Add __slots__

[harshil21]

This PR adds __slots__ to all classes (finally!). As discussed in #1109, __dict__ is currently added to slots for backward compatibility.

Some notes about the changes:

• __weakref__ was added in Dispatcher's __slots__ as we're creating weakrefs with it.
• Reworked to_dict() in base.py so it works with slots
• Also included CSI style comments wherever necessary.
• Make _request an instance attribute in updater.py.
• Private classes (not exposed for the users) don't have __dict__.
• TelegramDeprecationWarning is raised when users try to assign new attributes on classes which don't support it.
• See this comment for more changes.

Closes #1109

[harshil21]

Looks like in python 3.6 ABC's __slots__ has a __dict__, so those tests are failing. Other test fails are unrelated. Will fix in next commit

[Bibo-Joshi]

Hey, thanks for the big PR! I left a bunch of comments, though I didn't check the slots for ever class and skimmed the test :D

Regarding tests: You currently test for extra slots in each class's test. I would like to see another test that iterates over all modules and makes every class actually has slots. Could go to a standalone test_slots.py. If testing for additional slots can work on class.__slots__ instead of class instances, one could also think about moving the tests for extra slots to that test file. Would save duplication and we wouldn't have to remember putting the test_extra_slots in each test file …

Tests that I would also like to see and which could go into test_slots.py:

• Test that class.custom_attribute actually gives a warning
• Test that overwriting existing attributes still works and doesn't give a warning

[harshil21]

I would like to see another test that iterates over all modules and makes every class actually has slots.

Good idea. Although I'll have to exclude classes from filters as they are messy and we are refactoring them soon anyway.
Done.

Could go to a standalone test_slots.py. If testing for additional slots can work on class.__slots__ instead of class instances, one could also think about moving the tests for extra slots to that test file.

iirc, inspect or any other method for that matter doesn't get instance attributes without initializing the class, but I'll check that again...

• Test that class.custom_attribute actually gives a warning

I did that for the base classes, but it's probably best I do it for all, just to be sure... (and same for the other point)
Edit: Done

[harshil21]

Ok so the past few commits have allowed subclassing of classes like Handler, etc without raising TGDeprecation warning for setting a new instance attribute (useful for custom {filters, handlers, persistence, Bot, Updater, Dispatcher, Handler})

The slots tests now test:

• Extra slots
• Missing slots (by checking the instance's __dict__)
• Duplicate slots (if for eg. child class has same slot(s) as parent)
• If setting a custom attribute raises a warning, and setting non custom attribute does not.

and a test_slots.py, which tests if all classes have slots, and that each have a __dict__.

Also BasePersistence uses a __dict__ internally, since we there would be a clash if we set those attributes in the slots (same name as abstract methods, which python doesn't like), I thought about assigning them as class attributes, but can't get those tests to pass.

[Bibo-Joshi]

Hey, thanks for the updates :) Had a look and them and left some comments.

[Bibo-Joshi]

Hey. Thanks for the new updates. I had a look (only at the new changes for now) and left some comments. I'll have to review everything again and would also this PR to be reviewed by someone else before merging, though - tbh it became quite more involved than I had imagined 😬

[harshil21]

Hey. Thanks for the new updates. I had a look (only at the new changes for now) and left some comments. I'll have to review everything again and would also this PR to be reviewed by someone else before merging, though - tbh it became quite more involved than I had imagined 😬

I'm now gonna run all of the examples and some of my bots based on this branch to further verify that everything is working. Might also compare the memory usage for fun.

[Poolitzer]

Changes look good to me. They shouldn't break anything anyway, and we can add them later easily.