I presume this is about parsing a URL like

>>> urlsplit("//user:[@host")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/proj/python/cpython/Lib/urllib/parse.py", line 431, in urlsplit
    raise ValueError("Invalid IPv6 URL")
ValueError: Invalid IPv6 URL

Ideally the square bracket should be escaped as %5B. Related reports about parsing unescaped delimiters in a URL password are Issue 18140 (fragment #, query ?) and Issue 23328 (slash /).