Warn about string exceptions

[Tristan Seligmann]

These can take several forms:

   raise "shouldn't happen"
   raise 'Cannot determine type of callable: '+repr(callableObject)
   raise "Can't read input parameters file : %s" % options.input_file

Some of these will be harder to detect than others.

[Phil Frost (bitglue)]

Pyflakes still supports python 2.5, where string exceptions are still allowed. Pyflakes has no way to know what the target python version is, so must take the conservative approach and assume the code might be targeted at python 2.5 and string exceptions are valid.

For code targeted at later versions, I suggest catching these errors in unit tests.

[Tristan Seligmann (mithrandi)]

Unfortunately the most common occurrences of string exceptions are in exceptional code paths that are never (and perhaps have never) been tested; but perhaps pyflakes is not the right tool for dealing with this problem (although, since pyflakes already has to deal with Python 2 vs Python 3, I could imagine support being added for more precise version targets).

[Florent (florent.x)]

According to the Python documentation:

« Raising of strings as exceptions, as in the statement raise "Error occurred", is deprecated in Python 2.5 and will trigger a warning. The aim is to be able to remove the string-exception feature in a few releases. »
https://docs.python.org/2.7/whatsnew/2.5.html#pep-352-exceptions-as-new-style-classes

So we could probably enforce this deprecation in a linter tool which supports Python 2.5.

With the examples posted in the initial requests, I think that this checker will be easy to implement for "simple cases" as a warning in the "pep8.py" tool, which does non-AST analysis of the code.

[Adi Roiban (adiroiban)]

+1 for fixing this in pep8.py