确定Python类是抽象基类还是混凝土

My Python application contains many abstract classes and implementations. For example:

我的Python应用程序包含许多抽象类和实现。例如:

import abc
import datetime

class MessageDisplay(object):
    __metaclass__ = abc.ABCMeta

    @abc.abstractproperty
    def display(self, message):
        pass

class FriendlyMessageDisplay(MessageDisplay):
    def greet(self):
        hour = datetime.datetime.now().timetuple().tm_hour

        if hour < 7:
            raise Exception("Cannot greet while asleep.")
        elif hour < 12:
            self.display("Good morning!")
        elif hour < 18:
            self.display("Good afternoon!")
        elif hour < 20:
            self.display("Good evening!")
        else:
            self.display("Good night.")

class FriendlyMessagePrinter(FriendlyMessageDisplay):
    def display(self, message):
        print(message)

FriendlyMessagePrinter is a concrete class that we can use...

FriendlyMessagePrinter是一个我们可以使用的具体类......

FriendlyMessagePrinter().greet()

Good night.

...but MessageDisplay and FriendlyMessageDisplay are abstract classes and attempting to instantiate one would result in an error:

...但MessageDisplay和FriendlyMessageDisplay是抽象类,尝试实例化一个会导致错误:

TypeError: Can't instantiate abstract class MessageDisplay with abstract methods say

How can I check if a given class object is an (uninstantiatable) abstract class?

如何检查给定的类对象是否是(不可实例化的)抽象类?

3 个解决方案

#1

import inspect
print(inspect.isabstract(object))                  # False
print(inspect.isabstract(MessageDisplay))          # True
print(inspect.isabstract(FriendlyMessageDisplay))  # True
print(inspect.isabstract(FriendlyMessagePrinter))  # False

This checks that the internal flag TPFLAGS_IS_ABSTRACT is set in the class object, so it can't be fooled as easily as your implementation:

这将检查内部标志TPFLAGS_IS_ABSTRACT是否在类对象中设置,因此不能像实现那样容易上当。

class Fake:
    __abstractmethods__ = 'bluh'

print(is_abstract(Fake), inspect.isabstract(Fake)) # True, False

#2

Abstract classes and their concrete implementations have an __abstractmethods__ attribute containing the names of abstract methods and properties that have not been implemented. This behaviour is described in PEP 3199:

抽象类及其具体实现具有__abstractmethods__属性,该属性包含尚未实现的抽象方法和属性的名称。 PEP 3199中描述了此行为:

Implementation: The @abstractmethod decorator sets the function attribute __isabstractmethod__ to the value True. The ABCMeta.__new__ method computes the type attribute __abstractmethods__ as the set of all method names that have an __isabstractmethod__ attribute whose value is true. It does this by combining the __abstractmethods__ attributes of the base classes, adding the names of all methods in the new class dict that have a true __isabstractmethod__ attribute, and removing the names of all methods in the new class dict that don't have a true __isabstractmethod__ attribute. If the resulting __abstractmethods__ set is non-empty, the class is considered abstract, and attempts to instantiate it will raise TypeError. (If this were implemented in CPython, an internal flag Py_TPFLAGS_ABSTRACT could be used to speed up this check.)

实现:@abstractmethod装饰器将函数属性__isabstractmethod__设置为值True。 ABCMeta .__ new__方法将类型属性__abstractmethods__计算为具有__isabstractmethod__属性且值为true的所有方法名称的集合。它通过组合基类的__abstractmethods__属性,添加新类dict中具有true __isabstractmethod__属性的所有方法的名称,并删除新类dict中没有true的所有方法的名称来实现__isabstractmethod__属性。如果生成的__abstractmethods__集合为非空,则该类被视为抽象,并尝试实例化它将引发TypeError。 (如果这是在CPython中实现的,可以使用内部标志Py_TPFLAGS_ABSTRACT加速此检查。)

So in concrete classes, this attribute either will not exist or will be an empty set. This is easy to check:

因此,在具体类中,此属性将不存在或将为空集。这很容易检查:

def is_abstract(cls):
    if not hasattr(cls, "__abstractmethods__"):
        return False # an ordinary class
    elif len(cls.__abstractmethods__) == 0:
        return False # a concrete implementation of an abstract class
    else:
        return True # an abstract class

Or more succinctly:

或者更简洁:

def is_abstract(cls):
    return bool(getattr(cls, "__abstractmethods__", False))

print(is_abstract(object))                 # False
print(is_abstract(MessageDisplay))         # True
print(is_abstract(FriendlyMessageDisplay)) # True
print(is_abstract(FriendlyMessagePrinter)) # False

#3

You could do this with the _ast module. For example, if your example code were in foo.py you could invoked this function with "foo.py" and "FriendlyMessagePrinter" as arguments.

您可以使用_ast模块执行此操作。例如,如果您的示例代码位于foo.py中,则可以使用“foo.py”和“FriendlyMessagePrinter”作为参数调用此函数。

def is_abstract(filepath, class_name):
    astnode = compile(open(filename).read(), filename, 'exec', _ast.PyCF_ONLY_AST)
    for node in astnode.body:
        if isinstance(node, _ast.ClassDef) and node.name == class_name:
            for funcdef in node.body:
                if isinstance(funcdef, _ast.FunctionDef):
                    if any(not isinstance(n, _ast.Pass) for n in funcdef.body):
                        return False
            return True
    print 'class %s not found in file %s' %(class_name, filepath)