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

[Efanzh]

New submission from Efanzh :

The `headers` argument of the `__init__` method of `urllib.request.Request` 
class uses `{}` as default value, which is mutable. It is not a good idea.

--
components: Library (Lib)
messages: 355750
nosy: EFanZh
priority: normal
pull_requests: 16537
severity: normal
status: open
title: `urllib.request.Request` uses mutable value as default value
type: enhancement
versions: Python 3.9

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

[Efanzh]

Efanzh  added the comment:

How about changing `if headers` to `if headers is not None`?

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

[Efanzh]

Efanzh  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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