Importer Wall Of Shame

The following modules and packages violate protocol or best practice in some way:

  • They run magic during __init.py__ that makes life hard for Mitogen. Executing code during module import is always bad, and Mitogen is a concrete benchmark for why it’s bad.
  • They install crap in sys.modules that completely ignore or partially implement the protocols laid out in PEP-302.
  • They “vendor” a third party package, either incompletely, using hacks visible through the runtime’s standard interfaces, or with ancient versions of code that in turn mess with sys.modules in some horrible way.

Bugs will probably be filed for these in time, but it does not address the huge installed base of existing old software versions, so hacks are needed anyway.


It claims to use pkg_resources to read version information (_get_version_from_pkg_metadata()), which would result in PEP-302 being reused and everything just working wonderfully, but instead it actually does direct filesystem access.

What could it do instead?

  • pkg_resources.resource_stream()

What Mitogen is forced to do

When it sees pbr being loaded, it smodges the process environment with a PBR_VERSION variable to override any attempt to auto-detect the version. This will probably break code I haven’t seen yet.


Anything that imports pkg_resources will eventually cause pkg_resources to try and import and scan __main__ for its __requires__ attribute (pkg_resources/ This breaks any app that is not expecting its __main__ to suddenly be sucked over a network and injected into a remote process, like py.test.

A future version of Mitogen might have a more general hack that doesn’t import the master’s __main__ as __main__ in the slave, avoiding all kinds of issues like these.

What could it do instead?

  • Explicit is better than implicit: wait until the magical behaviour is explicitly requested (i.e. an API call).
  • Use get("__main__") on sys.modules rather than import, but this method isn’t general enough, it only really helps tools like Mitogen.

What Mitogen is forced to do

Examine the stack during every attempt to import __main__ and check if the requestee module is named pkg_resources, if so then refuse the import.


The six module makes some effort to conform to PEP-302, but it is missing several critical pieces, e.g. the __loader__ attribute. This not only breaks the Python standard library tooling (such as the inspect module), but also Mitogen. Newer versions of six improve things somewhat, but there are still outstanding issues preventing Mitogen from working with six.

This package is sufficiently popular that it must eventually be supported. See here for an example issue.

What could it do instead?

  • Any custom hacks installed into sys.modules should support the protocols laid out in PEP-302.

What Mitogen is forced to do

Vendored versions of six currently don’t work at all.