Using CMake with setup.py

Question

For a project I build a C library and implict Python bindings (via GObject introspection) with CMake. I also want to distribute some Python helper modules using distutils. I am able to build and install the module with this CMakeLists.txt

find_program(PYTHON "python")

if (PYTHON)
    set(SETUP_PY_IN "${CMAKE_CURRENT_SOURCE_DIR}/setup.py.in")
    set(SETUP_PY    "${CMAKE_CURRENT_BINARY_DIR}/setup.py")
    set(DEPS        "${CMAKE_CURRENT_SOURCE_DIR}/module/__init__.py")
    set(OUTPUT      "${CMAKE_CURRENT_BINARY_DIR}/build")

    configure_file(${SETUP_PY_IN} ${SETUP_PY})

    add_custom_command(OUTPUT ${OUTPUT}
                       COMMAND ${PYTHON}
                       ARGS setup.py build
                       DEPENDS ${DEPS})

    add_custom_target(target ALL DEPENDS ${OUTPUT})

    install(CODE "execute_process(COMMAND ${PYTHON} ${SETUP_PY} install)")
endif()

and the following setup.py.in:

from distutils.core import setup, Extension

if __name__ == '__main__':
    setup(name='foo',
          version='${PACKAGE_VERSION}',
          package_dir={ '': '${CMAKE_CURRENT_SOURCE_DIR}' },
          packages=['module'])

Unfortunately, the build step is executed each time I run make. I guess, the problem is related to the output of the custom command which is a directory rather than a file. Now, is there any way to tell CMake to run python setup.py build only when setup.py.in or one of the sources changed?

Answer 1

Only files, not directories, can be reliably used as OUTPUT and DEPENDS. You could modify your custom command to also produce a timestamp file, something like this:

add_custom_command(
  OUTPUT ${OUTPUT}/timestamp
  COMMAND ${PYTHON} setup.py build
  COMMAND ${CMAKE_COMMAND} -E touch ${OUTPUT}/timestamp
  DEPENDS ${DEPS}
)

add_custom_target(target ALL DEPENDS ${OUTPUT}/timestamp)