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.

    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

# 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()