What does "implements" mean in Python class definition?

Question

The bellow code sample is from a BeeWare suite example. (https://github.com/eliasdorneles/drawingapp-voc/blob/master/drawingapp/app.py)

What does the expression implements=android.view.View[OnClickListener] means? There should be (a list of) base class(es). Is it some incompatible special syntax of the framework or standard python I can not comprehend (and missing from the documentation)?

Moreover, we are using OnClickListener here which is never imported. There is no wildcard imports (*) and no module prefix before the symbol like android.Constants.OnClickListener or someting like that. How can the Python interpreter find the OnClickListener value in this case?

import android
from android.widget import LinearLayout, TextView, Button
import android.content.Context
from android.graphics import Bitmap, Canvas, Color, Paint, Path, PorterDuff
from android.view import MotionEvent, Gravity
import android.view

class ButtonClick(implements=android.view.View[OnClickListener]):
    def __init__(self, callback, *args, **kwargs):
        self.callback = callback

'''

In the readme it says that this python code is compiled to Java. So I belive that code is never supposed to be run on a standard interpreter and in fact raises an exception. — Wombatz, Jan 07 '19 at 22:12
Python's [class definition syntax](https://docs.python.org/3/reference/compound_stmts.html#class-definitions) lets you use `keyword=value` entries just as much as base classes. By default `metaclass=...` has meaning, and when using [`__init_subclass__`](https://docs.python.org/3/reference/datamodel.html#customizing-class-creation), other such keyword arguments are passed on to that method. Presumably the BeeWare suite is relying on that syntax. — Martijn Pieters, Jan 07 '19 at 22:12

jsbueno · Accepted Answer · 2019-01-07T23:06:33.687

So, the key to the mistery here is that this is not quite Python code- rather, it is a source file meant to be transpiled with VOC - that will generate Java bytecode, which will be further processed to work as an Android application using the standard Android Api.

As a transpiler, VOC relies on syntax which are valid Python, but it allows itself to take detours from the official language - like using keyword arguments on class definitions. That would only make sense in Python if it were inheriting a class which would implement the special method __init_subclass__, or use a custom metaclass to interpret these arguments - otherwise they'd have no effect. Also, VOC probably use these keywords (extends and implements) to indicate exactly these words as they are in Java class declaration syntax.

Also, as you put it, the name OnClickListener as it is in there would fail with a NameError in normal Python - it is likely that VOC defines additional names that behave like builtin names. In ordinary Python, if you create a function/method definition instead of a class definition, it is possible to makeuse of non-imported or undefined names as annotations. That is android.view.View[OnClickListener] would follow : instead of =, in a function definition. Annotations are lazily evaluated in Python 3.7 (but not in Python 3.6), so it would not cause an error. As it is, though, it would just raise a NameError in normal Python.

Here is a quick start for Python android apps using VOC in PyBee https://pybee.org/project/using/android-app/

update answer extensively reworded after I found out about VOC, though the first findings on "this is not Python" were correct)

score 2 · Answer 2 · answered Jan 07 '19 at 22:17

Python's class definition syntax lets you use keyword=value entries just as much as base classes. By default metaclass=... has meaning, and when using __init_subclass__, other such keyword arguments are passed on to that method. So using implements= in a class definition is fine from a syntax point of view.

Note that BeeWare doesn't run Python on Android, it uses a transpiler called VoC to translate Python code to Java. The implements= keyword argument in classes is used to let you define Java interfaces in Python code. Presumably the additional names like OnClickListener are available because VoC makes them available.

VoC translates Python bytecode to Java, so all that BeeWare compatible Python has to achieve is to be correct Python syntax. Runtime issues such as NameErrors don't have to be a problem, that's something the Java translator can compensate for.

What does "implements" mean in Python class definition?

2 Answers2