How to prevent python from using orphaned .pyc files? (ones with no matching .py files)

Question

Once in a while I run into a very difficult-to-debug problem: there's a leftover .pyc file somewhere in my $PYTHONPATH, and the matching .py file has been moved to somewhere else that's later in $PYTHONPATH - so when I try to import the module, the "orphaned" .pyc file is used and all changes to the "real" .py file are ignored, leaving me incredibly confused until I figure out that's what's happening.

Is there any way of making python not use "orphaned" .pyc files, or print a warning when using them?
Alternatively, does the fact that I have this problem at all mean I'm doing something wrong, and if so, what?

Answer 1

Try this (see here):

PYTHONDONTWRITEBYTECODE

If this is set, Python won’t try to write .pyc or .pyo files on the import of source modules. This is equivalent to specifying the -B option.

New in version 2.6.

But there is one issue associated with it: you will loose the benefit of having .pyc files (thus losing performance). The better, cleaner and friendlier way is to walk through the directory and clean orphaned .pyc files you do not need. You should do it with a script and be sure you do not have .pyc files that are not meant to be associated with .py ones (eg. for some level of obfuscation).

Answer 2

You cannot prevent Python from loading .pyc files if it finds them in it's path, no.

You have a few options:

1. Import the offending module and figure out what it's path is:

   >>> import borkenmod
   >>> import sys
   >>> sys.modules[borkenmod.__name__].__file__
   'lib/python2.7/borkenmod.pyc'
   

2. Use a script to walk your PYTHONPATH and delete stale bytecode files. The linked script removes all .pyc files for which there is no corresponding .py file present.

3. Wipe all .pyc files in your PYTHONPATH, and have Python regenerate them with the compileall module:

   python -m compileall [path]
   

   then remove that one file.

Do note that the last two options could result in the removal of legitimate python modules! Some python modules (especially commercially licenced) are distributed as compiled bytecode only.