Python MetaClass Magic - 代码杂货铺

在 Nova 代码中[0]发现一个神奇的应用，利用 decrator 来区分同名的函数，根据 decrator 上面定义的版本，来调用不同的方法。仔细学习了一下。它是用到了 MetaClass。主要思路是：

在 decrator 里面，把被装饰的方法存在类的一个属性里面(version_methods)
使用自定义的metaclass, 删除掉父类上的version_methods，并把这个属性放到子类中。这样做是为了防止子类共享父类的version_methods属性。
在子类里面，重写__getattribute__方法，当所需要的属性存在于version_methods时，根据特定的条件，返回期望的方法。如果没有，就调用父类的方法。

以下是示例代码

VERION_METHODS='version_methods' class VersionMethod(object): def __init__(self, version, func): self.version = version self.func = func class MetaClass(type): def __new__(cls, name, bases, cls_dict): for base in bases: version_methods = getattr(base, VERION_METHODS, None) if version_methods: cls_dict[VERION_METHODS] = version_methods delattr(base, VERION_METHODS) return super(MetaClass, cls).__new__(cls, name, bases, cls_dict) class Base(object): __metaclass__ = MetaClass def __getattribute__(self, name): def version_select(*args, **kwargs): if len(args) > 0: version = args[0]['version'] else: version = kwargs['req']['version'] for version_func in self.version_methods.get(name, None): if version_func.version == version: return version_func.func(self, *args, **kwargs) raise AttributeError try: version_methods = object.__getattribute__(self, VERION_METHODS) except AttributeError: return object.__getattribute__(self, name) if version_methods and \ name in version_methods: return version_select return object.__getattribute__(self, name) @classmethod def version(cls, version): def wrapper(func): name = func.func_name version_methods = getattr(cls, VERION_METHODS, {}) if not version_methods: setattr(cls, VERION_METHODS, version_methods) methods = version_methods.get(name, []) if not methods: version_methods[name] = methods methods.append(VersionMethod(version, func)) return func return wrapper class SubClass(Base): @Base.version("1.0") def foo(self, req): print "in SubClass 1.0 version: %s" % req @Base.version("2.0") def foo(self, req): print "in SubClass 2.0 version: %s" % req class SubClass2(Base): @Base.version("1.0") def foo(self, req): print "in SubClass2 1.0 version: %s" % req @Base.version("2.0") def foo(self, req): print "in SubClass2 2.0 version: %s" % req SubClass().foo({"version": "1.0"}) SubClass().foo({"version": "2.0"}) SubClass2().foo({"version": "1.0"}) SubClass2().foo({"version": "2.0"}) # Output # in SubClass 1.0 version: {'version': '1.0'} # in SubClass 2.0 version: {'version': '2.0'} # in SubClass2 1.0 version: {'version': '1.0'} # in SubClass2 2.0 version: {'version': '2.0'}

[0] https://github.com/openstack/nova/blob/c13f693d496f4ea5dbc78244fefd9ec2d6ea8de5/nova/api/openstack/wsgi.py#L1075

原始链接:http://xcodest.me/python-metaclass-magic.html

许可协议:"署名-非商用-相同方式共享 3.0" 转载请保留原文链接及作者。

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