isinstance（）和issubclass（）返回冲突的结果

Question

>>> class Hello:
    pass

和

>>> isinstance(Hello,object)
True
>>> issubclass(Hello,object)
False
>>> a = Hello()
>>> isinstance(a,object)
True

你怎么解释isinstance(Hello,object)返回True而issubclass(Hello,object)返回False

Answer 1

这是因为你使用旧式的类，所以它不是从object派生的。试试这个：

class Hello(object):
    pass

>>> issubclass(Hello,object)
True

旧式类已弃用，您不应再使用它们。

在Python 3.x中，所有类都是新式的，不再需要编写(object)。

Answer 2

接受的答案是正确的，但似乎错过了重要的一点。内置函数isinstance和issubclass提出了两个不同的问题。

isinstance（object，classinfo）询问对象是否是类的实例（或类的元组）。

issubclass（class，classinfo）询问一个类是否是另一个类（或其他类）的子类。

在任何一种方法中，classinfo都可以是“类，类型或元组和类似元组的类，类或元组”。

由于类本身就是对象，因此isinstance适用。我们还可以问一个类是否是另一个类的子类。但是，我们不一定期望从这两个问题得到同样的答案。

class Foo(object):
    pass

class Bar(Foo):
    pass

issubclass(Bar, Foo)
#>True
isinstance(Bar, Foo)
#>False

Bar是Foo的子类，而不是它的一个实例。 Bar是类型的实例，它是object的子类，因此类Bar是object的实例。

isinstance(Bar, type)
#>True
issubclass(type, object)
#>True
isinstance(Bar, object)
#>True

Answer 3

我的回答与Python 3有关。

为了扩展cbare的答案，下面的代码对我有帮助。

>>> class X:
...     pass
...     
>>> class Y(X):
...     pass
...     
>>> x = X()
>>> y = Y()
>>> isinstance(x, X)  # is object x an instance of class X (or any subclass)?
True
>>> isinstance(x, Y)  # is object x an instance of class Y (or any subclass)?
False
>>> isinstance(y, X)  # is object y an instance of class X (or any subclass)?
True
>>> isinstance(y, Y)  # is object y an instance of class Y (or any subclass)?
True

>>> issubclass(X, X)  # is class X a subclass of X (including class X)?
True
>>> issubclass(X, Y)  # is class X a subclass of Y (including class Y)?
False
>>> issubclass(Y, X)  # is class Y a subclass of X (including class X)?
True
>>> issubclass(Y, Y)  # is class Y a subclass of Y (including class Y)?
True

>>> issubclass(type(x), X)  # is class of object x a subclass of X (including class X)?
True
>>> issubclass(type(x), Y)  # is class of object x a subclass of Y (including class Y)?
False
>>> issubclass(type(y), X)  # is class of object y a subclass of X (including class X)?
True
>>> issubclass(type(y), Y)  # is class of object y a subclass of Y (including class Y)?
True

>>> issubclass(x.__class__, X)  # is class of object x a subclass of X (including class X)?
True
>>> issubclass(x.__class__, Y)  # is class of object x a subclass of Y (including class Y)?
False
>>> issubclass(y.__class__, X)  # is class of object y a subclass of X (including class X)?
True
>>> issubclass(y.__class__, Y)  # is class of object y a subclass of Y (including class Y)?
True

我们可以看到isinstance(object, class)正确地尊重继承/子类。