Pastebin

# List validation for Google App Engine
# by Andreas Blixt <andreas@blixt.org>
# MIT license

# Don't miss the === EXAMPLE === section below.

class ValidatingList(list):
    """A wrapper of the list class that will throw an exception if an attempt to
    add a value of any type other than the type specified in its constructor is
    made.
    """
    def __init__(self, type, sequence = None):
        self._type = type
        if sequence: self.extend(sequence)

    def __setitem__(self, i, y):
        self._validate(y)
        super(ValidatingList, self).__setitem__(i, y)

    def __setslice__(self, i, j, y):
        for v in y: self._validate(v)
        super(ValidatingList, self).__setslice__(i, j, y)

    def _validate(self, value):
        # Check whether the value that is being appended is of a valid type.
        if not isinstance(value, self._type):
            raise TypeError('Unexpected type \'%s\'; expected \'%s\'.' % (
                            type(value).__name__, self._type.__name__))

    def append(self, object):
        self._validate(object)
        super(ValidatingList, self).append(object)

    def extend(self, iterable):
        for v in iterable: self._validate(v)
        super(ValidatingList, self).extend(iterable)


# === EXAMPLE ===
class ListProperty(object):
    """Simplified ListProperty descriptor.
    """
    def __init__(self, item_type, default = None):
        if not isinstance(item_type, type):
            raise TypeError('\'item_type\' must be a type.')

        self.type = item_type

        # For the sake of simplicity, values are stored on the descriptor
        # instance. In the GAE db module, they are stored on the model instance.
        self.value = ValidatingList(item_type, default)

    def __get__(self, instance, owner):
        return self.value

    def __set__(self, instance, value):
        self.value = ValidatingList(self.type, value)

class StringListProperty(ListProperty):
    def __init__(self, default = None):
        super(StringListProperty, self).__init__(basestring, default)

class SimpleModel(object):
    list_of_strings = StringListProperty(default = ['1', '2', '3'])

test = SimpleModel()

# None of the following will throw an error:
test.list_of_strings[1:2] = ['3', '2']
test.list_of_strings.extend(['4', '5'])
test.list_of_strings.append('6')
test.list_of_strings = ['1', '2', '3']
test.list_of_strings += ['6', '7']
test.list_of_strings[2] = '8'

# All of the following will throw an error:
test.list_of_strings[1:2] = [3, 2]
test.list_of_strings.extend(['4', 5])
test.list_of_strings.append(6)
test.list_of_strings = ['1', 2, '3']
test.list_of_strings += [6, '7']
test.list_of_strings[2] = 8