Allow choice of specific python release (#1410) (#1424)

* Allow choice of specific python release

If you're building on a system with multiple versions of Python available,
it can be very hard to make OCIO build against a specific version. In
particular, https://cmake.org/cmake/help/latest/module/FindPython.html
explains that FindPython looks preferably for version 3 of Python.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

* Amendments based on review comments:

* Rename OCIO_PREFERRED_PYTHON_VERSION -> OCIO_PYTHON_VERSION.
* Move the cached variable to the top level CMakeLists.txt instead of in
  FindExtPackages.cmake.
* Warning if OCIO_PYTHON_VERSION is set but OCIO_BUILD_PYTHON is off.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Co-authored-by: Patrick Hodoul <patrick.hodoul@autodesk.com>

Co-authored-by: Larry Gritz <lg@larrygritz.com>
Co-authored-by: doug-walker <43830961+doug-walker@users.noreply.github.com>

CMakeLists.txt

@@ -118,6 +118,9 @@ option(OCIO_BUILD_FROZEN_DOCS "Specify whether to build frozen documentation, ne
 option(OCIO_BUILD_DOCS "Specify whether to build documentation" ${OCIO_BUILD_FROZEN_DOCS})
 
 option(OCIO_BUILD_PYTHON "Specify whether to build python bindings" ON)
+set (OCIO_PYTHON_VERSION "" CACHE STRING
+     "Preferred Python version (if any) in case multiple are available")
+
 option(OCIO_BUILD_JAVA "Specify whether to build java bindings" OFF)
 
 option(OCIO_WARNING_AS_ERROR "Set build error level for CI testing" OFF)

share/cmake/modules/FindExtPackages.cmake

@@ -52,18 +52,9 @@ if(OCIO_BUILD_APPS)
     find_package(lcms2 2.2 REQUIRED)
 endif()
 
-if(OCIO_BUILD_PYTHON)
-
-    # NOTE: Depending of the compiler version pybind11 2.4.3 does not compile 
-    # with C++17 so, if you change the pybind11 version update the code to 
-    # compile pybind11 and dependencies with C++17 or higher i.e. remove the 
-    # cap of C++ version in FindPybind11.cmake and 
-    # src/bindings/python/CMakeLists.txt.
-
-    # pybind11
-    # https://github.com/pybind/pybind11
-    find_package(pybind11 2.6.1 REQUIRED)
-endif()
+if (OCIO_PYTHON_VERSION AND NOT OCIO_BUILD_PYTHON)
+    message (WARNING "OCIO_PYTHON_VERSION=${OCIO_PYTHON_VERSION} but OCIO_BUILD_PYTHON is off.")
+endif ()
 
 if(OCIO_BUILD_PYTHON OR OCIO_BUILD_DOCS)
 
@@ -76,5 +67,12 @@ if(OCIO_BUILD_PYTHON OR OCIO_BUILD_DOCS)
     endif()
 
     # Python
-    find_package(Python REQUIRED COMPONENTS ${_Python_COMPONENTS})
+    find_package(Python ${OCIO_PYTHON_VERSION} REQUIRED
+                 COMPONENTS ${_Python_COMPONENTS})
+
+    if(OCIO_BUILD_PYTHON)
+        # pybind11
+        # https://github.com/pybind/pybind11
+        find_package(pybind11 2.6.1 REQUIRED)
+    endif()
 endif()