If someone is running a brew python and does "pip install numpy"  will pip find a binary wheel that will then not work? That would be bad, but maybe not our problem -- brew users should be using brew to build numpy anyway. 

 

No that is not how homebrew works. Brew does not pack anything that pip installable: https://github.com/Homebrew/homebrew/wiki/Homebrew-and-Python

so that would be a big issue. Especially since installing numpy with pip and brew python just works without any problem when building from source. 

There are unofficial brew taps (channels) with python packages but this not the recommended way.