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

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 None instead?

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}

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.