There is a problem with the standard library's urlsplit and urlparse functions, in Python 2.7 (module urlparse) and 3.2+ (module urllib.parse).

The documentation for these functions [1] does not explain how they behave when given an invalid URL.

One could try invoking them manually and conclude that they tolerate anything thrown at them:

>>> urlparse('http:////::\\\\!!::!!++///')
ParseResult(scheme='http', netloc='', path='//::\\\\!!::!!++///',
params='', query='', fragment='')

>>> urlparse(os.urandom(32).decode('latin-1'))
ParseResult(scheme='', netloc='', path='\x7f¼â1gdä»6\x82', params='',
query='', fragment='\n\xadJ\x18+fli\x9cÛ\x9ak*ÄÅ\x02³F\x85Ç\x18')

Without studying the source code, it is impossible to know that there is a very narrow class of inputs on which they raise ValueError [2]:

>>> urlparse('http://[')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.5/urllib/parse.py", line 295, in urlparse
    splitresult = urlsplit(url, scheme, allow_fragments)
  File "/usr/lib/python3.5/urllib/parse.py", line 345, in urlsplit
    raise ValueError("Invalid IPv6 URL")
ValueError: Invalid IPv6 URL

This could be viewed as a documentation issue. But it could also be viewed as an implementation issue. Instead of raising ValueError on those square brackets, urlsplit could simply consider them *invalid* parts of an RFC 3986 reg-name, and lump them into netloc, as it already does with other *invalid* characters:

>>> urlparse('http://\0\0æí\n/')
ParseResult(scheme='http', netloc='\x00\x00æí\n', path='/', params='',
query='', fragment='')

Note that the raising behavior was introduced in Python 2.7/3.2.

See also issue 8721 [3].


[1] https://docs.python.org/3/library/urllib.parse.html
[2] https://github.com/python/cpython/blob/e32ec93/Lib/urllib/parse.py#L406-L408
[3] http://bugs.python.org/issue8721

A note in the docs would be useful.  This API is far too well established to make any behavioral changes at this point.

I'm going to make a note in the documentation. I should have a PR for it in about 1 day.

New changeset f6e863d868a621594df2a8abe072b5d4766e7137 by Senthil Kumaran (Howie Benefiel) in branch 'master':
 bpo-29651 - Cover edge case of square brackets in urllib docs (#1128)
https://github.com/python/cpython/commit/f6e863d868a621594df2a8abe072b5d4766e7137

New changeset 72e5aa1ef812358b3b113e784e7365fec13dfd69 by Senthil Kumaran in branch '3.5':
 bpo-29651 - Cover edge case of square brackets in urllib docs (#1128) (#1597)
https://github.com/python/cpython/commit/72e5aa1ef812358b3b113e784e7365fec13dfd69

New changeset 75b8a54bcad70806d9dcbbe20786f4d9092ab39c by Senthil Kumaran in branch '3.6':
 bpo-29651 - Cover edge case of square brackets in urllib docs (#1128) (#1596)
https://github.com/python/cpython/commit/75b8a54bcad70806d9dcbbe20786f4d9092ab39c