added option to specify python version

CMakeLists.txt

@@ -78,6 +78,7 @@ endif()
 option(GTSAM_INSTALL_MATLAB_TOOLBOX      "Enable/Disable installation of matlab toolbox"  OFF)
 option(GTSAM_INSTALL_CYTHON_TOOLBOX      "Enable/Disable installation of Cython toolbox"  OFF)
 option(GTSAM_BUILD_WRAP                  "Enable/Disable building of matlab/cython wrap utility (necessary for matlab/cython interface)" ON)
+set(GTSAM_PYTHON_VERSION "Default" CACHE STRING "The version of python to build the cython wrapper or python module for (or Default)")
 
 # Check / set dependent variables for MATLAB wrapper
 if((GTSAM_INSTALL_MATLAB_TOOLBOX OR GTSAM_INSTALL_CYTHON_TOOLBOX) AND NOT GTSAM_BUILD_WRAP)
@@ -554,6 +555,7 @@ endif()
 
 message(STATUS "Cython toolbox flags                                      ")
 print_config_flag(${GTSAM_INSTALL_CYTHON_TOOLBOX}      "Install Cython toolbox         ")
+message(STATUS "  Python version                 : ${GTSAM_PYTHON_VERSION}")
 print_config_flag(${GTSAM_BUILD_WRAP}                  "Build Wrap                     ")
 message(STATUS "===============================================================")
 

cmake/FindNumPy.cmake

@@ -40,9 +40,17 @@
 
 # Finding NumPy involves calling the Python interpreter
 if(NumPy_FIND_REQUIRED)
+  if(GTSAM_PYTHON_VERSION STREQUAL "Default")
     find_package(PythonInterp REQUIRED)
+  else()
+      find_package(PythonInterp ${GTSAM_PYTHON_VERSION} EXACT REQUIRED)
+  endif()
 else()
+  if(GTSAM_PYTHON_VERSION STREQUAL "Default")
     find_package(PythonInterp)
+  else()
+    find_package(PythonInterp ${GTSAM_PYTHON_VERSION} EXACT)
+  endif()
 endif()
 
 if(NOT PYTHONINTERP_FOUND)

cmake/GtsamCythonWrap.cmake

@@ -5,8 +5,19 @@ unset(PYTHON_EXECUTABLE CACHE)
 unset(CYTHON_EXECUTABLE CACHE)
 unset(PYTHON_INCLUDE_DIR CACHE)
 unset(PYTHON_MAJOR_VERSION CACHE)
+
+if(GTSAM_PYTHON_VERSION STREQUAL "Default")
+  find_package(PythonLibs REQUIRED)
+else()
+  find_package(PythonLibs ${GTSAM_PYTHON_VERSION} EXACT REQUIRED)
+endif()
 find_package(Cython 0.25.2 REQUIRED)
 
+execute_process(COMMAND "${PYTHON_EXECUTABLE}" "-c"
+    "from __future__ import print_function;import sys;print(sys.version[0], end='')"
+    OUTPUT_VARIABLE PYTHON_MAJOR_VERSION
+)
+
 # User-friendly Cython wrapping and installing function.
 # Builds a Cython module from the provided interface_header.
 # For example, for the interface header gtsam.h,
@@ -31,16 +42,16 @@ endfunction()
 
 function(set_up_required_cython_packages)
   # Set up building of cython module
-  find_package(PythonLibs REQUIRED)
+  if(GTSAM_PYTHON_VERSION STREQUAL "Default")
+    find_package(PythonLibs REQUIRED)
+  else()
+    find_package(PythonLibs ${GTSAM_PYTHON_VERSION} EXACT REQUIRED)
+  endif()
   include_directories(${PYTHON_INCLUDE_DIRS})
   find_package(NumPy REQUIRED)
   include_directories(${NUMPY_INCLUDE_DIRS})
 endfunction()
 
-execute_process(COMMAND "${PYTHON_EXECUTABLE}" "-c"
-    "from __future__ import print_function;import sys;print(sys.version[0], end='')"
-    OUTPUT_VARIABLE PYTHON_MAJOR_VERSION
-)
 
 # Convert pyx to cpp by executing cython
 # This is the first step to compile cython from the command line

cython/README.md

@@ -2,7 +2,7 @@ This is the Cython/Python wrapper around the GTSAM C++ library.
 
 INSTALL
 =======
-- if you want to build the gtsam python library for python 3, use the `-DPython_ADDITIONAL_VERSIONS=3` option when running `cmake` otherwise the interpreter at `$ which python` will be used.
+- if you want to build the gtsam python library for a specific python version (eg 2.7), use the `-DGTSAM_PYTHON_VERSION=2.7` option when running `cmake` otherwise the default interpreter will be used.
     - If the interpreter is inside an environment (such as an anaconda environment or virtualenv environment) then the environment should be active while building gtsam.
 - This wrapper needs Cython(>=0.25.2), backports_abc>=0.5, and numpy. These can be installed as follows: