[Python-ideas] Predicate Sets

[Haoyi Li]

Doesn't

*all(map(list_of_filters, value))*

do what we want here, then? Maybe *imap* if you want the early bailout
behavior.

I'm all for having *all*, *map*, *any*, etc. be methods rather than
top-level-functions (yay, less namespace pollution!), but if we're talking
about a list of functions, it seems we can do exactly what we want very
concisely using normal list- and function- operations.


On Mon, Jan 20, 2014 at 6:30 PM, Steven D'Aprano <steve at pearwood.info>wrote:

> On Mon, Jan 20, 2014 at 05:51:36PM -0800, Haoyi Li wrote:
> > > What if you want to filter according to multiple conditions?
> >
> > What's wrong with
> >
> > lambda fname: func1(fname) and func2(fname) and func3(fname)
>
> That is a single compound condition, not multiple conditions.
>
> Think about a GUI application with a file selection dialog box, or a
> search utility. You might offer a rich set of filters, all optional, all
> selectable by the user at runtime:
>
>
> [x] Hidden dot files .foo
> [ ] Backup files foo~
> [x] File extensions:
>     [ ] Images
>     [x] Text files
>     [ ] Java code
>     [x] Custom: [ zip,tar,foo,bar,baz ]
> [x] File owner: [ steve               ]
> [ ] Group:      [                     ]
> [ ] Modified date between: [       ] and [       ]
>
>
> etc. It's not practical to create one single giant filter function that
> looks like this:
>
> def filter(name):
>     head, ext = os.path.splitext(name)
>     return (
>             (show_hidden_dot_files and name.startswith('.'))
>             and (show_backup_tilde_files and name.endswith('~'))
>             and (show_images and ext in list_of_image_extensions)
>             and ...
>             )
>
>
> It would be a pain to maintain and extend, and testing would be
> horrible. Better to have each setting provide a single filter function,
> then combine the active filters into a list:
>
> def filter(name, list_of_filters):
>     for f in list_of_filters:
>         if not f(name):
>             return False
>     return True
>
> One might even use a class to represent the list of filters, and give it
> "all" and "any" methods, and allow multiple lists to combine so you can
> say things like:
>
>   "show the file if *all* of these conditions are true, or if *any* of
>   these different conditions are true, but not if *any* of these
>   conditions are true"
>
> which of course is terribly overkill for a simple file selection dialog
> box, but might be useful for a more complex search engine.
>
> None of this should be read as supporting the original request to add
> PredicateSet into the standard library. But I encourage the OP to write
> his own library and put it on PyPI.
>
>
> --
> Steven
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> https://mail.python.org/mailman/listinfo/python-ideas
> Code of Conduct: http://python.org/psf/codeofconduct/
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20140121/1c1b5cea/attachment-0001.html>