Python中亟需留心的

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