[Python-Dev] os.path.normcase rationale? (original) (raw)

Guido van Rossum guido at python.org
Fri Sep 24 16:29:40 CEST 2010

On Fri, Sep 24, 2010 at 5:17 AM, R. David Murray <rdmurray at bitdance.com> wrote:

On Fri, 24 Sep 2010 11:13:46 +0100, Chris Withers <chris at simplistix.co.uk> wrote:

On 18/09/2010 23:36, Guido van Rossum wrote: > course, exists() and isdir() etc. do, and so does realpath(), but the > pure parsing functions don't.

Yes, but: H:>echo foo > TeSt.txt ...>>> import os.path  >>> os.path.realpath('test.txt') 'H:\test.txt'  >>> os.path.normcase('TeSt.txt') 'test.txt' Both feel unsatisfying to me :-S How can I get 'TeSt.txt' from 'test.txt' (which feels like the contract normcase should have...) You can't, and you shouldn't be able to.  "normalization" is something that happens without reference to existing objects, the whole point is to put the thing into "standard form" so that you can compare strings obtained from different sources and know that they will represent the same object on that filesystem.

Clearly there is another use case where people want to display the filename back to the user with the correct case. This is a reasonable request and I think it makes sense for us to add another API to os.path that does this by looking up the path on the filesystem, or making an OS-specific call.

> They can be used without a working > filesystem even. (E.g. you can import ntpath on a Unix box and happily > parse Windows paths.)

But what value does that add over just doing a .lower() on the path? It does what is appropriate for that....oh, yeah.  For that OS, not "for that filesystem".  (e.g. on Unix normcase does nothing since files with different cases but the same letters are different files.)

Yeah, which is wrong on Mac OS X -- that's Unix but the default filesystem is case-preserving (though apparently it's possible to mount case-sensitive filesystems too). I've heard that on Windows there are also case-sensitive filesystems (part of a POSIX compliance package?). And on Linux you can mount FAT32 filesystems which are case-preserving.

Being os specific rather than file system type specific is the usability bug.

Agreed.

But to fix it we'll need to introduce a 'filesystems' module enumerating the different file systems we support, with tools for figuring out what filesystem your program is talking to.  But normacase still, wouldn't (shouldn't) do what you want.

I don't think we should try to reimplement what the filesystem does. I think we should just ask the filesystem (how exactly I haven't figured out yet but I expect it will be more OS-specific than filesystem-specific). It will have to be a new API -- normcase() at least is intended to return a case-flattened name on OSes where case-preserving filesystems are the default, and changing it to look at the filesystem would break too much code. For a new use case we need a new API.

-- --Guido van Rossum (python.org/~guido)

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