This affects both Python 2 and 3. This is as expected:

>>> urlparse('abc:123.html')
ParseResult(scheme='abc', netloc='', path='123.html', params='', query='', fragment='')
>>> urlparse('123.html:abc')
ParseResult(scheme='123.html', netloc='', path='abc', params='', query='', fragment='')
>>> urlparse('abc:123/')
ParseResult(scheme='abc', netloc='', path='123/', params='', query='', fragment='')

This is NOT:

>>> urlparse('abc:123')
ParseResult(scheme='', netloc='', path='abc:123', params='', query='', fragment='')

Expected is path='123' and scheme='abc'. At least according to my reading of the rfc (https://tools.ietf.org/html/rfc1808.html) that is what should happen.

See issue 14072.  It may be time to look at this again, but we may still be constrained by backward compatibility.

The main backward compatibility consideration would be Issue 754016, but don’t agree with the changes made, and would support reverting them. The original bug reporter wanted urlparse("1.2.3.4:80", "http") to be treated as the URL http://1.2.3.4:80, but the IP address was being parsed as a scheme, so the default “http” scheme was ignored.

The original fix (r83701) affected any URL that had a digit 0–9 immediately after the “scheme:” prefix. In such URLs, the scheme component was no longer parsed. A test case for “path:80” was added, and a demonstration of not parsing any scheme from www.cwi.nl:80/%7Eguido/Python.html was added in the documentation.

Later, the logic was altered to test if the URL looked like an integer (revision 495d12196487, Issue 11467). This restored proper parsing of clsid:85bbd92o-42a0-1o69-a2e4-08002b30309d and mailto:1337@example.org, although another URL given, javascript:123, remains misparsed. The documentation was subsequently adjusted in Issue 16932 to just demonstrate www.cwi.nl/%7Eguido/Python.html being parsed as a path.

The logic was watered down to its current form by revision 9f6b7576c08c, Issue 14072. Now it tests for a non-digit anywhere after the scheme, so that tel:+31641044153 is again parsed properly. But it was pointed out that tel:1234 remains misparsed.

What’s the next step in the watering-down process? All the attempts so far break valid URLs in favour of special-casing inputs that are not valid URLs.

I hate to say it, but this may require a python-dev discussion.  We probably ought to be parsing valid urls correctly as our top priority, but if that breaks our parsing of "reasonable" non-valid URLs (that existing code is depending on), it's going to be a backward compatibility problem.

On second thought, what are the chances that special casing something that looks like an IP address in the scheme position would maintain backward compatibility?

Depends on how you define “looks like an IP address”. Does the www.cwi.nl:80 case look like an IP address? What about “path:80” or “localhost:80”? If there is any code relying on the bug, it may just as easily involve host name as a numeric IP address.

Ah, good point, I misread the scope of the problem.

Based on discussion in issue 16932, I agree that reverting the parsing decisions from issue 754016 (as Martin suggested in msg271719) seems appropriate. I created a pull request that does that.

New changeset 5a88d50ff013a64fbdb25b877c87644a9034c969 by Senthil Kumaran (Tim Graham) in branch 'master':
bpo-27657: Fix urlparse() with numeric paths (#661)
https://github.com/python/cpython/commit/5a88d50ff013a64fbdb25b877c87644a9034c969

New changeset 82b5f6b16e051f8a2ac6e87ba86b082fa1c4a77f by Miss Islington (bot) in branch '3.7':
bpo-27657: Fix urlparse() with numeric paths (GH-661)
https://github.com/python/cpython/commit/82b5f6b16e051f8a2ac6e87ba86b082fa1c4a77f

New changeset 0f3187c1ce3b3ace60f6c1691dfa3d4e744f0384 by Senthil Kumaran in branch '3.8':
[3.8] bpo-27657: Fix urlparse() with numeric paths (GH-661) (#16839)
https://github.com/python/cpython/commit/0f3187c1ce3b3ace60f6c1691dfa3d4e744f0384

This issue got fixes, so I close it.

This is a surprising change to put in a minor release. This change totally changes the semantics of parsing scheme-less URLs with ports in them and ended up breaking a significant amount of my software. It turns out that urls like `example.com:80` are more common than one might hope, and a lot of software has always assumed that `example.com:80` would get parsed as the netloc and the software can guess the scheme based on the port...

@James - Originally the issue was considered a revert and the versions were set for the merge, but I certainly recognize the problem when parsing can fail for simple URLs like `localhost:8000` which is very common.

Another developer had raised the concerns with the change in this PR: https://github.com/python/cpython/pull/16839#issuecomment-570758153 

I am reopening this issue, and re-read the arguments again to understand and propose the next steps.

Just to add to the list of places this is causing a regression. This has broken the target host determination routines in gabbi: https://github.com/cdent/gabbi/issues/277

While the original fix may have been strictly correct in some ways, it results in a terrible UX, and as several others have noted violated backwards compatibility.

Hi Lukaz / Ned:

I will like to revert the backports done in 3.8 and 3.7.

Preferably in 3.8.2 and 3.7.7, so that this undesirable behavior exists only for a single release. 

I have set this is a release blocker to catch your attention.

New changeset 505b6015a1579fc50d9697e4a285ecc64976397a by Senthil Kumaran in branch '3.7':
Revert "bpo-27657: Fix urlparse() with numeric paths (GH-661)" (#18526)
https://github.com/python/cpython/commit/505b6015a1579fc50d9697e4a285ecc64976397a

New changeset ea316fd21527dec53e704a5b04833ac462ce3863 by Senthil Kumaran in branch '3.8':
Revert "[3.8] bpo-27657: Fix urlparse() with numeric paths (GH-16839)" (GH-18525)
https://github.com/python/cpython/commit/ea316fd21527dec53e704a5b04833ac462ce3863