Add advanced example that installs a CMake package (#8)

* Add advanced example that installs a CMake package

This advanced example, unlike the other examples, builds a separate
shared library and installs a CMake package to use that shared library
with the Python package.

The example uses pybind11 to implement the C++-Python binding.
The example has Python tests, but is missing C++ tests.

* Change example to use packaged pybind11 cmakefiles

* Apply suggestions from code review

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

* Run cmake-format

* tests: run tests on CMake package

* ci: drop Windows

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

Author: ktbarrett

noxfile.py

@@ -1,7 +1,12 @@
+import sys
+
 import nox
 
-hello_list = "hello-pure", "hello-cpp", "hello-pybind11", "hello-cython"
-long_hello_list = hello_list + ("pen2-cython",)
+
+hello_list = ["hello-pure", "hello-cpp", "hello-pybind11", "hello-cython"]
+if not sys.platform.startswith("win"):
+    hello_list.append("hello-cmake-package")
+long_hello_list = hello_list + ["pen2-cython"]
 
 
 @nox.session

projects/hello-cmake-package/CMakeLists.txt

@@ -0,0 +1,109 @@
+cmake_minimum_required(VERSION 3.14...3.19)
+
+project(
+  hello
+  VERSION "0.1.0"
+  LANGUAGES CXX)
+
+include(GNUInstallDirs)
+
+# define the C++ library "hello"
+add_library(hello SHARED "${PROJECT_SOURCE_DIR}/src/hello.cpp")
+
+target_include_directories(
+  hello PUBLIC $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
+               $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+
+install(DIRECTORY "${PROJECT_SOURCE_DIR}/include/"
+        DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+
+# Standard installation subdirs for the C++ library are used. The files will end
+# up in the specified subdirectories under the Python package root. For example,
+# "<python package prefix>/hello/lib/" if the destination is "lib/".
+#
+# Installing the objects in the package provides encapsulation and will become
+# important later for binary redistribution reasons.
+install(
+  TARGETS hello
+  EXPORT helloTargets
+  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR})
+
+# The CMake package config and target files are installed under the Python
+# package root. This is necessary to ensure that all the relative paths in the
+# helloTargets.cmake resolve correctly. It also provides encapsulation.
+#
+# The actual path used must be selected so that consuming projects can locate it
+# via `find_package`. To support finding CMake packages in the Python package
+# prefix, using `find_package`s default search path of
+# `<prefix>/<name>/share/<name>*/cmake/` is reasonable. Adding the Python
+# package installation prefix to CMAKE_PREFIX_PATH in combination with this path
+# will allow `find_package` to find this package and any other package installed
+# via a Python package if the CMake and Python packages are named the same.
+set(HELLO_CMAKE_PACKAGE_INSTALL_SUBDIR "share/hello/cmake")
+
+install(
+  EXPORT helloTargets
+  NAMESPACE hello::
+  DESTINATION ${HELLO_CMAKE_PACKAGE_INSTALL_SUBDIR})
+
+include(CMakePackageConfigHelpers)
+
+write_basic_package_version_file(
+  helloConfigVersion.cmake
+  VERSION ${PROJECT_VERSION}
+  COMPATIBILITY SameMinorVersion)
+
+configure_package_config_file(
+  "${PROJECT_SOURCE_DIR}/cmake/helloConfig.cmake.in" helloConfig.cmake
+  INSTALL_DESTINATION ${HELLO_CMAKE_PACKAGE_INSTALL_SUBDIR})
+
+install(FILES "${PROJECT_BINARY_DIR}/helloConfig.cmake"
+              "${PROJECT_BINARY_DIR}/helloConfigVersion.cmake"
+        DESTINATION ${HELLO_CMAKE_PACKAGE_INSTALL_SUBDIR})
+
+# We are using the SKBUILD variable, which is defined when scikit-build is
+# running the CMake build, to control building the Python wrapper. This allows
+# the C++ project to be installed, standalone, when using the standard CMake
+# build flow.
+if(DEFINED SKBUILD)
+
+  # prevent an unused variable warning
+  set(ignoreMe "${SKBUILD}")
+
+  # call pybind11-config to obtain the root of the cmake package
+  execute_process(COMMAND ${PYTHON_EXECUTABLE} -m pybind11 --cmakedir
+                  OUTPUT_VARIABLE pybind11_ROOT_RAW)
+  string(STRIP ${pybind11_ROOT_RAW} pybind11_ROOT)
+  find_package(pybind11)
+
+  pybind11_add_module(_hello MODULE
+                      "${PROJECT_SOURCE_DIR}/src/hello/hello_py.cpp")
+
+  target_link_libraries(_hello PRIVATE hello)
+
+  # Installing the extension module to the root of the package
+  install(TARGETS _hello DESTINATION .)
+
+  configure_file("${PROJECT_SOURCE_DIR}/src/hello/__main__.py.in"
+                 "${PROJECT_BINARY_DIR}/src/hello/__main__.py")
+
+  install(FILES "${PROJECT_BINARY_DIR}/src/hello/__main__.py" DESTINATION .)
+
+  # The extension module must load the hello library as a dependency when the
+  # extension module is loaded. The easiest way to locate the hello library is
+  # via RPATH. Absolute RPATHs are possible, but they make the resulting
+  # binaries not redistributable to other Python installations (conda is broke,
+  # wheel reuse is broke, and more!).
+  #
+  # Placing the hello library in the package and using relative RPATHs that
+  # doesn't point outside of the package means that the built package is
+  # relocatable. This allows for safe binary redistribution.
+  if(APPLE)
+    set_target_properties(
+      _hello PROPERTIES INSTALL_RPATH "@loader_path/${CMAKE_INSTALL_LIBDIR}")
+  else()
+    set_target_properties(_hello PROPERTIES INSTALL_RPATH
+                                            "$ORIGIN/${CMAKE_INSTALL_LIBDIR}")
+  endif()
+
+endif()

projects/hello-cmake-package/README.md

@@ -0,0 +1,33 @@
+# Hello
+
+This is an example project demonstrating the use of scikit-build for distributing a standalone C library, *hello*;
+a CMake package for that library; and a Python wrapper implemented in pybind11.
+
+The example assume some familiarity with CMake and pybind11, only really going into detail on the scikit-build parts.
+pybind11 is used to implement the biding, but anything is possible: swig, C API library, etc.
+
+To install the package run in the project directory
+
+```bash
+pip install .
+```
+
+To run the Python tests, first install the package then in the project directory run
+
+```bash
+pytest
+```
+
+To run the C++ test, first install the package, then configure and build the project
+
+```bash
+cmake -S test/cpp -B build/ -Dhello_ROOT=$(python -m hello --cmakefiles)
+cmake --build build/
+```
+
+Then run ctest in the build dir
+
+```bash
+cd build/
+ctest
+```

projects/hello-cmake-package/cmake/helloConfig.cmake.in

@@ -0,0 +1,3 @@
+@PACKAGE_INIT@
+
+include("${CMAKE_CURRENT_LIST_DIR}/helloTargets.cmake")

projects/hello-cmake-package/include/hello.hpp

@@ -0,0 +1,14 @@
+#ifndef HELLO_HPP
+#define HELLO_HPP
+
+#include <iostream>
+
+namespace hello {
+
+void hello();
+
+int return_two();
+
+}  // namespace hello
+
+#endif

projects/hello-cmake-package/pyproject.toml

@@ -0,0 +1,10 @@
+[build-system]
+requires = [
+    "setuptools>=42",
+    "wheel",
+    "scikit-build",
+    "cmake>=3.14",
+    "ninja",
+    "pybind11>=2.6"
+]
+build-backend = "setuptools.build_meta"

projects/hello-cmake-package/setup.py

@@ -0,0 +1,14 @@
+from skbuild import setup
+
+
+setup(
+    name="hello-cmake-package",
+    version="0.1.0",
+    packages=['hello'],
+    package_dir={'': 'src'},
+    cmake_install_dir='src/hello')
+
+# When building extension modules `cmake_install_dir` should always be set to the
+# location of the package you are building extension modules for.
+# Specifying the installation directory in the CMakeLists subtley breaks the relative
+# paths in the helloTargets.cmake file to all of the library components.

projects/hello-cmake-package/src/hello.cpp

@@ -0,0 +1,9 @@
+#include "hello.hpp"
+
+namespace hello {
+
+void hello() { std::cout << "Hello, World!" << std::endl; }
+
+int return_two() { return 2; }
+
+}  // namespace hello

projects/hello-cmake-package/src/hello/__init__.py

@@ -0,0 +1,3 @@
+from ._hello import hello, return_two
+
+__all__ = ("hello", "return_two")

projects/hello-cmake-package/src/hello/__main__.py.in

@@ -0,0 +1,30 @@
+import argparse
+import sys
+from typing import List, Any
+from pathlib import Path
+import hello
+
+
+def main(argv: List[Any]) -> int:
+
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--cmakefiles", action='store_true', help="Print hello project CMake module directory. Useful for setting hello_ROOT in CMake.")
+    parser.add_argument("--prefix", action='store_true', help="Print hello package installation prefix. Useful for setting CMAKE_PREFIX_PATH in CMake.")
+
+    args = parser.parse_args(args=argv[1:])
+
+    if not argv[1:]:
+        parser.print_help()
+        return
+
+    prefix = Path(hello.__file__).parent
+
+    if args.cmakefiles:
+        print(prefix / "@HELLO_CMAKE_PACKAGE_INSTALL_SUBDIR@")
+
+    if args.prefix:
+        print(prefix)
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))