cmake: Remove ENABLE_PYTHON3 flag and simplify Python detection

- Makes use of more modern find_package(Python2/3) if available
- Moves almost all Python-related code to UHDPython.cmake
- ENABLE_PYTHON3 is no longer necessary

5 files changed, 160 insertions, 69 deletions

diff --git a/host/CMakeLists.txt b/host/CMakeLists.txt
index 2d2e465d1..916138ee9 100644
--- a/host/CMakeLists.txt
+++ b/host/CMakeLists.txt
@@ -6,11 +6,6 @@
 #
 
 ########################################################################
-#if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
-#    message(FATAL_ERROR "Prevented in-tree built. This is bad practice.")
-#endif(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
-
-########################################################################
 # Project setup
 ########################################################################
 # NOTE! If you change the version in the following line, also change
@@ -38,6 +33,7 @@ set(MSVC_MIN_VERSION_READABLE "14.0")
 # This Python version gets used for Python API (if requested) as well as
 # all the build-time Python scripts
 set(PYTHON_MIN_VERSION "2.7")
+set(PYTHON3_MIN_VERSION "3.5")
 # Other deps
 set(BOOST_MIN_VERSION "1.58")
 set(NUMPY_MIN_VERSION "1.7")
@@ -279,15 +275,6 @@ if(WIN32)
 endif(WIN32)
 
 ########################################################################
-# Choose a version of Python
-########################################################################
-option(ENABLE_PYTHON3 "Enable Python 3. Default is Python 2" OFF)
-set(PYBIND11_PYTHON_VERSION 2.7)
-if(${ENABLE_PYTHON3})
-    set(PYBIND11_PYTHON_VERSION 3)
-endif()
-
-########################################################################
 # Setup Boost
 ########################################################################
 message(STATUS "")
@@ -327,7 +314,6 @@ set(Boost_ADDITIONAL_VERSIONS
     "1.64" "1.65" "1.66" "1.67" "1.68" "1.69"
 )
 
-#Python API requirements
 message(STATUS "Looking for optional Boost components...")
 find_package(Boost ${BOOST_MIN_VERSION} OPTIONAL_COMPONENTS ${BOOST_OPTIONAL_COMPONENTS})
 
@@ -352,18 +338,11 @@ include(UHDLog)
 ########################################################################
 # Check Python Modules
 ########################################################################
-find_package(PythonInterp ${PYBIND11_PYTHON_VERSION})
-find_package(PythonLibs ${PYBIND11_PYTHON_VERSION})
 include(UHDPython)
 
-set(PYTHON_VERSION "platform.python_version() >= '2.7' and platform.python_version() < '3.0'")
-if(${ENABLE_PYTHON3})
-    set(PYTHON_VERSION "platform.python_version() >= '3.0'")
-endif()
-
 PYTHON_CHECK_MODULE(
-    "Python version ${PYBIND11_PYTHON_VERSION} or greater"
-    "platform" ${PYTHON_VERSION}
+    "Python version ${PYTHON_MIN_VERSION} or greater"
+    "platform" "platform.python_version() >= '${PYTHON_MIN_VERSION}'"
     HAVE_PYTHON_PLAT_MIN_VERSION
 )
 
@@ -385,8 +364,6 @@ PYTHON_CHECK_MODULE(
     HAVE_PYTHON_MODULE_NUMPY
 )
 
-set(PYTHON_ADDITIONAL_VERSIONS 2.7 3.4 3.5)
-
 ########################################################################
 # Create Uninstall Target
 ########################################################################
@@ -415,9 +392,9 @@ UHD_INSTALL(FILES
 LIBUHD_REGISTER_COMPONENT("LibUHD" ENABLE_LIBUHD ON "Boost_FOUND;HAVE_PYTHON_PLAT_MIN_VERSION;HAVE_PYTHON_MODULE_MAKO" OFF ON)
 LIBUHD_REGISTER_COMPONENT("LibUHD - C API" ENABLE_C_API ON "ENABLE_LIBUHD" OFF OFF)
 if(WIN32)
-    LIBUHD_REGISTER_COMPONENT("LibUHD - Python API" ENABLE_PYTHON_API OFF "ENABLE_LIBUHD;HAVE_PYTHON_MODULE_NUMPY;PythonLibs_FOUND" OFF OFF)
+  LIBUHD_REGISTER_COMPONENT("LibUHD - Python API" ENABLE_PYTHON_API OFF "ENABLE_LIBUHD;HAVE_PYTHON_MODULE_NUMPY;HAVE_PYTHON_LIBS" OFF OFF)
 else()
-    LIBUHD_REGISTER_COMPONENT("LibUHD - Python API" ENABLE_PYTHON_API ON "ENABLE_LIBUHD;HAVE_PYTHON_MODULE_NUMPY;PythonLibs_FOUND" OFF OFF)
+  LIBUHD_REGISTER_COMPONENT("LibUHD - Python API" ENABLE_PYTHON_API ON "ENABLE_LIBUHD;HAVE_PYTHON_MODULE_NUMPY;HAVE_PYTHON_LIBS" OFF OFF)
 endif(WIN32)
 LIBUHD_REGISTER_COMPONENT("Examples" ENABLE_EXAMPLES ON "ENABLE_LIBUHD" OFF OFF)
 LIBUHD_REGISTER_COMPONENT("Utils" ENABLE_UTILS ON "ENABLE_LIBUHD" OFF OFF)
diff --git a/host/cmake/Modules/UHDPython.cmake b/host/cmake/Modules/UHDPython.cmake
index 5ac2d991b..a93761131 100644
--- a/host/cmake/Modules/UHDPython.cmake
+++ b/host/cmake/Modules/UHDPython.cmake
@@ -1,6 +1,7 @@
 #
 # Copyright 2010-2011 Ettus Research LLC
 # Copyright 2018 Ettus Research, a National Instruments Company
+# Copyright 2019 Ettus Research, a National Instruments Company
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 #
@@ -9,62 +10,121 @@ if(NOT DEFINED INCLUDED_UHD_PYTHON_CMAKE)
 set(INCLUDED_UHD_PYTHON_CMAKE TRUE)
 
 ########################################################################
-# Setup Python
+# Setup Python Part 1: Find the interpreters
 ########################################################################
 message(STATUS "")
-message(STATUS "Configuring the python interpreter...")
+message(STATUS "Configuring the Python interpreter...")
 #this allows the user to override PYTHON_EXECUTABLE
 if(PYTHON_EXECUTABLE)
-
     set(PYTHONINTERP_FOUND TRUE)
+endif(PYTHON_EXECUTABLE)
 
-#otherwise if not set, try to automatically find it
-else(PYTHON_EXECUTABLE)
-
-    #use the built-in find script
-    if(ENABLE_PYTHON3)
-        find_package(PythonInterp 3.0)
-    else(ENABLE_PYTHON3)
-        find_package(PythonInterp 2.0)
-    endif(ENABLE_PYTHON3)
-
-    #and if that fails use the find program routine
-    if(NOT PYTHONINTERP_FOUND)
-        if(ENABLE_PYTHON3)
-            find_program(PYTHON_EXECUTABLE NAMES python3 python3.5 python3.6)
-        else(ENABLE_PYTHON3)
-            find_program(PYTHON_EXECUTABLE NAMES python2 python2.7)
-        endif(ENABLE_PYTHON3)
-
-        if(PYTHON_EXECUTABLE)
-            set(PYTHONINTERP_FOUND TRUE)
-        endif(PYTHON_EXECUTABLE)
-    endif(NOT PYTHONINTERP_FOUND)
+# We always try to find Py3k first. Once we only support Py3k we can remove
+# most of this.
+if(NOT PYTHONINTERP_FOUND)
+    find_package(Python3 ${PYTHON3_MIN_VERSION} QUIET)
+    if(Python3_Interpreter_FOUND)
+        set(PYTHON_MIN_VERSION ${PYTHON3_MIN_VERSION})
+        set(PYTHON_VERSION ${Python3_VERSION})
+        set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE})
+        set(PYTHONINTERP_FOUND TRUE)
+    endif(Python3_Interpreter_FOUND)
+endif(NOT PYTHONINTERP_FOUND)
 
-endif(PYTHON_EXECUTABLE)
+if(NOT PYTHONINTERP_FOUND)
+    find_package(PythonInterp ${PYTHON3_MIN_VERSION} QUIET)
+    if(PYTHONINTERP_FOUND)
+        set(PYTHON_MIN_VERSION ${PYTHON3_MIN_VERSION})
+        set(PYTHON_VERSION ${PYTHON_VERSION_STRING})
+    endif(PYTHONINTERP_FOUND)
+endif(NOT PYTHONINTERP_FOUND)
 
-#make the path to the executable appear in the cmake gui
-set(PYTHON_EXECUTABLE ${PYTHON_EXECUTABLE} CACHE FILEPATH
-    "python buildtime interpreter")
+# Next, try and find Py2k.
+if(NOT PYTHONINTERP_FOUND)
+    find_package(Python2 ${PYTHON_MIN_VERSION} QUIET)
+    if(Python2_Interpreter_FOUND)
+        set(PYTHON_VERSION ${Python2_VERSION})
+        set(PYTHON_EXECUTABLE ${Python2_EXECUTABLE})
+        set(PYTHONINTERP_FOUND TRUE)
+    endif(Python2_Interpreter_FOUND)
+endif(NOT PYTHONINTERP_FOUND)
 
-message(STATUS "Python interpreter: ${PYTHON_EXECUTABLE}")
-message(STATUS "Override with: -DPYTHON_EXECUTABLE=<path-to-python>")
+if(NOT PYTHONINTERP_FOUND)
+    find_package(PythonInterp ${PYTHON_MIN_VERSION} QUIET)
+    set(PYTHON_VERSION ${PYTHON_VERSION_STRING})
+endif(NOT PYTHONINTERP_FOUND)
 
+# If that fails, try using the build-in find program routine.
+if(NOT PYTHONINTERP_FOUND)
+    message(STATUS "Attempting to find Python without CMake...")
+    find_program(PYTHON_EXECUTABLE NAMES python3 python3.5 python3.6 python3.7 python3.8)
+    if(PYTHON_EXECUTABLE)
+        set(PYTHONINTERP_FOUND TRUE)
+        set(PYTHON_MIN_VERSION ${PYTHON3_MIN_VERSION})
+    endif(PYTHON_EXECUTABLE)
+endif(NOT PYTHONINTERP_FOUND)
+
+if(NOT PYTHONINTERP_FOUND)
+    message(STATUS "Attempting to find Python without CMake...")
+    find_program(PYTHON_EXECUTABLE NAMES python2 python2.7)
+    if(PYTHON_EXECUTABLE)
+        set(PYTHONINTERP_FOUND TRUE)
+    endif(PYTHON_EXECUTABLE)
+endif(NOT PYTHONINTERP_FOUND)
+
+if(NOT PYTHON_VERSION)
+    message(STATUS "Manually determining build Python version...")
+    execute_process(COMMAND ${PYTHON_EXECUTABLE} -c "
+from __future__ import print_function
+import sys
+print('{}.{}.{}'.format(
+    sys.version_info.major,
+    sys.version_info.minor,
+    sys.version_info.micro))"
+        OUTPUT_VARIABLE PYTHON_VERSION
+        OUTPUT_STRIP_TRAILING_WHITESPACE)
+endif(NOT PYTHON_VERSION)
+
+# If we still haven't found a Python interpreter, then we're done.
 if(NOT PYTHONINTERP_FOUND)
     message(FATAL_ERROR "Error: Python interpreter required by the build system.")
 endif(NOT PYTHONINTERP_FOUND)
+if(NOT PYTHON_EXECUTABLE)
+    message(FATAL_ERROR "Error: Python interpreter required by the build system.")
+endif(NOT PYTHON_EXECUTABLE)
+
+#make the path to the executable appear in the cmake gui
+set(PYTHON_EXECUTABLE ${PYTHON_EXECUTABLE} CACHE FILEPATH
+    "python buildtime interpreter")
+
+message(STATUS "Python interpreter: ${PYTHON_EXECUTABLE} Version: ${PYTHON_VERSION}")
+message(STATUS "Override with: -DPYTHON_EXECUTABLE=<path-to-python>")
 
 #this allows the user to override RUNTIME_PYTHON_EXECUTABLE
 if(NOT RUNTIME_PYTHON_EXECUTABLE)
     #default to the buildtime interpreter
     set(RUNTIME_PYTHON_EXECUTABLE ${PYTHON_EXECUTABLE})
+    set(RUNTIME_PYTHON_VERSION ${PYTHON_VERSION})
 endif(NOT RUNTIME_PYTHON_EXECUTABLE)
 
+if(NOT RUNTIME_PYTHON_VERSION)
+    message(STATUS "Manually determining runtime Python version...")
+    execute_process(COMMAND ${PYTHON_EXECUTABLE} -c "
+from __future__ import print_function
+import sys
+print('{}.{}.{}'.format(
+    sys.version_info.major,
+    sys.version_info.minor,
+    sys.version_info.micro))"
+        OUTPUT_VARIABLE RUNTIME_PYTHON_VERSION
+        OUTPUT_STRIP_TRAILING_WHITESPACE)
+endif(NOT RUNTIME_PYTHON_VERSION)
+
 #make the path to the executable appear in the cmake gui
 set(RUNTIME_PYTHON_EXECUTABLE ${RUNTIME_PYTHON_EXECUTABLE} CACHE FILEPATH
     "python runtime interpreter")
 
-message(STATUS "Python runtime interpreter: ${RUNTIME_PYTHON_EXECUTABLE}")
+message(STATUS "Python runtime interpreter: ${RUNTIME_PYTHON_EXECUTABLE} Version: ${RUNTIME_PYTHON_VERSION}")
 message(STATUS "Override with: -DRUNTIME_PYTHON_EXECUTABLE=<path-to-python>")
 
 macro(PYTHON_CHECK_MODULE desc mod cmd have)
@@ -97,4 +157,60 @@ exit(0)
     endif()
 endmacro(PYTHON_CHECK_MODULE)
 
+###############################################################################
+# Part 2: Python Libraries
+###############################################################################
+# The libraries must match the RUNTIME_PYTHON_EXECUTABLE's version.
+# - Figure out version
+# - See if Python3_LIBRARIES is already set (or Python2_LIBRARIES)
+if(NOT PYTHON_LIBRARIES OR NOT PYTHON_INCLUDE_DIRS)
+    message(STATUS "Finding Python Libraries...")
+    find_package(PythonLibs ${RUNTIME_PYTHON_VERSION} EXACT QUIET)
+    if(RUNTIME_PYTHON_VERSION VERSION_LESS 3)
+        if(NOT PYTHON_LIBRARIES OR NOT PYTHON_INCLUDE_DIRS)
+            find_package(Python3 ${RUNTIME_PYTHON_VERSION}
+                EXACT
+                QUIET
+                COMPONENTS Interpreter Development)
+            if(Python3_Development_FOUND)
+                set(PYTHON_LIBRARIES ${Python3_LIBRARIES})
+                set(PYTHON_INCLUDE_DIRS ${Python3_INCLUDE_DIRS})
+            endif(Python3_Development_FOUND)
+        endif(NOT PYTHON_LIBRARIES OR NOT PYTHON_INCLUDE_DIRS)
+    else(RUNTIME_PYTHON_VERSION VERSION_LESS 3)
+        if(NOT PYTHON_LIBRARIES OR NOT PYTHON_INCLUDE_DIRS)
+            find_package(Python2 ${RUNTIME_PYTHON_VERSION}
+                EXACT
+                QUIET
+                COMPONENTS Interpreter Development)
+            if(Python2_Development_FOUND)
+                set(PYTHON_LIBRARIES ${Python2_LIBRARIES})
+                set(PYTHON_INCLUDE_DIRS ${Python2_INCLUDE_DIRS})
+            endif(Python2_Development_FOUND)
+        endif(NOT PYTHON_LIBRARIES OR NOT PYTHON_INCLUDE_DIRS)
+    endif(RUNTIME_PYTHON_VERSION VERSION_LESS 3)
+    if(NOT PYTHON_LIBRARIES OR NOT PYTHON_INCLUDE_DIRS)
+        message(STATUS "Could not find Python Libraries.")
+    endif(NOT PYTHON_LIBRARIES OR NOT PYTHON_INCLUDE_DIRS)
+endif(NOT PYTHON_LIBRARIES OR NOT PYTHON_INCLUDE_DIRS)
+
+if(PYTHON_LIBRARIES AND PYTHON_INCLUDE_DIRS)
+    set(HAVE_PYTHON_LIBS TRUE)
+    message(STATUS "Python Libraries: ${PYTHON_LIBRARIES}")
+    message(STATUS "Python include directories: ${PYTHON_INCLUDE_DIRS}")
+else(PYTHON_LIBRARIES AND PYTHON_INCLUDE_DIRS)
+    set(HAVE_PYTHON_LIBS FALSE)
+endif(PYTHON_LIBRARIES AND PYTHON_INCLUDE_DIRS)
+
+if(NOT PYTHON_LIBRARY)
+    set(PYTHON_LIBRARIES ${PYTHON_LIBRARIES} CACHE FILEPATH
+        "Python libraries")
+    mark_as_advanced(PYTHON_LIBRARIES)
+endif(NOT PYTHON_LIBRARY)
+if(NOT PYTHON_INCLUDE_DIR)
+    set(PYTHON_INCLUDE_DIRS ${PYTHON_INCLUDE_DIRS} CACHE FILEPATH
+        "Python include dirs")
+    mark_as_advanced(PYTHON_INCLUDE_DIRS)
+endif(NOT PYTHON_INCLUDE_DIR)
+
 endif(NOT DEFINED INCLUDED_UHD_PYTHON_CMAKE)
diff --git a/host/docs/build.dox.in b/host/docs/build.dox.in
index 9e4f91ce4..1d9d9a0fe 100644
--- a/host/docs/build.dox.in
+++ b/host/docs/build.dox.in
@@ -220,10 +220,8 @@ extensions:
 
 \subsection cmake_win Generate the project with CMake
 
-- Enable Python3 option in the CMake list if the version of installed Python is >=3, otherwise go to the next step.
-  - Go to the source code: `<uhd-repo-path>/host`.
-  - Open 'CMakeLists.txt'.
-  - Find 'ENABLE_PYTHON3' option and set it to 'ON', e.g. 'option(ENABLE_PYTHON3 "Enable Python 3. Default is Python 2" ON)'.
+- Make sure the version of Python that CMake detects matches the version you
+  want it to be (check the PYTHON_VERSION variable)
 - Open the CMake GUI.
 - Set the path to the source code: `<uhd-repo-path>/host`.
 - Set the path to the build directory (this may require creating the folder \build\): `<uhd-repo-path>/host/build`.
diff --git a/host/docs/pythonapi.dox b/host/docs/pythonapi.dox
index 3f73c3a74..c8f504049 100644
--- a/host/docs/pythonapi.dox
+++ b/host/docs/pythonapi.dox
@@ -14,16 +14,16 @@ provide that functionality. On Windows, these headers always get installed when
 using the binary installers provided on https://www.python.org/downloads/windows/.
 
 If CMake can't find the Python headers or library, specify
-the `PYTHON_INCLUDE_DIRS` and/or `PYTHON_LIBRARY` CMake variables manually.
+the `PYTHON_INCLUDE_DIR` and/or `PYTHON_LIBRARY` CMake variables manually.
 
 
 \subsection python_install_2v3 Python 2 vs. 3
 
 The Python API supports both Python 2 and 3, but if you have both versions
 installed, CMake might require some hints which version is the desired one.
-To force Python 3, UHD has a CMake variable `ENABLE_PYTHON3`. If you set it,
-e.g., by running `cmake -DENABLE_PYTHON3=ON`, it will force the usage of
-Python 3.
+To force a particular version, it helps to set `-DPYTHON_EXECUTABLE=/usr/bin/python3`,
+or whatever interpreter you are expecting. The variables PYTHON_LIBRARY and
+PYTHON_INCLUDE_DIR may also need to be set.
 
 \subsection python_install_windows Installing on Windows
 
diff --git a/host/python/CMakeLists.txt b/host/python/CMakeLists.txt
index 7aae1dfeb..5598dda24 100644
--- a/host/python/CMakeLists.txt
+++ b/host/python/CMakeLists.txt
@@ -36,7 +36,7 @@ target_include_directories(pyuhd PUBLIC
     ${PYBIND11_INCLUDE_DIR}
 )
 
-target_link_libraries(pyuhd ${Boost_LIBRARIES} ${PYTHON_LIBRARY} uhd)
+target_link_libraries(pyuhd ${Boost_LIBRARIES} ${PYTHON_LIBRARIES} uhd)
 # Copy pyuhd library to the staging directory
 if(WIN32)
     set(PYUHD_LIBRARY_NAME libpyuhd.pyd)