Python List Predicates

list_predicates.py

from types import FunctionType
from UserList import UserList

class PredicateList(UserList):
    def Exists(self, predicate):
        """Returns true if the predicate is satisfied by at least one list member."""
        if type(predicate) is FunctionType:
            for item in self.data:
                if predicate(item):
                    return True
            return False
        else:
            return predicate in self.data

    def GetIndex(self, predicate, reversed):
        """Common functionality shared by FirstIndex and LastIndex."""
        xr = xrange(len(self.data))
        if reversed:
            xr = xr.__reversed__()
        for i in xr:
            try:
                if type(predicate) == FunctionType:
                    if predicate(self.data[i]):
                        return i
                else:
                    if self.data[i] == predicate:
                        return i
            # Function predicates that check object properties will fail on simple 
            # types like strings. This is rather unelegant, but works.
            except AttributeError:
                pass
        return None
    
    def FirstIndex(self, predicate):
        """Returns the first integer position in the list that satisfies the predicate."""
        return self.GetIndex(predicate, False)
    
    def FirstValue(self, predicate):
        """Returns the first value in the list that satisfies the predicate."""
        firstObjectIndex = self.FirstIndex(predicate)
        if not firstObjectIndex is None:
            return self.data[firstObjectIndex]
        else:
            return None
    
    def LastIndex(self, predicate):
        """Returns the last integer position in the list that satisfies the predicate."""
        return self.GetIndex(predicate, True)
    
    def LastValue(self, predicate):
        """Returns the last value in the list that satisfies the predicate."""
        lastObjectIndex = self.LastIndex(predicate)
        if not lastObjectIndex is None:
            return self.data[lastObjectIndex]
        else:
            return None

class Person():
    def __init__(self, firstName, lastName):
        self.firstName = firstName
        self.lastName = lastName
    def __str__(self):
        return '%s %s' % (self.firstName, self.lastName)

sampleList = PredicateList()
sampleList.append('3')
sampleList.append('2')
sampleList.append('1')
sampleList.append(Person('Joe', 'Shmoe'))
sampleList.append('5')
sampleList.append('2')
print sampleList

# Existence checks
print sampleList.Exists('2')
print sampleList.Exists(lambda x: x == '2')

# First index checks
print sampleList.FirstIndex('2')
print sampleList.FirstIndex(lambda x: x == '2')

# First object checks
print sampleList.FirstValue('2')
print sampleList.FirstValue(lambda person: person.firstName == 'Joe')

# Last index checks
print sampleList.LastIndex('2')
print sampleList.LastIndex(lambda x: x == '2')

# Last object checks
print sampleList.LastValue('2')
print sampleList.LastValue(lambda person: person.lastName == 'Shmoe')