keep in mind that python is an interpreted language. What is happening here is when the function "f" is defined, it creates the list and assigns it to the default parameter "L" of function "f". Later, when you call this function, the same list is used as the default parameter. In short, the code on the "def" line, only gets executed once when the function is defined. This is a common python pitfall, of which I have fallen in myself.

def f(a, L=[]):
    L.append(a)
    return L

print f(1)
print f(2)
print f(3)

There have been suggestions for idioms in other answers here to fix this issue. The one I would suggest is as follows:

def f(a, L=None):
    L = L or []
    L.append(a)
    return L

This uses the or short circuit to either take the "L" that was passed, or create a new list.

The answer to your scope question is the "L" only has a scope within the function "f", but because the default parameters are only assigned once to a single list instead of every time you call the function it behaves as if the default parameter "L" has a global scope.