实例方法的Python装饰器可以访问该类吗？

Question

您好我有类似以下的内容。基本上我需要从定义中的实例方法使用的装饰器访问实例方法的类。

def decorator(view):
    # do something that requires view's class
    print view.im_class
    return view

class ModelA(object):
    @decorator
    def a_method(self):
        # do some stuff
        pass

代码按原样提供

AttributeError: 'function' object has no attribute 'im_class'

我发现了类似的问题/答案 - Python decorator makes function forget that it belongs to a class和Get class in Python decorator - 但这些问题依赖于通过抢夺第一个参数在运行时抓取实例的解决方法。在我的情况下，我将根据从其类中收集的信息调用该方法，因此我不能等待来电。

谢谢。

Answer 1

如果您使用的是Python 2.6或更高版本，则可以使用类装饰器，可能是这样的（警告：未经测试的代码）。

def class_decorator(cls):
   for name, method in cls.__dict__.iteritems():
        if hasattr(method, "use_class"):
            # do something with the method and class
            print name, cls
   return cls

def method_decorator(view):
    # mark the method as something that requires view's class
    view.use_class = True
    return view

@class_decorator
class ModelA(object):
    @method_decorator
    def a_method(self):
        # do some stuff
        pass

方法装饰器通过添加“use_class”属性将方法标记为感兴趣的方法 - 函数和方法也是对象，因此您可以将附加元数据附加到它们。

在创建类之后，类装饰器会遍历所有方法，并对已标记的方法执行任何操作。

如果您希望所有方法都受到影响，那么您可以省略方法装饰器并使用类装饰器。

Answer 2

正如其他人所指出的那样，在调用装饰器时尚未创建类。 但是，可以使用装饰器参数注释函数对象，然后在元类的__new__方法中重新装饰该函数。您需要直接访问函数的__dict__属性，至少对我来说，func.foo = 1导致了AttributeError。

Answer 3

正如马克所说：

1. 任何装饰者都被称为BEFORE类，因此装饰者不知道。
2. 我们可以标记这些方法，并在以后进行必要的后续处理。
3. 我们有两种后处理选项：自动在类定义的末尾或应用程序运行之前的某个位置。我更喜欢使用基类的第一个选项，但你也可以遵循第二种方法。

此代码显示了如何使用自动后处理：

def expose(**kw):
    "Note that using **kw you can tag the function with any parameters"
    def wrap(func):
        name = func.func_name
        assert not name.startswith('_'), "Only public methods can be exposed"

        meta = func.__meta__ = kw
        meta['exposed'] = True
        return func

    return wrap

class Exposable(object):
    "Base class to expose instance methods"
    _exposable_ = None  # Not necessary, just for pylint

    class __metaclass__(type):
        def __new__(cls, name, bases, state):
            methods = state['_exposed_'] = dict()

            # inherit bases exposed methods
            for base in bases:
                methods.update(getattr(base, '_exposed_', {}))

            for name, member in state.items():
                meta = getattr(member, '__meta__', None)
                if meta is not None:
                    print "Found", name, meta
                    methods[name] = member
            return type.__new__(cls, name, bases, state)

class Foo(Exposable):
    @expose(any='parameter will go', inside='__meta__ func attribute')
    def foo(self):
        pass

class Bar(Exposable):
    @expose(hide=True, help='the great bar function')
    def bar(self):
        pass

class Buzz(Bar):
    @expose(hello=False, msg='overriding bar function')
    def bar(self):
        pass

class Fizz(Foo):
    @expose(msg='adding a bar function')
    def bar(self):
        pass

print('-' * 20)
print("showing exposed methods")
print("Foo: %s" % Foo._exposed_)
print("Bar: %s" % Bar._exposed_)
print("Buzz: %s" % Buzz._exposed_)
print("Fizz: %s" % Fizz._exposed_)

print('-' * 20)
print('examine bar functions')
print("Bar.bar: %s" % Bar.bar.__meta__)
print("Buzz.bar: %s" % Buzz.bar.__meta__)
print("Fizz.bar: %s" % Fizz.bar.__meta__)

输出产量：

Found foo {'inside': '__meta__ func attribute', 'any': 'parameter will go', 'exposed': True}
Found bar {'hide': True, 'help': 'the great bar function', 'exposed': True}
Found bar {'msg': 'overriding bar function', 'hello': False, 'exposed': True}
Found bar {'msg': 'adding a bar function', 'exposed': True}
--------------------
showing exposed methods
Foo: {'foo': <function foo at 0x7f7da3abb398>}
Bar: {'bar': <function bar at 0x7f7da3abb140>}
Buzz: {'bar': <function bar at 0x7f7da3abb0c8>}
Fizz: {'foo': <function foo at 0x7f7da3abb398>, 'bar': <function bar at 0x7f7da3abb488>}
--------------------
examine bar functions
Bar.bar: {'hide': True, 'help': 'the great bar function', 'exposed': True}
Buzz.bar: {'msg': 'overriding bar function', 'hello': False, 'exposed': True}
Fizz.bar: {'msg': 'adding a bar function', 'exposed': True}

请注意，在此示例中：

1. 我们可以用任意参数注释任何函数。
2. 每个班级都有自己的曝光方法。
3. 我们也可以继承暴露的方法。
4. 方法可以覆盖，因为更新了曝光功能。

希望这有帮助

Answer 4

正如蚂蚁所说，你不能从班级中获得对班级的引用。但是，如果您想要区分不同的类（不操纵实际的类类型对象），则可以为每个类传递一个字符串。您还可以使用类样式装饰器将您喜欢的任何其他参数传递给装饰器。

class Decorator(object):
    def __init__(self,decoratee_enclosing_class):
        self.decoratee_enclosing_class = decoratee_enclosing_class
    def __call__(self,original_func):
        def new_function(*args,**kwargs):
            print 'decorating function in ',self.decoratee_enclosing_class
            original_func(*args,**kwargs)
        return new_function


class Bar(object):
    @Decorator('Bar')
    def foo(self):
        print 'in foo'

class Baz(object):
    @Decorator('Baz')
    def foo(self):
        print 'in foo'

print 'before instantiating Bar()'
b = Bar()
print 'calling b.foo()'
b.foo()

打印：

before instantiating Bar()
calling b.foo()
decorating function in  Bar
in foo

此外，see Bruce Eckel's page on decorators.

Answer 5

问题是当调用装饰器时类尚不存在。试试这个：

def loud_decorator(func):
    print("Now decorating %s" % func)
    def decorated(*args, **kwargs):
        print("Now calling %s with %s,%s" % (func, args, kwargs))
        return func(*args, **kwargs)
    return decorated

class Foo(object):
    class __metaclass__(type):
        def __new__(cls, name, bases, dict_):
            print("Creating class %s%s with attributes %s" % (name, bases, dict_))
            return type.__new__(cls, name, bases, dict_)

    @loud_decorator
    def hello(self, msg):
        print("Hello %s" % msg)

Foo().hello()

该程序将输出：

Now decorating <function hello at 0xb74d35dc>
Creating class Foo(<type 'object'>,) with attributes {'__module__': '__main__', '__metaclass__': <class '__main__.__metaclass__'>, 'hello': <function decorated at 0xb74d356c>}
Now calling <function hello at 0xb74d35dc> with (<__main__.Foo object at 0xb74ea1ac>, 'World'),{}
Hello World

如你所见，你将不得不找出一种不同的方式来做你想做的事。

Answer 6

flask-classy做的是创建一个临时缓存，它存储在方法上，然后它使用其他东西（Flask将使用register类方法注册类的事实）来实际包装方法

您可以重复使用此模式，这次使用元类，以便您可以在导入时包装该方法。

def route(rule, **options):
    """A decorator that is used to define custom routes for methods in
    FlaskView subclasses. The format is exactly the same as Flask's
    `@app.route` decorator.
    """

    def decorator(f):
        # Put the rule cache on the method itself instead of globally
        if not hasattr(f, '_rule_cache') or f._rule_cache is None:
            f._rule_cache = {f.__name__: [(rule, options)]}
        elif not f.__name__ in f._rule_cache:
            f._rule_cache[f.__name__] = [(rule, options)]
        else:
            f._rule_cache[f.__name__].append((rule, options))

        return f

    return decorator

在实际的类上（你可以使用元类做同样的事情）：

@classmethod
def register(cls, app, route_base=None, subdomain=None, route_prefix=None,
             trailing_slash=None):

    for name, value in members:
        proxy = cls.make_proxy_method(name)
        route_name = cls.build_route_name(name)
        try:
            if hasattr(value, "_rule_cache") and name in value._rule_cache:
                for idx, cached_rule in enumerate(value._rule_cache[name]):
                    # wrap the method here

来源：https://github.com/apiguy/flask-classy/blob/master/flask_classy.py

Answer 7

这是一个简单的例子：

def mod_bar(cls):
    # returns modified class

    def decorate(fcn):
        # returns decorated function

        def new_fcn(self):
            print self.start_str
            print fcn(self)
            print self.end_str

        return new_fcn

    cls.bar = decorate(cls.bar)
    return cls

@mod_bar
class Test(object):
    def __init__(self):
        self.start_str = "starting dec"
        self.end_str = "ending dec" 

    def bar(self):
        return "bar"

输出结果为：

>>> import Test
>>> a = Test()
>>> a.bar()
starting dec
bar
ending dec

Answer 8

从python 3.6开始，您可以使用object.__set_name__以一种非常简单的方式完成此操作。该文档指出__set_name__是“在创建拥有类 owner 时调用的”。 这是一个示例：

class class_decorator:
    def __init__(self, fn):
        self.fn = fn

    def __set_name__(self, owner, name):
        # do something with owner, i.e.
        print(f"decorating {self.fn} and using {owner}")
        self.fn.class_name = owner.__name__

        # then replace ourself with the original method
        setattr(owner, name, self.fn)

请注意，它是在创建类时被调用的：

>>> class A:
...     @class_decorator
...     def hello(self, x=42):
...         return x
...
decorating <function A.hello at 0x7f9bedf66bf8> and using <class '__main__.A'>
>>> A.hello
<function __main__.A.hello(self, x=42)>
>>> A.hello.class_name
'A'
>>> a = A()
>>> a.hello()
42

如果您想了解有关如何创建类的更多信息，尤其是确切地何时调用__set_name__的信息，可以参考documentation on "Creating the class object"。

Answer 9

这是一个古老的问题，但遇到了venusian。 http://venusian.readthedocs.org/en/latest/

它似乎有能力装饰方法，并允许您同时访问类和方法。 请注意，调用setattr(ob, wrapped.__name__, decorated)并不是使用venusian的典型方式，并且有些失败了。

无论哪种方式......下面的例子都已完成并应该运行。

import sys
from functools import wraps
import venusian

def logged(wrapped):
    def callback(scanner, name, ob):
        @wraps(wrapped)
        def decorated(self, *args, **kwargs):
            print 'you called method', wrapped.__name__, 'on class', ob.__name__
            return wrapped(self, *args, **kwargs)
        print 'decorating', '%s.%s' % (ob.__name__, wrapped.__name__)
        setattr(ob, wrapped.__name__, decorated)
    venusian.attach(wrapped, callback)
    return wrapped

class Foo(object):
    @logged
    def bar(self):
        print 'bar'

scanner = venusian.Scanner()
scanner.scan(sys.modules[__name__])

if __name__ == '__main__':
    t = Foo()
    t.bar()

Answer 10

正如其他答案指出的那样，decorator是一种函数式的东西，由于尚未创建该类，因此您无法访问此方法所属的类。但是，完全可以使用装饰器来“标记”函数，然后再使用元类技术来处理该方法，因为在__new__阶段，该类是由其元类创建的。

这是一个简单的例子：

我们使用@field将方法标记为特殊字段，并在元类中对其进行处理。

def field(fn):
    """Mark the method as an extra field"""
    fn.is_field = True
    return fn

class MetaEndpoint(type):
    def __new__(cls, name, bases, attrs):
        fields = {}
        for k, v in attrs.items():
            if inspect.isfunction(v) and getattr(k, "is_field", False):
                fields[k] = v
        for base in bases:
            if hasattr(base, "_fields"):
                fields.update(base._fields)
        attrs["_fields"] = fields

        return type.__new__(cls, name, bases, attrs)

class EndPoint(metaclass=MetaEndpoint):
    pass


# Usage

class MyEndPoint(EndPoint):
    @field
    def foo(self):
        return "bar"

e = MyEndPoint()
e._fields  # {"foo": ...}

Answer 11

您可以在装饰器应该返回的修饰方法中访问要在其上调用方法的对象的类。像这样：

def decorator(method):
    # do something that requires view's class
    def decorated(self, *args, **kwargs):
        print 'My class is %s' % self.__class__
        method(self, *args, **kwargs)
    return decorated

使用您的ModelA类，这是它的作用：

>>> obj = ModelA()
>>> obj.a_method()
My class is <class '__main__.ModelA'>

Answer 12

当装饰器代码运行时，函数不知道它是否是定义点的方法。只有当它通过类/实例标识符访问时，它才能知道它的类/实例。要克服此限制，您可以按描述符对象进行装饰，以将实际装饰代码延迟到访问/调用时间：

class decorated(object):
    def __init__(self, func, type_=None):
        self.func = func
        self.type = type_

    def __get__(self, obj, type_=None):
        func = self.func.__get__(obj, type_)
        print('accessed %s.%s' % (type_.__name__, func.__name__))
        return self.__class__(func, type_)

    def __call__(self, *args, **kwargs):
        name = '%s.%s' % (self.type.__name__, self.func.__name__)
        print('called %s with args=%s kwargs=%s' % (name, args, kwargs))
        return self.func(*args, **kwargs)

这允许您装饰单个（静态|类）方法：

class Foo(object):
    @decorated
    def foo(self, a, b):
        pass

    @decorated
    @staticmethod
    def bar(a, b):
        pass

    @decorated
    @classmethod
    def baz(cls, a, b):
        pass

class Bar(Foo):
    pass

现在你可以使用装饰器代码进行内省......

>>> Foo.foo
accessed Foo.foo
>>> Foo.bar
accessed Foo.bar
>>> Foo.baz
accessed Foo.baz
>>> Bar.foo
accessed Bar.foo
>>> Bar.bar
accessed Bar.bar
>>> Bar.baz
accessed Bar.baz

...以及改变功能行为：

>>> Foo().foo(1, 2)
accessed Foo.foo
called Foo.foo with args=(1, 2) kwargs={}
>>> Foo.bar(1, b='bcd')
accessed Foo.bar
called Foo.bar with args=(1,) kwargs={'b': 'bcd'}
>>> Bar.baz(a='abc', b='bcd')
accessed Bar.baz
called Bar.baz with args=() kwargs={'a': 'abc', 'b': 'bcd'}

Answer 13

我只想添加我的示例，因为它包含了从装饰方法访问类时可以想到的所有内容。它使用@tyrion建议的描述符。装饰器可以接受参数并将其传递给描述符。它既可以处理类中的方法，也可以处理不具有类的函数。

import datetime as dt
import functools

def dec(arg1):
    class Timed(object):
        local_arg = arg1
        def __init__(self, f):
            functools.update_wrapper(self, f)
            self.func = f

        def __set_name__(self, owner, name):
            # doing something fancy with owner and name
            print('owner type', owner.my_type())
            print('my arg', self.local_arg)

        def __call__(self, *args, **kwargs):
            start = dt.datetime.now()
            ret = self.func(*args, **kwargs)
            time = dt.datetime.now() - start
            ret["time"] = time
            return ret
        
        def __get__(self, instance, owner):
            from functools import partial
            return partial(self.__call__, instance)
    return Timed

class Test(object):
    def __init__(self):
        super(Test, self).__init__()

    @classmethod
    def my_type(cls):
        return 'owner'

    @dec(arg1='a')
    def decorated(self, *args, **kwargs):
        print(self)
        print(args)
        print(kwargs)
        return dict()

    def call_deco(self):
        self.decorated("Hello", world="World")

@dec(arg1='a function')
def another(*args, **kwargs):
    print(args)
    print(kwargs)
    return dict()

if __name__ == "__main__":
    t = Test()
    ret = t.call_deco()
    another('Ni hao', world="shi jie")
    

Answer 14

@asterio 冈萨雷斯

我更喜欢你的方法，但是为了符合新的元类处理，Python 3 必须稍微改变一下（还有一些打印语句缺少括号）：

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Mon Aug  9 15:27:30 2021

@author: yves
"""

def expose(**kw):
    "Note that using **kw you can tag the function with any parameters"
    def wrap(func):
        name = func.__name__
        assert not name.startswith('_'), "Only public methods can be exposed"

        meta = func.__meta__ = kw
        meta['exposed'] = None
        return func

    return wrap

class ExposableMetaclass(type):
    def __new__(cls, name, bases, state):
        methods = state['_exposed_'] = dict()

        # inherit bases exposed methods
        for base in bases:
            methods.update(getattr(base, '_exposed_', {}))

        for name, member in state.items():
            meta = getattr(member, '__meta__', None)
            if meta is not None:
                print("Found", name, meta)
                methods[name] = member
        return type.__new__(cls, name, bases, state)

class Exposable(metaclass=ExposableMetaclass):
    "Base class to expose instance methods"
    _exposable_ = None  # Not necessary, just for pylint

class Foo(Exposable):
    @expose(any='parameter will go', inside='__meta__ func attribute')
    def foo(self):
        pass

class Bar(Exposable):
    @expose(hide=True, help='the great bar function')
    def bar(self):
        pass

class Buzz(Bar):
    @expose(hello=False, msg='overriding bar function')
    def bar(self):
        pass

class Fizz(Foo):
    @expose(msg='adding a bar function')
    def bar(self):
        pass

print('-' * 20)
print("showing exposed methods")
print("Foo: %s" % Foo._exposed_)
print("Bar: %s" % Bar._exposed_)
print("Buzz: %s" % Buzz._exposed_)
print("Fizz: %s" % Fizz._exposed_)

满足我的需求！