Created by: wangkuiyi

I see that the executable paddle_trainer is built by paddle/trainer/CMakeLists.txt using

add_paddle_exe(paddle_trainer TrainerMain.cpp)

where add_paddle_exe calls link_paddle_exe defined in cmake/util.cmake and specifies Python as a dependency:

    if(WITH_PYTHON)
        target_link_libraries(${TARGET_NAME}
            ${PYTHON_LIBRARIES})
    endif()

I am wondering how cmake knows the value of PYTHON_LIBRARIES. After grep-ing the whole source code repo, I noticed that it is defined in paddle/api/paddle_api_config.py.in, and paddle_api_config.py is imported by other Python modules. The importing sequence is as follows:

1. in paddle/CMakeLists.txt, cmake converts setup.py.in into setup.py and then add_subdirectory(api). Note that paddle/setup.py is listed in paddle/.gitignore.

2. paddle/setup.py.in calls import api.paddle_ld_flags, where paddle/api/paddle_ld_flags.py runs from paddle_api_config import *

3. where paddle/api/paddle_api_config.py is generated by paddle/api/CMakeLists.txt from paddle/api/paddle_api_config.py.in.

4. In paddle/api/paddle_api_config.py.in, there is PYTHON_LIBRARIES="@PYTHON_LIBRARIES@". This implies that cmake should have known the value of PYTHON_LIBRARIES before it can execute the .in template.

But how does cmake knows the value of PYTHON_LIBRARIES?!

I'd thought that it was set by cmake standard macro FindPythonInterp, but the document doesn't say so.