为什么空字典是 Python 中危险的默认值？
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Why is the empty dictionary a dangerous default value in Python?
提问by tscizzle
I put a dict as the default value for an optional argument to a Python function, and pylint (using Sublime package) told me it was dangerous. Can someone explain why this is the case? And is a better alternative to use Noneinstead?
我将 dict 作为 Python 函数的可选参数的默认值，pylint（使用 Sublime 包）告诉我这很危险。有人可以解释为什么会这样吗？是更好的替代品None吗？
采纳答案by John Zwinck
It's dangerous only if your function will modify the argument. If you modify a default argument, it will persist until the next call, so your "empty" dict will start to contain values on calls other than the first one.
仅当您的函数将修改参数时才危险。如果您修改默认参数，它将持续到下一次调用，因此您的“空”字典将开始包含除第一个调用之外的其他调用的值。
Yes, using Noneis both safe and conventional in such cases.
是的，None在这种情况下使用既安全又传统。
回答by Bill Lynch
Let's look at an example:
让我们看一个例子：
def f(value, key, hash={}):
hash[value] = key
return hash
print f('a', 1)
print f('b', 2)
Which you probably expect to output:
您可能希望输出：
{'a': 1}
{'b': 2}
But actually outputs:
但实际上输出：
{'a': 1}
{'a': 1, 'b': 2}
