[easy] Enhance the protocol checker

[pylint-bot]

Originally reported by: Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore)

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We could improve the protocol checker, there are a lot of special methods which must return something of a given type in order work properly, other special methods require the presence of another method defined and so on: https://docs.python.org/3/reference/datamodel.html

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• Bitbucket: https://bitbucket.org/logilab/pylint/issue/560

[pylint-bot]

Original comment by Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore):

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Here's a short list of checks that can be implemented:

• __index__ returned non-int
• __repr__ must return string
• __str__ must return string
• __bytes__ must return bytes
• __getnewargs_ex__ should return a tuple of length 2
• first item of __getnewargs_ex__ must be a tuple
• second item of __getnewargs_ex__ must be a dict
• __getnewargs__ should return a tuple
• __bool__ should return bool
• __hash__ should return integer
• __lenght_hint__ must be an integer
• __length_hint__ should return >=0
• __format__ must return string

[craig-sh]

Hi, I'm at the Pycon canada sprints and I like to try to fix this issue. I'm thinking of following the same approach as in #845, is that a good place to start? Thanks

[PCManticore]

Hi @craig-sh Yeah, that's a good place to start! Thanks for taking a look at it!

[craig-sh]

I added some code to check if __bool__returns a bool. If you get a chance could you take a look at the WIP pull request and let me know if I'm on the right track?
Thanks!