Quelle  _immutable.py   Sprache: Python

import sys


def immutable(members='', name='Immutable', verbose=False):
    """
    Produces a class that either can be used standalone or as a base class for persistent classes.

    This is a thin wrapper around a named tuple.

    Constructing a type and using it to instantiate objects:

    >>> Point = immutable('x, y', name='Point')
    >>> p = Point(1, 2)
    >>> p2 = p.set(x=3)
    >>> p
    Point(x=1, y=2)
    >>> p2
    Point(x=3, y=2)

    Inheriting from a constructed type. In this case no type name needs to be supplied:

    >>> class PositivePoint(immutable('x, y')):
    ...     __slots__ = tuple()
    ...     def __new__(cls, x, y):
    ...         if x > 0 and y > 0:
    ...             return super(PositivePoint, cls).__new__(cls, x, y)
    ...         raise Exception('Coordinates must be positive!')
    ...
    >>> p = PositivePoint(1, 2)
    >>> p.set(x=3)
    PositivePoint(x=3, y=2)
    >>> p.set(y=-3)
    Traceback (most recent call last):
    Exception: Coordinates must be positive!

    The persistent class also supports the notion of frozen members. The value of a frozen member
    cannot be updated. For example it could be used to implement an ID that should remain the same
    over time. A frozen member is denoted by a trailing underscore.

    >>> Point = immutable('x, y, id_', name='Point')
    >>> p = Point(1, 2, id_=17)
    >>> p.set(x=3)
    Point(x=3, y=2, id_=17)
    >>> p.set(id_=18)
    Traceback (most recent call last):
    AttributeError: Cannot set frozen members id_
    """

    if isinstance(members, str):
        members = members.replace(',', ' ').split()

    def frozen_member_test():
        frozen_members = ["'%s'" % f for f in members if f.endswith('_')]
        if frozen_members:
            return """
        frozen_fields = fields_to_modify & set([{frozen_members}])
        if frozen_fields:
            raise AttributeError('Cannot set frozen members %s' % ', '.join(frozen_fields))
            """.format(frozen_members=', '.join(frozen_members))

        return ''

    quoted_members = ', '.join("'%s'" % m for m in members)
    template = """
class {class_name}(namedtuple('ImmutableBase', [{quoted_members}])):
    __slots__ = tuple()

    def __repr__(self):
        return super({class_name}, self).__repr__().replace('ImmutableBase', self.__class__.__name__)

    def set(self, **kwargs):
        if not kwargs:
            return self

        fields_to_modify = set(kwargs.keys())
        if not fields_to_modify <= {member_set}:
            raise AttributeError("'%s' is not a member" % ', '.join(fields_to_modify - {member_set}))

        {frozen_member_test}

        return self.__class__.__new__(self.__class__, *map(kwargs.pop, [{quoted_members}], self))
""".format(quoted_members=quoted_members,
               member_set="set([%s])" % quoted_members if quoted_members else 'set()',
               frozen_member_test=frozen_member_test(),
               class_name=name)

    if verbose:
        print(template)

    from collections import namedtuple
    namespace = dict(namedtuple=namedtuple, __name__='pyrsistent_immutable')
    try:
        exec(template, namespace)
    except SyntaxError as e:
        raise SyntaxError(str(e) + ':\n' + template) from e

    return namespace[name]