Add support for Python 3 in CMake build

CMakeLists.txt

@@ -152,14 +152,46 @@ if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   )
 endif()
 
-if(APPLE)
-  # Ensure that the python2 executable or link is used in preference to the
-  # python and python2.7 executables or links.
-  find_program(PYTHON_EXECUTABLE NAMES python2)
+# The supported Python major/minor versions should match those listed in both
+# doc/developers.rst and tools/workspace/python/repository.bzl.
+set(SUPPORTED_PYTHON_VERSIONS 2 2.7 3 3.6 3.7)
+
+if(PYTHON_EXECUTABLE)
+  find_package(PythonInterp MODULE REQUIRED)
+else()
+  string(REPLACE ";" " " SUPPORTED_PYTHON_VERSIONS_STRING
+    "${SUPPORTED_PYTHON_VERSIONS}"
+  )
+  set(WITH_PYTHON_VERSION 2 CACHE STRING
+    "Choose the version of Python to use, options are ${SUPPORTED_PYTHON_VERSIONS_STRING}"
+  )
+  set_property(CACHE WITH_PYTHON_VERSION PROPERTY
+    STRINGS "${SUPPORTED_PYTHON_VERSIONS}"
+  )
+
+  string(REPLACE "." ";" WITH_PYTHON_VERSION_LIST "${WITH_PYTHON_VERSION}")
+  list(GET WITH_PYTHON_VERSION_LIST 0 WITH_PYTHON_VERSION_MAJOR)
+
+  if(APPLE AND WITH_PYTHON_VERSION_MAJOR EQUAL 2)
+    # Ensure that the python2 executable or link is used in preference to the
+    # python and python2.7 executables or links, which correspond to the
+    # system version of Python installed on macOS. There is no system python3
+    # as of macOS Mojave 10.14.
+    find_program(PYTHON_EXECUTABLE NAMES python2)
+  endif()
+
+  find_package(PythonInterp ${WITH_PYTHON_VERSION} EXACT MODULE REQUIRED)
 endif()
 
-# TODO(jamiesnape) Ensure that we can switch between supported versions.
-find_package(PythonInterp 2.7 EXACT MODULE REQUIRED)
+set(PYTHON_VERSION_MAJOR_MINOR
+  "${PYTHON_VERSION_MAJOR}.${PYTHON_VERSION_MINOR}"
+)
+
+if(NOT PYTHON_VERSION_MAJOR_MINOR IN_LIST SUPPORTED_PYTHON_VERSIONS)
+  message(WARNING
+    "Python ${PYTHON_VERSION_MAJOR_MINOR} is NOT supported. Python code in project drake_cxx_python may fail at runtime."
+  )
+endif()
 
 if(CMAKE_BUILD_TYPE STREQUAL Debug)
   set(BAZEL_COMPILATION_MODE dbg)

doc/developers.rst

@@ -99,7 +99,6 @@ The following configurations are presently untested in continuous integration:
 
 -   macOS Mojave: C++, Python
 -   macOS, Ubuntu Bionic: MATLAB
--   Ubuntu Bionic: Python 3
 
 .. _configuration-management-non-determinism:
 

doc/python_bindings.rst

@@ -11,8 +11,6 @@ class which is exposed to C++ has been explicitly enumerated in one of the
 source files inside the ``bindings/pydrake`` folder. These bindings are
 installed as a single package called ``pydrake``.
 
-Python 2.7 is currently the only supported version for these bindings.
-
 .. _python-bindings-binary:
 
 Binary Installation for Python
@@ -45,6 +43,9 @@ Next, ensure that your ``PYTHONPATH`` is properly configured:
 See :ref:`below <using-python-bindings>` for usage instructions. If using
 macOS, pay special attention to :ref:`this note <using-python-mac-os-path>`.
 
+Python 2.7 is currently the only supported version for the bindings supplied
+by the binary packages.
+
 Building the Python Bindings
 ----------------------------
 
@@ -64,6 +65,7 @@ Please note the additional CMake options which affect the Python bindings:
 *   ``-DWITH_GUROBI={ON, [OFF]}`` - Build with Gurobi enabled.
 *   ``-DWITH_MOSEK={ON, [OFF]}`` - Build with MOSEK enabled.
 *   ``-DWITH_SNOPT={ON, [OFF]}`` - Build with SNOPT enabled.
+*   ``-DWITH_PYTHON_VERSION={[2], 3}`` - Build with a specific version of Python.
 
 ``{...}`` means a list of options, and the option surrounded by ``[...]`` is
 the default option. An example of building ``pydrake`` with both Gurobi and

tools/workspace/pybind11/package-create-cps.py

@@ -21,18 +21,10 @@
   "Version": "%(VERSION_MAJOR)s.%(VERSION_MINOR)s.%(VERSION_PATCH)s",
   "Requires": {
     "PythonInterp": {
-      "Version": "2.7",
-      "X-CMake-Find-Args": [
-        "EXACT",
-        "MODULE"
-      ]
+      "X-CMake-Find-Args": ["MODULE"]
     },
     "PythonLibs": {
-      "Version": "2.7",
-      "X-CMake-Find-Args": [
-        "EXACT",
-        "MODULE"
-      ]
+      "X-CMake-Find-Args": ["MODULE"]
     }
   },
   "Default-Components": [":pybind11"],

tools/workspace/python/repository.bzl

@@ -34,6 +34,8 @@ Arguments:
 load("@drake//tools/workspace:execute.bzl", "execute_or_fail", "which")
 load("@drake//tools/workspace:os.bzl", "determine_os")
 
+# The supported Python versions should match those listed in both the root
+# CMakeLists.txt and doc/developers.rst.
 _VERSION_SUPPORT_MATRIX = {
     "ubuntu:16.04": ["2.7"],
     "ubuntu:18.04": ["2.7", "3.6"],