在类中定义装饰器，这在类定义中也是有用的

Question

我正在尝试在Python中实现一个“子命令”系统作为一个可继承的类。我的预期用例类似于：

from command import Command
import sys

class MyCommand(Command):
    @Command.subcommand
    def foo(self):
        print "this can be run as a subcommand"

    def bar(self):
        print "but this is a plain method and isn't exposed to the CLI"

MyCommand()(*sys.argv)

# at the command line, the user runs "mycommand.py foo"

我将Command.subcommand实现为一个静态方法，一切都运行得很好，直到我尝试向父类添加子命令，这使我获得了TypeError: 'staticmethod' object is not callable。事后看来，很明显，this不会工作：

class Command(object):
    @staticmethod
    def subcommand(method):
        method.is_subcommand = True

        return method

    @subcommand
    def common(self):
        print "this subcommand is available to all child classes"

到目前为止，我找到的唯一替代方法是在父类外部声明subcommand装饰器，然后在类定义完成后注入它。

def subcommand(method):
    method.is_subcommand = True

    return method

class Command(object):
    @subcommand
    def common(self):
        print "this subcommand is available to all child classes"

Command.subcommand = staticmethod(subcommand)
del subcommand

然而，作为一个在添加装饰器之前从未使用过Python的人，这对我来说感觉非常笨拙。有没有更优雅的方式来实现这一点呢？

Answer 1

对于这个问题，我能想到两种解决方案。最简单的方法是在父类中使用完之后，将它变成一个静态方法：

class Command(object):
    def subcommand(method): # Regular function in class definition scope.
        method.is_subcommand = True

        return method

    @subcommand
    def common(self):
        print "this subcommand is available to all child classes"

    subcommand = staticmethod(subcommand)
    # Now a static method. Can no longer be called during class definition phase.

这有点脆弱，因为在将其设置为静态方法之后，您不能在父类中使用它。更健壮的方法是添加一个中间类：

class Command(object):
    @staticmethod
    def subcommand(method):
        method.is_subcommand = True

        return method

class CommandBase(Command):

    @Command.subcommand
    def common(self):
        print "this subcommand is available to all child classes"

现在，您可以从CommandBase而不是Command继承所有类。