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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]