Python中需要留心的
Important warning: 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. 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)
This will print
If you don’t want the default to be shared between subsequent calls, you can write the function like this instead:
def f(a, L=None):
if L is None:
L = []
L.append(a)
return L
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