Why is the empty dictionary a dangerous default value in Python? [duplicate]

问题

This question already has answers here:

“Least Astonishment” and the Mutable Default Argument (31 answers)

Python methods: default parameter values are evaluated ONCE (3 answers)

Closed 5 years ago.

I put empty braces 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 None instead?

回答1:

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.

Yes, using None is both safe and conventional in such cases.

回答2:

Let's look at an example:

def f(value, key, hash={}):
    hash[value] = key
    return hash

print f('a', 1)
print f('b', 2)

Which you probably expect to output:

{'a': 1}
{'b': 2}

But actually outputs:

{'a': 1}
{'a': 1, 'b': 2}

来源：https://stackoverflow.com/questions/26320899/why-is-the-empty-dictionary-a-dangerous-default-value-in-python