I missed #7229 but after discussing with @NelleV I would like to suggest another approach for working on the dev version of matplotlib, that I have always used myself.

I personally think that editable installs are pretty bad because you get the worst of both worlds: you mess up your everyday-use install (or you have to work in a {v,conda}env -- if anything I think suggesting setting up such an environment should come first, not as "you may want to consider ..."), and you still need to remember to build extensions every once in a while. (See extensive discussion in #7229.)

Instead, you can simply run python setup.py build_ext --inplace and then do whatever you want prefixed with PYTHONPATH=/path/to/mpl/lib.

I will readily acknowledge that there is one flaw with this approach: until recently, you could not work on the mpl_toolkits namespace package by setting PYTHONPATH, because the -nspkg.pth approach used by setuptools for namespace packages meant that PYTHONPATH would be ignored (see e.g. https://mail.python.org/pipermail/distutils-sig/2014-March/024050.html). Fortunately, this issue has been solved recently by setuptools 28.3.0 (https://github.com/pypa/setuptools/blob/master/CHANGES.rst#v2830): for Python3.3+, setuptools just relies on PEP420 support, which does respect PYTHONPATH.

attn @NelleV @tacaswell