[Python-Dev] pathlib - current status of discussions (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Wed Apr 13 23:27:41 EDT 2016

On 14 April 2016 at 13:14, Ethan Furman <ethan at stoneleaf.us> wrote:

On 04/13/2016 07:57 PM, Nikolaus Rath wrote:

Either I haven't understood your answer, or you haven't understood my question. I'm concerned about this case:

class Special(bytes): def fspath(self): return 'str-val' obj = Special('bytes-val', 'utf8') pathobj = fspath(obj, allowbytes=True) With #2, pathobj == 'bytes-val'. With #3, pathobj == 'str-val'. I misunderstood your question. That is... an interesting case. ;)

In this kind of case, inheritance tends to trump protocol. For example, int subclasses can't override operator.index:

from operator import index class NotAnInt(): ... def index(self): ... return 42 ... index(NotAnInt()) 42 class MyInt(int): ... def index(self): ... return 42 ... index(MyInt(53)) 53

The reasons for that behaviour are more pragmatic than philosophical: builtins and their subclasses are extensively special-cased for speed reasons, and those shortcuts are encountered before the interpreter even considers using the general protocol.

In cases where the magic method return types are polymorphic (so subclasses may want to override them) we'll use more restrictive exact type checks for the shortcuts, but that argument doesn't apply for typechecked protocols where the result is required to be an instance of a particular builtin type (but subclasses are considered acceptable).

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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