Permit building python bindings separately from libsdformat library

README.md

@@ -165,9 +165,32 @@ 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.
+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

@@ -1,3 +1,20 @@
+# Detect if we are doing a standalone build of the bindings, using an external gz-math
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  cmake_minimum_required(VERSION 3.16)
+  set(SDF_VER 15)
+  set(GZ_UTILS_VER 3)
+  project(sdformat${SDF_VER}-python VERSION ${SDF_VER})
+  find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
+  find_package(pybind11 REQUIRED)
+  find_package(sdformat${SDF_VER} REQUIRED)
+  set(PROJECT_LIBRARY_TARGET_NAME "sdformat${PROJECT_VERSION_MAJOR}::sdformat${PROJECT_VERSION_MAJOR}")
+  include(GNUInstallDirs)
+  include(CTest)
+  if(BUILD_TESTING)
+    enable_testing()
+  endif()
+endif()
+
 if(USE_SYSTEM_PATHS_FOR_PYTHON_INSTALLATION)
   if(NOT Python3_SITEARCH)
     # Get variable from Python3 module
@@ -12,7 +29,7 @@ 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