[Python-ideas] Predicate Sets

[Haoyi Li]

> *all(map(list_of_filters, value))*

Scratch that, what I actually want is

*all(map(lambda f: f(value), list_of_filters))*

I always mix up the order of things going into *map* =(


On Tue, Jan 21, 2014 at 12:50 AM, Haoyi Li <haoyi.sg at gmail.com> wrote:

> 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
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>
>
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