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:

de         


        

Answer 1

The scope of the L variable is behaving as you expect.

The "problem" is with the list you're creating with []. Python does not create a new list each time you call the function. L gets assigned the same list each time you call which is why the function "remembers" previous calls.

So in effect this is what you have:

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

The Python Tutorial puts it this way:

The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes.

and suggests the following way to code the expected behaviour:

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