Which $path is needed so g++/pybind11 could locate Python.h?

Question

I started using pybind11 (on Ubuntu 16.04 with Python 2.7).

To test the waters, I created a do-nothing wrapper around my c++ library. Alas, the compilation is unable to find Python.h:

$ g++ -std=c++0x -fPIC -pedantic -g -Wno-missing-field-initializers -Wno-switch -Wno-multichar   -ftree-vectorize  -ftree-vectorize -mssse3  backend.h uvc-v4l2.cpp wrap.cpp  -o wrap.so
backend.h:4:9: warning: #pragma once in main file
 #pragma once
         ^
In file included from /usr/local/include/pybind11/pytypes.h:12:0,
                 from /usr/local/include/pybind11/cast.h:13,
                 from /usr/local/include/pybind11/attr.h:13,
                 from /usr/local/include/pybind11/pybind11.h:36,
                 from wrap.cpp:1:
/usr/local/include/pybind11/common.h:72:20: fatal error: Python.h: No such file or directory
 #include <Python.h>
                    ^
compilation terminated.

However, I do seem to have the file:

$ find /usr -name Python.h
/usr/include/python2.7/Python.h
/usr/include/python3.5m/Python.h

Which (path?) should I correct so g++ would be able to locate Python.h?

Notes:

$ apt list | grep -iE -- '^python.-dev|^python-dev'

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

python-dev/xenial,now 2.7.11-1 amd64 [installed]
python3-dev/xenial,now 3.5.1-3 amd64 [installed]

$ dpkg -S Python.h
libpython2.7-dev:amd64: /usr/include/python2.7/Python.h
libpython3.5-dev:amd64: /usr/include/python3.5m/Python.h

$ dpkg -L python2.7-dev
/.
/usr
/usr/bin
/usr/share
/usr/share/man
/usr/share/man/man1
/usr/share/doc
/usr/share/doc/python2.7
/usr/share/doc/python2.7/x86_64-linux-gnu
/usr/share/doc/python2.7/x86_64-linux-gnu/test_results.gz
/usr/share/doc/python2.7/x86_64-linux-gnu/pybench.log.gz
/usr/share/doc/python2.7/gdbinit.gz
/usr/share/doc/python2.7/HISTORY.gz
/usr/share/doc/python2.7/README.valgrind.gz
/usr/share/doc/python2.7/README.maintainers
/usr/bin/python2.7-config
/usr/share/man/man1/python2.7-config.1.gz
/usr/share/doc/python2.7-dev

Answer 1

-I/usr/include/python2.7

The -I flag adds the directory to the list of directories to be searched for header files. If you were to instead want the python3.5 equivalent, you'd replace 2.7 with 3.5m

A more reliable way to get this flag is to use pkg-config. For instance:

$ pkg-config --cflags python
-I/usr/include/python2.7 -I/usr/include/x86_64-linux-gnu/python2.7
$ pkg-config --cflags python3
-I/usr/include/python3.6m -I/usr/include/x86_64-linux-gnu/python3.6m