It is possible to get valid ParseResult from the urlparse function even for a non-numeric port value. Only by requesting the port it fails[1].
Would it be an improvement if _checknetloc[2] validates the value of port properly?


// code snippet
Python 3.8.5 (default, Jan 27 2021, 15:41:15) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from urllib.parse import urlparse
>>> uri = 'xx://foo:bar'
>>> uri_parts = urlparse(uri)
>>> uri_parts.netloc
'foo:bar'
>>> uri_parts.port
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/urllib/parse.py", line 174, in port
    raise ValueError(message) from None
ValueError: Port could not be cast to integer value as 'bar'
// code snippet

[1] https://github.com/python/cpython/blob/e1903e11a3d42512effe336026e0c67f602e5848/Lib/urllib/parse.py#L172
[2] https://github.com/python/cpython/blob/e1903e11a3d42512effe336026e0c67f602e5848/Lib/urllib/parse.py#L416

I guess moving port validation logic to parsing time is done as part of https://github.com/python/cpython/pull/16780

Treating this as bug in itself might be a better idea than waiting for a ipv6 scope introduction, which had few caveats. 

> Would it be an improvement if _checknetloc[2] validates the value of port properly?

Yes, we could check if it is an int. That should be sufficient.

Thank you for your swift response and your willingness to add port validation to _checknetloc.

I think the validation itself should compound both exceptional branches implemented in port[3]
* port is an int
* port is in the range

[3] https://github.com/python/cpython/blob/e1903e11a3d42512effe336026e0c67f602e5848/Lib/urllib/parse.py#L173-L178

I also think the validation logic should be ran as early as possible.

I gave it a shot and implemented it. 

I appreciate any reviews: https://github.com/python/cpython/pull/25774

Got some ideas from https://github.com/python/cpython/pull/16780