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

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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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