在Python中，创建一个不变的类型，该类型能够对自身的哈希值和相等性进行排序，但可以轻松地进行子类化。尝试使用namedtuple：
class Command(namedtuple('Command', 'cmd_string')):
def valid_msg(msg):
return True
def make_command(msg):
if self.valid_msg(msg):
return '%s:%s' % (self.cmd_string, msg)
else:
raise ValueError(INVALID_MSG)
但是，这种方法不支持子类化。直接子类化意味着元组的名称保持不变(对于打印来说，这不是一个很大的问题)，但更重要的是，无法添加字段：
class LimitedLengthCommand(Command):
# 想拥有 self.length！放在哪里？
def valid_msg(msg):
return len(msg) <= self.length
简单地创建另一个named tuple(根据文档)意味着不会继承任何方法！
如何以最简单和最容易的方式实现如下效果？
-
有多个Command的子类(例如，十六进制文本，1 或 0 等)，但没有复杂的内容。
-
良好地处理多重继承不是必需的。
-
解决方案 以下是一个满足需求的元类：
import collections as co
import abc
class ImmutableMeta(abc.ABCMeta):
_classes = {}
def __new__(meta, clsname, bases, clsdict):
attributes = clsdict.pop('_attributes_')
if bases[0] is object:
# 'new' class
methods = clsdict
else:
# we're 'inheriting' from an existing class
base = bases[0]
attributes = meta._classes[base]['attributes'] + ' ' + attributes
base_methods = meta._classes[base]['methods'].copy()
base_methods.update(clsdict)
methods = base_methods
# construct the actual base class and create the return class
new_base = co.namedtuple(clsname + 'Base', attributes)
cls = super(ImmutableMeta, meta).__new__(meta, clsname, (new_base,),
methods)
# register the data necessary to 'inherit' from the class
# and make sure that it passes typechecking
meta._classes[cls] = {'attributes': attributes,
'methods': methods}
if bases[0] is not object:
base.register(cls)
return cls
然后，可以照着以下方式构造几个类：
class Foo(object):
__metaclass__ = ImmutableMeta
_attributes_ = 'a b'
def sayhi(self):
print "Hello from {0}".format(type(self).__name__)
class Bar(Foo):
_attributes_ = 'c'
def saybye(self):
print "Goodbye from {0}".format(type(self).__name__)
以下是一些测试代码：
a = Foo(1, 2)
a.sayhi()
b = Bar(1, 2, 3)
b.sayhi() # 'inherited' from class Foo
b.saybye()
try:
b.c = 1 # will raise an AttributeError
except AttributeError:
print "Immutable"
print "issubclass(Bar, Foo): {0}".format(issubclass(Bar, Foo))
try:
d = {b: 1} # No problems
except TypeError:
print "Cant put it in a dict"
else:
print "Can put it in a dict"
输出结果：
Hello from Foo
Hello from Bar
Goodbye from Bar
Immutable
issubclass(Bar, Foo): True
Can put it in a dict
