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mitogen/docs/shame.rst

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7 years ago
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.
7 years ago
* They install crap in :py:data:`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 :py:data:`sys.modules` in some horrible way.
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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.
``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.
**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.
``pkg_resources``
=================
7 years ago
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).
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* Use ``get("__main__")`` on :py:data:`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.
``six``
=======
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 :py:mod:`inspect`
module), but also Mitogen. Newer versions of ``six`` improve things somewhat,
but there are still outstanding issues preventing Mitogen from working with
``six``.
7 years ago
This package is sufficiently popular that it must eventually be supported. See
`here for an example issue`_.
.. _here for an example issue: https://github.com/dw/mitogen/issues/31
**What could it do instead?**
* Any custom hacks installed into :py:data:`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.