Permit building python bindings separately from libsdformat library (backport #1491)

CMakeLists.txt

@@ -97,16 +97,6 @@ if (BUILD_SDF)
     )
     if (NOT Python3_Development_FOUND)
       GZ_BUILD_WARNING("Python development libraries are missing: Python interfaces are disabled.")
-    else()
-      set(PYBIND11_PYTHON_VERSION 3)
-      find_package(pybind11 2.4 CONFIG QUIET)
-
-      if (pybind11_FOUND)
-        message (STATUS "Searching for pybind11 - found version ${pybind11_VERSION}.")
-      else()
-        GZ_BUILD_WARNING("pybind11 is missing: Python interfaces are disabled.")
-        message (STATUS "Searching for pybind11 - not found.")
-      endif()
     endif()
   endif()
 
@@ -159,7 +149,7 @@ if (BUILD_SDF)
   add_subdirectory(sdf)
   add_subdirectory(conf)
   add_subdirectory(doc)
-  if (pybind11_FOUND AND NOT SKIP_PYBIND11)
+  if (Python3_Development_FOUND AND NOT SKIP_PYBIND11)
     add_subdirectory(python)
   endif()
 endif(BUILD_SDF)

README.md

@@ -165,9 +165,34 @@ sdformat supported cmake parameters at configuring time:
 
 | Name                  | Type | Default  | Description                          |
 |-----------------------|------|----------|--------------------------------------|
+| `SKIP_PYBIND11`       | BOOL | False    | Skip generating Python bindings via pybind11 |
 | `USE_INTERNAL_URDF`   | BOOL | False    | Use an internal copy of urdfdom 1.0.0 instead of looking for one installed in the system |
 | `USE_UPSTREAM_CFLAGS` | BOOL | True     | Use the sdformat team compilation flags instead of the common set defined by cmake.      |
 
+### Build python bindings separately from main library
+
+If you want to build Python bindings separately from the main libsdformat library
+(for example if you want to build Python bindings for multiple versions of Python),
+you can invoke cmake on the `python` folder instead of the root folder.
+This requires cmake version 3.22.1 due to the use of a `CMAKE_REQUIRE_FIND_PACKAGE_*`
+variable, which is newer than the minimum required version of cmake for sdformat14.
+Specify the path to the python executable with which you wish to build bindings
+in the `Python3_EXECUTABLE` cmake variable.
+Specify the install path for the bindings in the `CMAKE_INSTALL_PREFIX`
+variable, and be sure to set your `PYTHONPATH` accordingly after install.
+
+```bash
+cd sdformat
+mkdir build_python3
+cd build_python3
+cmake ../python \
+    -DPython3_EXECUTABLE=/usr/local/bin/python3.12 \
+    -DCMAKE_INSTALL_PREFIX=<prefix>
+```
+
+See the homebrew [sdformat15 formula](https://github.com/osrf/homebrew-simulation/blob/027d06f5be49da1e40d01180aedae7f76dc7ff47/Formula/sdformat15.rb#L12-L56)
+for an example of building bindings for multiple versions of Python.
+
 ## Uninstallation
 
 To uninstall the software installed with the previous steps:

python/CMakeLists.txt

@@ -1,15 +1,39 @@
-if(WIN32 AND CMAKE_BUILD_TYPE STREQUAL "Debug")
-  # pybind11 logic for setting up a debug build when both a debug and release
-  # python interpreter are present in the system seems to be pretty much broken.
-  # This works around the issue.
-  set(PYTHON_LIBRARIES "${PYTHON_DEBUG_LIBRARIES}")
+# Detect if we are doing a standalone build of the bindings, using an external sdformat
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  cmake_minimum_required(VERSION 3.22.1)
+  set(SDF_VER 14)
+  project(sdformat${SDF_VER}-python VERSION ${SDF_VER})
+  find_package(sdformat${SDF_VER} REQUIRED)
+  set(PROJECT_LIBRARY_TARGET_NAME "sdformat${PROJECT_VERSION_MAJOR}::sdformat${PROJECT_VERSION_MAJOR}")
+  # require python dependencies to be found
+  find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
+  set(CMAKE_REQUIRE_FIND_PACKAGE_pybind11 TRUE)
+  include(GNUInstallDirs)
+  include(CTest)
+elseif(${CMAKE_VERSION} VERSION_LESS "3.12.0")
+  # TODO: remove once the minimum CMake version is increased
+  if(WIN32 AND CMAKE_BUILD_TYPE STREQUAL "Debug")
+    # pybind11 logic for setting up a debug build when both a debug and release
+    # python interpreter are present in the system seems to be pretty much broken.
+    # This works around the issue.
+    set(PYTHON_LIBRARIES "${PYTHON_DEBUG_LIBRARIES}")
+  endif()
 endif()
 
+set(PYBIND11_PYTHON_VERSION 3)
+find_package(pybind11 2.4 CONFIG QUIET)
+
+if (pybind11_FOUND)
+  message (STATUS "Searching for pybind11 - found version ${pybind11_VERSION}.")
+else()
+  message(WARNING "pybind11 is missing: Python interfaces are disabled.")
+  return()
+endif()
 
 if(USE_SYSTEM_PATHS_FOR_PYTHON_INSTALLATION)
   if(${CMAKE_VERSION} VERSION_LESS "3.12.0")
     execute_process(
-      COMMAND "${PYTHON_EXECUTABLE}" -c "if True:
+      COMMAND "${Python3_EXECUTABLE}" -c "if True:
   from distutils import sysconfig as sc
   print(sc.get_python_lib(plat_specific=True))"
       OUTPUT_VARIABLE Python3_SITEARCH
@@ -28,7 +52,7 @@ if(USE_SYSTEM_PATHS_FOR_PYTHON_INSTALLATION)
   endif()
 else()
   # If not a system installation, respect local paths
-  set(GZ_PYTHON_INSTALL_PATH ${GZ_LIB_INSTALL_DIR}/python)
+  set(GZ_PYTHON_INSTALL_PATH ${CMAKE_INSTALL_LIBDIR}/python)
 endif()
 
 # Set the build location and install location for a CPython extension
@@ -124,7 +148,6 @@ if (BUILD_TESTING AND NOT WIN32)
 
   target_link_libraries(sdformattest PRIVATE
     ${PROJECT_LIBRARY_TARGET_NAME}
-    gz-utils${GZ_UTILS_VER}::gz-utils${GZ_UTILS_VER}
   )
 
   set(python_tests