[Python-Dev] PEP 544: Protocols (original) (raw)

Ivan Levkivskyi levkivskyi at gmail.com
Tue Mar 21 12:36:45 EDT 2017

On 21 March 2017 at 17:09, Matthias Kramm <kramm at google.com> wrote:

The one thing that isn't clear to me is how type checkers will distinguish between 1.) Protocol methods in A that need to implemented in B so that B is considered a structural subclass of A. 2.) Extra methods you get for free when you explicitly inherit from A. To provide a more concrete example: Since Mapping implements eq, do I also have to implement eq if I want my class to be (structurally) compatible with Mapping?

An implicit subtype should implement all methods, so that yes, in this case eq should be implemented for Mapping.

There was an idea to make some methods "non-protocol" (i.e. not necessary to implement), but it was rejected, since this complicates things. Briefly, consider this function:

def fun(m: Mapping): m.keys()

The question is should this be an error? I think most people would expect this to be valid. The same applies to most other methods in Mapping, people expect that they are provided my Mapping. Therefore, to be on the safe side, we need to require these methods to be implemented. If you look at definitions in collections.abc, there are very few methods that could be considered "non-protocol". Therefore, it was decided to not introduce "non-protocol" methods.

There is only one downside for this: it will require some boilerplate for implicit subtypes of Mapping etc. But, this applies to few "built-in" protocols (like Mapping and Sequence) and people already subclass them. Also, such style will be discouraged for user defined protocols. It will be recommended to create compact protocols and combine them. (This was discussed, but it looks like we forgot to add an explicit statement about this.)

I will add a section on non-protocol methods to rejected/postponed ideas.

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