http.client: Usage of mutable default headers={} in HTTPConnection.request

[pareshjoshij]

Bug report

Bug description:

Hi everyone,

I noticed that the request method defines headers with a mutable default argument ({}):

# Lib/http/client.py
def request(self, method, url, body=None, headers={}, *, encode_chunked=False):

I know the Python docs have a specific warning about this (in Section 4.9.1) because mutable defaults can sometimes accumulate values across calls if they get modified.

It looks like the current code is actually safe because _send_request just reads from the dictionary and doesn't write to it, so I doubt this is causing any bugs right now.

But I was just wondering—is this kept as {} for backward compatibility (maybe for tools inspecting the signature)? Or would it be better to update it to None to follow the standard best practice?

If you think it's worth fixing, I’d be happy to put together a PR for it. If it's better to just leave it alone to be safe, I totally understand!

Thanks!

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

[albertedwardson]

might be fixed by #141508 with changing default value to frozendict

cc @vstinner

[serhiy-storchaka]

There is nothing wrong with using mutable defaults if they are not mutated.

It is not worth fixing because there is no issue here.