Add ctypes support for python3's recipe

Should be mentioned that the current test app for python3 has been modified by adding libffi to the requirements because the ui for the app has some button to test the ctypes module.

doc/source/buildoptions.rst

@@ -27,6 +27,10 @@ and works with any recent version of the Android NDK.
 Select Python 3 by adding it to your requirements,
 e.g. ``--requirements=python3``.
 
+.. note:: ctypes is not included automatically, if you would like to use it
+          then add libffi to your requirements,
+          e.g. ``--requirements=kivy,libffi,python3``.
+
 
 CrystaX python3
 ###############

pythonforandroid/bootstraps/sdl2/build/src/main/java/org/kivy/android/PythonUtil.java

@@ -37,6 +37,7 @@ protected static ArrayList<String> getLibraries(File filesDir) {
         ArrayList<String> libsList = new ArrayList<String>();
         addLibraryIfExists(libsList, "crystax", libsDir);
         addLibraryIfExists(libsList, "sqlite3", libsDir);
+        addLibraryIfExists(libsList, "ffi", libsDir);
         libsList.add("SDL2");
         libsList.add("SDL2_image");
         libsList.add("SDL2_mixer");

pythonforandroid/recipes/python3/__init__.py

@@ -1,4 +1,4 @@
-from pythonforandroid.recipe import TargetPythonRecipe
+from pythonforandroid.recipe import TargetPythonRecipe, Recipe
 from pythonforandroid.toolchain import shprint, current_directory
 from pythonforandroid.logger import logger, info, error
 from pythonforandroid.util import ensure_dir, walk_valid_filens
@@ -33,16 +33,38 @@
 
 
 class Python3Recipe(TargetPythonRecipe):
+    '''
+    .. note::
+        In order to build certain python modules, we need to add some extra
+        recipes to our build requirements:
+
+            - ctypes: you must add the recipe for ``libffi``.
+    '''
     version = '3.7.1'
     url = 'https://www.python.org/ftp/python/{version}/Python-{version}.tgz'
     name = 'python3'
 
     depends = ['hostpython3']
     conflicts = ['python3crystax', 'python2']
+    opt_depends = ['libffi']
 
     # This recipe can be built only against API 21+
     MIN_NDK_API = 21
 
+    def set_libs_flags(self, env, arch):
+        '''Takes care to properly link libraries with python depending on our
+        requirements and the attribute :attr:`opt_depends`.
+        '''
+        if 'libffi' in self.ctx.recipe_build_order:
+            info('Activating flags for libffi')
+            recipe = Recipe.get_recipe('libffi', self.ctx)
+            include = ' -I' + ' -I'.join(recipe.get_include_dirs(arch))
+            ldflag = ' -L' + join(recipe.get_build_dir(arch.arch),
+                                  recipe.get_host(arch), '.libs') + ' -lffi'
+            env['CPPFLAGS'] = env.get('CPPFLAGS', '') + include
+            env['LDFLAGS'] = env.get('LDFLAGS', '') + ldflag
+        return env
+
     def build_arch(self, arch):
         if self.ctx.ndk_api < self.MIN_NDK_API:
             error('Target ndk-api is {}, but the python3 recipe supports only {}+'.format(
@@ -119,6 +141,8 @@ def build_arch(self, arch):
 
             env['SYSROOT'] = sysroot
 
+            env = self.set_libs_flags(env, arch)
+
             if not exists('config.status'):
                 shprint(sh.Command(join(recipe_build_dir, 'configure')),
                         *(' '.join(('--host={android_host}',

testapps/setup_testapp_python3.py

@@ -2,7 +2,7 @@
 from distutils.core import setup
 from setuptools import find_packages
 
-options = {'apk': {'requirements': 'sdl2,pyjnius,kivy,python3',
+options = {'apk': {'requirements': 'libffi,sdl2,pyjnius,kivy,python3',
                    'android-api': 27,
                    'ndk-api': 21,
                    'dist-name': 'bdisttest_python3_googlendk',