[crystax] Improve docs and inline comments

By shortening a little some long texts, thanks @Jonast and @inclement!!

Author: opacam

doc/source/buildoptions.rst

@@ -35,18 +35,12 @@ e.g. ``--requirements=python3``.
 CrystaX python3
 ~~~~~~~~~~~~~~~
 
-python-for-android originally supported Python 3 using the CrystaX NDK.  Since
-we have a working python3 recipe, we don't support CrystaX NDK anymore. If you
-were using `python3crystax`, we recommend to give it a try to the new `python3`
-recipe.
-
-.. note:: Since we don't support `python3crystax` anymore, the old instructions
-          has been removed from here. If you, still have the need to make use
-          of this old recipe, you should do it with an old `python-for-android`
-          release. Probably, a good starting point would be `version 0.7.0
-          <https://github.com/kivy/python-for-android/archive/0.7.0.zip>`__ or
-          if that doesn't work , then go for `version 0.6.0
-          <https://github.com/kivy/python-for-android/archive/0.6.0.zip>`__
+python-for-android no longer supports building for Python 3 using the CrystaX
+NDK. Instead, use the python3 recipe, which can be built using the normal
+Google NDK.
+
+.. note:: The last python-for-android version supporting CrystaX was `0.7.0.
+          <https://github.com/kivy/python-for-android/archive/0.7.0.zip>`__
 
 .. _bootstrap_build_options:
 

pythonforandroid/recipe.py

@@ -729,20 +729,14 @@ class PythonRecipe(Recipe):
 
     depends = [('python2', 'python3')]
     '''
-    .. note:: it's important to keep this depends as a class attribute, outside
-              `__init__` because, sometimes, we only initialize the object, so
-              the `__init__` call it won't be called, which will lead to not
-              have the python versions as a dependencies and it will cause a
-              tremendous `test_graph` error (difficult to track) and also, the
-              build order for dependencies will not be computed as expected (if
-              computed...). So be very careful with this line!!
-
-    .. warning:: this `depends` may be overwrote in inherited classes of
-                 `PythonRecipe`, so we make sure that any sub class will
-                 contain python as a dependency. We do this by checking the
-                 dependencies in meth:`PythonRecipe.__init__` method and adding
-                 them again in case that is necessary, so don't forget to call
-                 `super` in any inherited class of this class.
+    .. note:: it's important to keep this depends as a class attribute outside
+              `__init__` because sometimes we only initialize the class, so the
+              `__init__` call won't be called and the deps would be missing
+              (which breaks the dependency graph computation)
+
+    .. warning:: don't forget to call `super().__init__()` in any recipe's
+                 `__init__`, or otherwise it may not be ensured that it depends
+                 on python2 or python3 which can break the dependency graph
     '''
 
     def __init__(self, *args, **kwargs):
@@ -754,11 +748,10 @@ def __init__(self, *args, **kwargs):
                 if d in self.depends
             ]
         ):
-            # we overwrote `depends` in inherited recipe, so we must add it
-            # again the python versions as dependencies, but we only do this in
-            # case that the sub classes recipe does not contain any python
-            # version as dependency because it may be some recipes only
-            # compatible with a single version of python
+            # We ensure here that the recipe depends on python even it overrode
+            # `depends`. We only do this if it doesn't already depend on any
+            # python, since some recipes intentionally don't depend on/work
+            # with all python variants
             depends = self.depends
             depends.append(('python2', 'python3'))
             depends = list(set(depends))