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

Nick Coghlan ncoghlan at gmail.com
Thu Apr 14 02:09:17 EDT 2016

On 14 April 2016 at 14:05, Random832 <random832 at fastmail.com> wrote:

On Wed, Apr 13, 2016, at 23:27, Nick Coghlan wrote:

In this kind of case, inheritance tends to trump protocol. For example, int subclasses can't override operator.index: ... 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). Then why aren't we doing it for str? Because "try: path = path.fspath()" is more idiomatic than the alternative?

The sketches Brett posted will bear little resemblance to the actual implementation - that will be in C and use similar idioms to those we use for other abstract protocols (such as shortcuts for instances of builtin types, and doing the method lookup via the passed in object's type, rather than on the instance).

Cheers, Nick.

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

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