Conditionally include Python in the C++ builds. (#5190)

* Conditionally include Python in the C++ builds.

Added an option `USE_PYTHON` that allows users to decide to link to Python3 or not.

* Also conditionally include this defintion.

* Define `USE_PYTHON` for the Windows builds.

* Remove Python3 reference in CMake test as it's no longer required.

* Accidentally removed the closing > from the decl.

* Also add `USE_PYTHON` when building the separate image library on Windows.

* Also conditionally include this depening on USE_PYTHON.

* Go back to require Python for this example.

* Find Python in the hello world example.

* Add a paragraph documenting the `USE_PYTHON` option.
Co-authored-by: Prabhat Roy <prabhatroy@fb.com>

CMakeLists.txt

@@ -4,6 +4,7 @@ set(CMAKE_CXX_STANDARD 14)
 file(STRINGS version.txt TORCHVISION_VERSION)
 
 option(WITH_CUDA "Enable CUDA support" OFF)
+option(USE_PYTHON "Link to Python when building" OFF)
 
 if(WITH_CUDA)
   enable_language(CUDA)
@@ -17,7 +18,10 @@ if(WITH_CUDA)
   endif()
 endif()
 
-find_package(Python3 COMPONENTS Development)
+if (USE_PYTHON)
+  add_definitions(-DUSE_PYTHON)
+  find_package(Python3 REQUIRED COMPONENTS Development)
+endif()
 
 find_package(Torch REQUIRED)
 find_package(PNG REQUIRED)
@@ -76,7 +80,12 @@ FOREACH(DIR ${ALLOW_LISTED})
 ENDFOREACH()
 
 add_library(${PROJECT_NAME} SHARED ${ALL_SOURCES})
-target_link_libraries(${PROJECT_NAME} PRIVATE ${TORCH_LIBRARIES} ${PNG_LIBRARY} ${JPEG_LIBRARIES} Python3::Python)
+target_link_libraries(${PROJECT_NAME} PRIVATE ${TORCH_LIBRARIES} ${PNG_LIBRARY} ${JPEG_LIBRARIES})
+
+if (USE_PYTHON)
+  target_link_libraries(${PROJECT_NAME} PRIVATE Python3::Python)
+endif()
+
 set_target_properties(${PROJECT_NAME} PROPERTIES
   EXPORT_NAME TorchVision
   INSTALL_RPATH ${TORCH_INSTALL_PREFIX}/lib)

README.rst

@@ -157,6 +157,10 @@ so make sure that it is also available to cmake via the ``CMAKE_PREFIX_PATH``.
 
 For an example setup, take a look at ``examples/cpp/hello_world``.
 
+Python linking is disabled by default when compiling TorchVision with CMake, this allows you to run models without any Python 
+dependency. In some special cases where TorchVision's operators are used from Python code, you may need to link to Python. This 
+can be done by passing ``-DUSE_PYTHON=on`` to CMake.
+
 TorchVision Operators
 ---------------------
 In order to get the torchvision operators registered with torch (eg. for the JIT), all you need to do is to ensure that you

cmake/TorchVisionConfig.cmake.in

@@ -28,8 +28,10 @@ include("${CMAKE_CURRENT_LIST_DIR}/${PN}Targets.cmake")
 if(NOT TARGET torch_library)
 find_package(Torch REQUIRED)
 endif()
-if(NOT TARGET Python3::Python)
-find_package(Python3 COMPONENTS Development)
+if (@USE_PYTHON@)
+  if(NOT TARGET Python3::Python)
+    find_package(Python3 COMPONENTS Development)
+  endif()
 endif()
 
 set_target_properties(TorchVision::TorchVision PROPERTIES INTERFACE_INCLUDE_DIRECTORIES "${${PN}_INCLUDE_DIR}" INTERFACE_LINK_LIBRARIES "torch;Python3::Python" )

examples/cpp/hello_world/CMakeLists.txt

@@ -6,6 +6,10 @@ project(hello-world)
 # so there is no need to also add `find_package(Torch)` here.
 find_package(TorchVision REQUIRED)
 
+# This due to LibTorch's version is the one included in the Python
+# package that links to Python.
+find_package(Python3 COMPONENTS Development)
+
 add_executable(hello-world main.cpp)
 
 # We now need to link the TorchVision library to our executable.

setup.py

@@ -201,7 +201,7 @@ def get_extensions():
 
     if sys.platform == "win32":
         define_macros += [("torchvision_EXPORTS", None)]
-
+        define_macros += [("USE_PYTHON", None)]
         extra_compile_args["cxx"].append("/MP")
 
     debug_mode = os.getenv("DEBUG", "0") == "1"
@@ -254,6 +254,9 @@ def get_extensions():
     image_library = []
     image_link_flags = []
 
+    if sys.platform == "win32":
+        image_macros += [("USE_PYTHON", None)]
+
     # Locating libPNG
     libpng = distutils.spawn.find_executable("libpng-config")
     pngfix = distutils.spawn.find_executable("pngfix")

torchvision/csrc/io/image/image.cpp

 #include "image.h"
 
 #include <ATen/core/op_registration/op_registration.h>
+#ifdef USE_PYTHON
 #include <Python.h>
+#endif
 
 // If we are in a Windows environment, we need to define
 // initialization functions for the _custom_ops extension
+#ifdef USE_PYTHON
 #ifdef _WIN32
 PyMODINIT_FUNC PyInit_image(void) {
   // No need to do anything.
   return NULL;
 }
 #endif
+#endif // USE_PYTHON
 
 namespace vision {
 namespace image {

torchvision/csrc/io/video_reader/video_reader.cpp

 #include "video_reader.h"
 
+#ifdef USE_PYTHON
 #include <Python.h>
+#endif
 
 #include "../decoder/memory_buffer.h"
 #include "../decoder/sync_decoder.h"
 
+#ifdef USE_PYTHON
 // If we are in a Windows environment, we need to define
 // initialization functions for the _custom_ops extension
 #ifdef _WIN32
@@ -13,6 +16,7 @@ PyMODINIT_FUNC PyInit_video_reader(void) {
   return NULL;
 }
 #endif
+#endif // USE_PYTHONs
 
 using namespace ffmpeg;
 

torchvision/csrc/vision.cpp

 #include "vision.h"
 
 #ifndef MOBILE
+#ifdef USE_PYTHON
 #include <Python.h>
 #endif
+#endif
 #include <torch/library.h>
 
 #ifdef WITH_CUDA
@@ -16,10 +18,12 @@
 // initialization functions for the _custom_ops extension.
 // For PyMODINIT_FUNC to work, we need to include Python.h
 #if !defined(MOBILE) && defined(_WIN32)
+#ifdef USE_PYTHON
 PyMODINIT_FUNC PyInit__C(void) {
   // No need to do anything.
   return NULL;
 }
+#endif // USE_PYTHON
 #endif // !defined(MOBILE) && defined(_WIN32)
 
 namespace vision {