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Info about inconsistent detection of Python version between pybind11 … (#1093)

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* Info about inconsistent detection of Python version between pybind11 and CMake in FAQ
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@ -242,6 +242,39 @@ that that were ``malloc()``-ed in another shared library, using data
structures with incompatible ABIs, and so on. pybind11 is very careful not
to make these types of mistakes.
Inconsistent detection of Python version in CMake and pybind11
==============================================================
The functions ``find_package(PythonInterp)`` and ``find_package(PythonLibs)`` provided by CMake
for Python version detection are not used by pybind11 due to unreliability and limitations that make
them unsuitable for pybind11's needs. Instead pybind provides its own, more reliable Python detection
CMake code. Conflicts can arise, however, when using pybind11 in a project that *also* uses the CMake
Python detection in a system with several Python versions installed.
This difference may cause inconsistencies and errors if *both* mechanisms are used in the same project. Consider the following
Cmake code executed in a system with Python 2.7 and 3.x installed:
.. code-block:: cmake
find_package(PythonInterp)
find_package(PythonLibs)
find_package(pybind11)
It will detect Python 2.7 and pybind11 will pick it as well.
In contrast this code:
.. code-block:: cmake
find_package(pybind11)
find_package(PythonInterp)
find_package(PythonLibs)
will detect Python 3.x for pybind11 and may crash on ``find_package(PythonLibs)`` afterwards.
It is advised to avoid using ``find_package(PythonInterp)`` and ``find_package(PythonLibs)`` from CMake and rely
on pybind11 in detecting Python version. If this is not possible CMake machinery should be called *before* including pybind11.
How to cite this project?
=========================
@ -256,4 +289,3 @@ discourse:
note = {https://github.com/pybind/pybind11},
title = {pybind11 -- Seamless operability between C++11 and Python}
}