mitogen/docs/shame.rst

Module Import Wall Of Shame
---------------------------

The following modules and packages 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.


``pkg_resources``
=================

Anything that imports ``pkg_resources`` will eventually cause
pkg_resources to try and import and scan ``__main__`` for its ``__requires__``
attribute (``pkg_resources/__init__.py::_build_master()``). 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 :py:attr:`sys.modules` rather than ``import``, but
  this method isn't general enough, it only really helps tools like Mitogen.


``pbr``
=======

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. So we smodge the environment with a ``PBR_VERSION``
environment variable to override any version that was defined. This will
probably break code I haven't seen yet.

**What could it do instead?**

* ``pkg_resources.get_resource_stream()``