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

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“Least Astonishment” and the Mutable Default Argument 30 answers

Python methods: default parameter values are evaluated ONCE 3 answers

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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.

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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}

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