Las clases que heredan de la siguiente Immutableclase son inmutables, como son sus instancias, una vez que su __init__método termina de ejecutarse. Como se trata de Python puro, como han señalado otros, no hay nada que impida que alguien use los métodos especiales de mutación de la base objectytype , pero esto es suficiente para detener a nadie de la mutación de una clase / instancia por accidente.
Funciona secuestrando el proceso de creación de clases con una metaclase.
"""Subclasses of class Immutable are immutable after their __init__ has run, in
the sense that all special methods with mutation semantics (in-place operators,
setattr, etc.) are forbidden.
"""
# Enumerate the mutating special methods
mutation_methods = set()
# Arithmetic methods with in-place operations
iarithmetic = '''add sub mul div mod divmod pow neg pos abs bool invert lshift
rshift and xor or floordiv truediv matmul'''.split()
for op in iarithmetic:
mutation_methods.add('__i%s__' % op)
# Operations on instance components (attributes, items, slices)
for verb in ['set', 'del']:
for component in '''attr item slice'''.split():
mutation_methods.add('__%s%s__' % (verb, component))
# Operations on properties
mutation_methods.update(['__set__', '__delete__'])
def checked_call(_self, name, method, *args, **kwargs):
"""Calls special method method(*args, **kw) on self if mutable."""
self = args[0] if isinstance(_self, object) else _self
if not getattr(self, '__mutable__', True):
# self told us it's immutable, so raise an error
cname= (self if isinstance(self, type) else self.__class__).__name__
raise TypeError('%s is immutable, %s disallowed' % (cname, name))
return method(*args, **kwargs)
def method_wrapper(_self, name):
"Wrap a special method to check for mutability."
method = getattr(_self, name)
def wrapper(*args, **kwargs):
return checked_call(_self, name, method, *args, **kwargs)
wrapper.__name__ = name
wrapper.__doc__ = method.__doc__
return wrapper
def wrap_mutating_methods(_self):
"Place the wrapper methods on mutative special methods of _self"
for name in mutation_methods:
if hasattr(_self, name):
method = method_wrapper(_self, name)
type.__setattr__(_self, name, method)
def set_mutability(self, ismutable):
"Set __mutable__ by using the unprotected __setattr__"
b = _MetaImmutable if isinstance(self, type) else Immutable
super(b, self).__setattr__('__mutable__', ismutable)
class _MetaImmutable(type):
'''The metaclass of Immutable. Wraps __init__ methods via __call__.'''
def __init__(cls, *args, **kwargs):
# Make class mutable for wrapping special methods
set_mutability(cls, True)
wrap_mutating_methods(cls)
# Disable mutability
set_mutability(cls, False)
def __call__(cls, *args, **kwargs):
'''Make an immutable instance of cls'''
self = cls.__new__(cls)
# Make the instance mutable for initialization
set_mutability(self, True)
# Execute cls's custom initialization on this instance
self.__init__(*args, **kwargs)
# Disable mutability
set_mutability(self, False)
return self
# Given a class T(metaclass=_MetaImmutable), mutative special methods which
# already exist on _MetaImmutable (a basic type) cannot be over-ridden
# programmatically during _MetaImmutable's instantiation of T, because the
# first place python looks for a method on an object is on the object's
# __class__, and T.__class__ is _MetaImmutable. The two extant special
# methods on a basic type are __setattr__ and __delattr__, so those have to
# be explicitly overridden here.
def __setattr__(cls, name, value):
checked_call(cls, '__setattr__', type.__setattr__, cls, name, value)
def __delattr__(cls, name, value):
checked_call(cls, '__delattr__', type.__delattr__, cls, name, value)
class Immutable(object):
"""Inherit from this class to make an immutable object.
__init__ methods of subclasses are executed by _MetaImmutable.__call__,
which enables mutability for the duration.
"""
__metaclass__ = _MetaImmutable
class T(int, Immutable): # Checks it works with multiple inheritance, too.
"Class for testing immutability semantics"
def __init__(self, b):
self.b = b
@classmethod
def class_mutation(cls):
cls.a = 5
def instance_mutation(self):
self.c = 1
def __iadd__(self, o):
pass
def not_so_special_mutation(self):
self +=1
def immutabilityTest(f, name):
"Call f, which should try to mutate class T or T instance."
try:
f()
except TypeError, e:
assert 'T is immutable, %s disallowed' % name in e.args
else:
raise RuntimeError('Immutability failed!')
immutabilityTest(T.class_mutation, '__setattr__')
immutabilityTest(T(6).instance_mutation, '__setattr__')
immutabilityTest(T(6).not_so_special_mutation, '__iadd__')
.ay.b? Para eso parecen existir las propiedades después de todo.