Using via scikit-build

scikit-build provides two separate concepts geared towards the users of Python extension modules.

1. A setuptools replacement (legacy behaviour)

2. A series of cmake modules with definitions which help building Python extensions

Note

It is possible to use scikit-build’s cmake modules to bypass the cmake setup mechanism completely, and to write targets which call f2py -c. This usage is not recommended since the point of these build system documents are to move away from the internal numpy.distutils methods.

For situations where no setuptools replacements are required or wanted (i.e. if wheels are not needed), it is recommended to instead use the vanilla cmake setup described in Using via cmake.

Fibonacci walkthrough (F77)

We will consider the fib example from Three ways to wrap - getting started section.

C FILE: FIB1.F
      SUBROUTINE FIB(A,N)
C
C     CALCULATE FIRST N FIBONACCI NUMBERS
C
      INTEGER N
      REAL*8 A(N)
      DO I=1,N
         IF (I.EQ.1) THEN
            A(I) = 0.0D0
         ELSEIF (I.EQ.2) THEN
            A(I) = 1.0D0
         ELSE 
            A(I) = A(I-1) + A(I-2)
         ENDIF
      ENDDO
      END
C END FILE FIB1.F

CMake modules only

Consider using the following CMakeLists.txt.

### setup project ###
cmake_minimum_required(VERSION 3.9)

project(fibby
  VERSION 1.0
  DESCRIPTION "FIB module"
  LANGUAGES C Fortran
  )

# Safety net
if(PROJECT_SOURCE_DIR STREQUAL PROJECT_BINARY_DIR)
  message(
    FATAL_ERROR
      "In-source builds not allowed. Please make a new directory (called a build directory) and run CMake from there.\n"
  )
endif()

# Ensure scikit-build modules
if (NOT SKBUILD)
  find_package(PythonInterp 3.8 REQUIRED)
  # Kanged --> https://github.com/Kitware/torch_liberator/blob/master/CMakeLists.txt
  # If skbuild is not the driver; include its utilities in CMAKE_MODULE_PATH
  execute_process(
    COMMAND "${PYTHON_EXECUTABLE}"
    -c "import os, skbuild; print(os.path.dirname(skbuild.__file__))"
    OUTPUT_VARIABLE SKBLD_DIR
    OUTPUT_STRIP_TRAILING_WHITESPACE
  )
  list(APPEND CMAKE_MODULE_PATH "${SKBLD_DIR}/resources/cmake")
  message(STATUS "Looking in ${SKBLD_DIR}/resources/cmake for CMake modules")
endif()

# scikit-build style includes
find_package(PythonExtensions REQUIRED) # for ${PYTHON_EXTENSION_MODULE_SUFFIX}

# Grab the variables from a local Python installation
# NumPy headers
execute_process(
  COMMAND "${PYTHON_EXECUTABLE}"
  -c "import numpy; print(numpy.get_include())"
  OUTPUT_VARIABLE NumPy_INCLUDE_DIRS
  OUTPUT_STRIP_TRAILING_WHITESPACE
)
# F2PY headers
execute_process(
  COMMAND "${PYTHON_EXECUTABLE}"
  -c "import numpy.f2py; print(numpy.f2py.get_include())"
  OUTPUT_VARIABLE F2PY_INCLUDE_DIR
  OUTPUT_STRIP_TRAILING_WHITESPACE
)

# Prepping the module
set(f2py_module_name "fibby")
set(fortran_src_file "${CMAKE_SOURCE_DIR}/fib1.f")
set(f2py_module_c "${f2py_module_name}module.c")

# Target for enforcing dependencies
add_custom_target(genpyf
  DEPENDS "${fortran_src_file}"
)
add_custom_command(
  OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/${f2py_module_c}"
  COMMAND ${PYTHON_EXECUTABLE}  -m "numpy.f2py"
                   "${fortran_src_file}"
                   -m "fibby"
                   --lower # Important
  DEPENDS fib1.f # Fortran source
)

add_library(${CMAKE_PROJECT_NAME} MODULE
            "${f2py_module_name}module.c"
            "${F2PY_INCLUDE_DIR}/fortranobject.c"
            "${fortran_src_file}")

target_include_directories(${CMAKE_PROJECT_NAME} PUBLIC
                           ${F2PY_INCLUDE_DIR}
                           ${NumPy_INCLUDE_DIRS}
                           ${PYTHON_INCLUDE_DIRS})
set_target_properties(${CMAKE_PROJECT_NAME} PROPERTIES SUFFIX "${PYTHON_EXTENSION_MODULE_SUFFIX}")
set_target_properties(${CMAKE_PROJECT_NAME} PROPERTIES PREFIX "")

# Linker fixes
if (UNIX)
  if (APPLE)
    set_target_properties(${CMAKE_PROJECT_NAME} PROPERTIES
    LINK_FLAGS  '-Wl,-dylib,-undefined,dynamic_lookup')
  else()
    set_target_properties(${CMAKE_PROJECT_NAME} PROPERTIES
  LINK_FLAGS  '-Wl,--allow-shlib-undefined')
  endif()
endif()

add_dependencies(${CMAKE_PROJECT_NAME} genpyf)

install(TARGETS ${CMAKE_PROJECT_NAME} DESTINATION fibby)

Much of the logic is the same as in Using via cmake, however notably here the appropriate module suffix is generated via sysconfig.get_config_var("SO"). The resulting extension can be built and loaded in the standard workflow.

ls .
# CMakeLists.txt fib1.f
cmake -S . -B build
cmake --build build
cd build
python -c "import numpy as np; import fibby; a = np.zeros(9); fibby.fib(a); print (a)"
# [ 0.  1.  1.  2.  3.  5.  8. 13. 21.]

setuptools replacement

Note

As of November 2021

The behavior described here of driving the cmake build of a module is considered to be legacy behaviour and should not be depended on.

The utility of scikit-build lies in being able to drive the generation of more than extension modules, in particular a common usage pattern is the generation of Python distributables (for example for PyPI).

The workflow with scikit-build straightforwardly supports such packaging requirements. Consider augmenting the project with a setup.py as defined:

from skbuild import setup

setup(
    name="fibby",
    version="0.0.1",
    description="a minimal example package (fortran version)",
    license="MIT",
    packages=['fibby'],
    python_requires=">=3.7",
)

Along with a commensurate pyproject.toml

[build-system]
requires = ["setuptools>=42", "wheel", "scikit-build", "cmake>=3.9", "numpy>=1.21"]
build-backend = "setuptools.build_meta"

Together these can build the extension using cmake in tandem with other standard setuptools outputs. Running cmake through setup.py is mostly used when it is necessary to integrate with extension modules not built with cmake.

ls .
# CMakeLists.txt fib1.f pyproject.toml setup.py
python setup.py build_ext --inplace
python -c "import numpy as np; import fibby.fibby; a = np.zeros(9); fibby.fibby.fib(a); print (a)"
# [ 0.  1.  1.  2.  3.  5.  8. 13. 21.]

Where we have modified the path to the module as --inplace places the extension module in a subfolder.