Default Argument Values: default value is evaluated only once
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#This makes a difference when the default is a mutable object such as a list,
#dictionary, or instances of most classes. For example, the following function
#accumulates the arguments passed to it on subsequent calls:
def f(a, L=[]):
L.append(a)
return L
print f(1)
print f(2)
print f(3)
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