Issue understanding Warning: Unbrewed header files were found in /usr/local/include

Question

After running brew doctor, I get this error related to a header file that I'm not sure about. I don't know what greenlet is, so I'm apprehensive to remove it as suggested in this post.

Warning: Unbrewed header files were found in /usr/local/include.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.

Unexpected header files:
    /usr/local/include/python2.7/greenlet/greenlet.h

Please answer the following questions:

1. What does the error Unbrewed header files actually mean?

2. Should I remove the file listed in the error?

Answer 1

"Unbrewed header files" indicates that there are header files in the Homebrew tree (by default, /usr/local) that Homebrew itself did not put there. Likely they were put there by some other software you installed (in this case, python-greenlet).

If you are sure you don't have the software that requires those header files installed anymore, you can safely remove them. Otherwise, you should leave them alone. They could potentially cause problems if Homebrew has to install software with files with the same path, but that probably won't happen.