[Python-ideas] Predicate Sets

[Ian Foote]

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On 19/01/14 23:41, Daniel da Silva wrote:
> Below is a description of a very simple but immensely useful class 
> called a "predicate set". In combination with the set and list 
> comprehensions they would allow another natural layer of reasoning
> with mathematical set logic in Python.
> 
> In my opinion, a concept like this would be best located in the 
> functools module.
> 
> 
> *Overview:* Sets in mathematics can be defined by a list of
> elements without repetitions, and alternatively by a predicate
> (function) that determines inclusion. A predicate set would be a
> set-like class that is instantiated with a predicate function that
> is called to determine ``a in the_predicate_set''.
> 
>>> myset = predicateset(lambda s: s.startswith('a')) 'xyz' in
>>> myset
> False
>>> 'abc' in myset
> True
>>> len(myself)
> Traceback (most recent call last): [...] TypeError * * *Example
> Uses:* # Dynamic excludes in searching foo_files =
> search_files('foo', exclude=set(['a.out', 'Makefile'])) bar_files =
> search_files('bar', exclude=predicateset(lambda fname: not 
> fname.endswith('~'))) # exclude *~
> 
> # Use in place of a set with an ORM validusernames =
> predicateset(lambda s: re.match(s, '[a-zA-Z0-9]+'))
> 
> class Users(db.Model): username =
> db.StringProperty(choices=validusernames) password =
> db.StringProperty()
> 
> 

Hi Daniel,

That's an interesting idea. I'm not sure it would be used enough to
include in the standard library though. Have you considered releasing
an implementation on PyPI? That has the advantage that people can
start using it earlier than would be possible if it was added to the
standard library.

Regards,
Ian

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