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Using complex expressions is deceitful. In Python functions the default value is evaluated only once, at function creation time, but inspect.signature will evaluate it every time. For example foo(x={}) and foo(x=dict()) means the same in function declaration, but different in signature.
It could also affect security, because allow arbitrary code execution at the place where it was not allowed before.
I think this issue should be discussed on Python-Dev. I'm not sure that it is pythonic.