[Python-Dev] semantics of subclassing things from itertools (original) (raw)

Maciej Fijalkowski fijall at gmail.com
Sun Sep 13 09:49:23 CEST 2015

On Fri, Sep 11, 2015 at 1:48 AM, Raymond Hettinger <raymond.hettinger at gmail.com> wrote:

On Sep 10, 2015, at 3:23 AM, Maciej Fijalkowski <fijall at gmail.com> wrote:

I would like to know what are the semantics if you subclass something from itertools (e.g. islice). Right now it's allowed and people do it, which is why the documentation is incorrect. It states "equivalent to: a function-or a generator", but you can't subclass whatever it is equivalent to, which is why in PyPy we're unable to make it work in pure python. I would like some clarification on that. The docs should say "roughly equivalent to" not "exactly equivalent to". The intended purpose of the examples in the itertools docs is to use pure python code to help people better understand each tool. It is not is intended to dictate that tool x is a generator or is a function. The intended semantics are that the itertools are classes (not functions and not generators). They are intended to be sub-classable (that is why they have PyTPFLAGSBASETYPE defined).

Ok, so what's completely missing from the documentation is what are the semantics of subclasses of those classes? Can you override any magic methods? Can you override next (which is or isn't a magic method depending how you look)? Etc.

The documentation on this is completely missing and it's left guessing with "whatever cpython happens to be doing".

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