why are these not the same?

[Lowell Kirsh]

On a webpage (see link below) I read that the following 2 forms are not 
the same and that the second should be avoided. They look the same to 
me. What's the difference?

Lowell
----
def functionF(argString="abc", argList = None):
        if argList is None: argList = []
        ...
def functionF(argString="abc", argList=None):
        argList = argList or []
        ...
http://www.ferg.org/projects/python_gotchas.html (number 5)
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