When there is a file called "sys" in the local directory of a python script and warning.warn is called with an invalid stacklevel, python tries to import that file and throws an error like the following:

>>> import warnings
>>> warnings.warn("foo", Warning, stacklevel=2)
Traceback (most recent call last):
  File "/usr/lib/python3.5/tokenize.py", line 392, in find_cookie
    line_string = line.decode('utf-8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 24: invalid start byte

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.5/warnings.py", line 18, in showwarning
    file.write(formatwarning(message, category, filename, lineno, line))
  File "/usr/lib/python3.5/warnings.py", line 26, in formatwarning
    line = linecache.getline(filename, lineno) if line is None else line
  File "/usr/lib/python3.5/linecache.py", line 16, in getline
    lines = getlines(filename, module_globals)
  File "/usr/lib/python3.5/linecache.py", line 47, in getlines
    return updatecache(filename, module_globals)
  File "/usr/lib/python3.5/linecache.py", line 136, in updatecache
    with tokenize.open(fullname) as fp:
  File "/usr/lib/python3.5/tokenize.py", line 456, in open
    encoding, lines = detect_encoding(buffer.readline)
  File "/usr/lib/python3.5/tokenize.py", line 433, in detect_encoding
    encoding = find_cookie(first)
  File "/usr/lib/python3.5/tokenize.py", line 397, in find_cookie
    raise SyntaxError(msg)
SyntaxError: invalid or missing encoding declaration for 'sys'

In this case "sys" is a binary that belongs to openafs (/usr/bin/sys) and of course it is no valid python.

"import sys" produces no error though, so typically python is able to distinguish between its sys module and this file.

A workaround is to run python with the "-I" parameter.

Expected output:

>>> import warnings
>>> warnings.warn("foo", Warning, stacklevel=2)
sys:1: Warning: foo

This bug was spotted in Gnome's pygobject bindings. Here is the corresponding bug report:
https://bugzilla.gnome.org/show_bug.cgi?id=757184