[issue38654] `urllib.request.Request` uses mutable value as default value

[Efanzh]

Efanzh <efanzh at gmail.com> added the comment:

I agree that this doesn’t fix any bug. But is is easy to make mistakes by using mutable default values (See https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments). Both PyCharm and pylint show warnings about mutable default arguments even there is no bug in the code.

Using `{}` as default value, by looking at the function signature, I might wonder whether this argument stays the same on each function call, since it is mutable. But by using `None` as default value, I am know this argument will always be `None` on each function call. Using `None` as default value makes the function interface more easy to understand.

Also, if some new Python coders saw `[]` or `{}` being used as default values in the standard library, they might think “I’ll do it too since the standard library does it”.

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<https://bugs.python.org/issue38654>
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