[Python-Dev] Pathlib enhancements - acceptable inputs and outputs for fspath and os.fspath() (original) (raw)

Stephen J. Turnbull stephen at xemacs.org
Sat Apr 16 07:21:54 EDT 2016

Nick Coghlan writes:

On 15 April 2016 at 00:52, Stephen J. Turnbull <stephen at xemacs.org> wrote:

Nick Coghlan writes:

The use case for returning bytes from fspath is DirEntry, so you can write things like this in low level code:

def myscandir(dirpath):
    for entry in os.scandir(dirpath):
        if entry.is_file():
            with open(entry) as f:
                # do something

Excuse me, but that is not a use case for returning bytes from DirEntry.fspath. open() is perfectly happy taking str (including surrogate-encoded rawbytes).

That results in a different type for the file object's name:

open("README.md").name 'README.md' open(b"README.md").name b'README.md'

OK, you win, fspath needs to be polymorphic.

But you've just shifted me to -1 on "os.fspath": it's an attractive nuisance. EIBTI, applications and high-level library functions should use os.fsdecode or os.fsencode. Functions that take a polymorphic argument and want preserve type should invoke fspath on the argument. That will visually signal that the caller is not merely low-level, but is explicitly a boundary function. (You could rename the generic function as "os._fspath", I guess, but I really want to deprecate calling the polymorphic version in user code. _fspath can be added if experience shows that polymorphic usage is very desireable outside the stdlib. This remark is in my not-so-Dutch opinion, of course.)

The guarantee we want to provide those folks is that if they're operating in the binary domain they'll stay there.

Et tu, Nick? "Guarantee"?! You can't guarantee any such thing with an implicitly invoked polymorphic API like this one -- unless you consider a crashed program to be in the binary domain. ;-) Note that the current proposala don't even do that for the binary domain, only for the text domain!

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