问题:对于新样式的类,super()引发“ TypeError:必须为类型,而不是classobj”

以下用法super()引发TypeError:为什么?

>>> from  HTMLParser import HTMLParser
>>> class TextParser(HTMLParser):
...     def __init__(self):
...         super(TextParser, self).__init__()
...         self.all_data = []
...         
>>> TextParser()
(...)
TypeError: must be type, not classobj

在StackOverflow上有一个类似的问题:Python super()引发TypeError,该错误由用户类不是新型类的事实来解释。但是,上面的类是一种新式的类,因为它继承自object

>>> isinstance(HTMLParser(), object)
True

我想念什么?我如何super()在这里使用?

使用HTMLParser.__init__(self)代替super(TextParser, self).__init__()可以工作,但是我想了解TypeError。

PS:Joachim指出,成为一个新类实例并不等同于成为一个实例object。我读了很多相反的书,因此感到困惑(基于object实例测试的新型类实例测试的示例:https : //stackoverflow.com/revisions/2655651/3)。

The following use of super() raises a TypeError: why?

>>> from  HTMLParser import HTMLParser
>>> class TextParser(HTMLParser):
...     def __init__(self):
...         super(TextParser, self).__init__()
...         self.all_data = []
...         
>>> TextParser()
(...)
TypeError: must be type, not classobj

There is a similar question on StackOverflow: Python super() raises TypeError, where the error is explained by the fact that the user class is not a new-style class. However, the class above is a new-style class, as it inherits from object:

>>> isinstance(HTMLParser(), object)
True

What am I missing? How can I use super(), here?

Using HTMLParser.__init__(self) instead of super(TextParser, self).__init__() would work, but I would like to understand the TypeError.

PS: Joachim pointed out that being a new-style-class instance is not equivalent to being an object. I read the opposite many times, hence my confusion (example of new-style class instance test based on object instance test: https://stackoverflow.com/revisions/2655651/3).


回答 0

好吧,这是通常的“ super()不能与老式类一起使用”。

但是,重要的一点是对“这是一个新的实例(即对象)吗?” 的正确测试。是

>>> class OldStyle: pass
>>> instance = OldStyle()
>>> issubclass(instance.__class__, object)
False

而不是(如问题所示):

>>> isinstance(instance, object)
True

对于,正确的“这是新型类”测试是:

>>> issubclass(OldStyle, object)  # OldStyle is not a new-style class
False
>>> issubclass(int, object)  # int is a new-style class
True

关键的一点是,与老式类的的实例和它的类型是不同的。在这里,OldStyle().__class__is OldStyle,它不继承自object,而type(OldStyle())is instance类型,它确实继承自object。基本上,旧式类仅创建类型的对象instance(而新式类将创建类型为类本身的对象)。这大概就是为什么实例OldStyle()object:其type()从继承object(事实上,它的类并没有继承object不计数:老式类只是构建类型的新对象instance)。部分参考:https://stackoverflow.com/a/9699961/42973

PS:新式类和旧式类之间的区别还可以通过以下方式看到:

>>> type(OldStyle)  # OldStyle creates objects but is not itself a type
classobj
>>> isinstance(OldStyle, type)
False
>>> type(int)  # A new-style class is a type
type

(旧式类不是类型,因此它们不能是其实例的类型)。

Alright, it’s the usual “super() cannot be used with an old-style class”.

However, the important point is that the correct test for “is this a new-style instance (i.e. object)?” is

>>> class OldStyle: pass
>>> instance = OldStyle()
>>> issubclass(instance.__class__, object)
False

and not (as in the question):

>>> isinstance(instance, object)
True

For classes, the correct “is this a new-style class” test is:

>>> issubclass(OldStyle, object)  # OldStyle is not a new-style class
False
>>> issubclass(int, object)  # int is a new-style class
True

The crucial point is that with old-style classes, the class of an instance and its type are distinct. Here, OldStyle().__class__ is OldStyle, which does not inherit from object, while type(OldStyle()) is the instance type, which does inherit from object. Basically, an old-style class just creates objects of type instance (whereas a new-style class creates objects whose type is the class itself). This is probably why the instance OldStyle() is an object: its type() inherits from object (the fact that its class does not inherit from object does not count: old-style classes merely construct new objects of type instance). Partial reference: https://stackoverflow.com/a/9699961/42973.

PS: The difference between a new-style class and an old-style one can also be seen with:

>>> type(OldStyle)  # OldStyle creates objects but is not itself a type
classobj
>>> isinstance(OldStyle, type)
False
>>> type(int)  # A new-style class is a type
type

(old-style classes are not types, so they cannot be the type of their instances).


回答 1

super()仅可用于新型类,这意味着根类需要从’object’类继承。

例如,顶级类需要像这样:

class SomeClass(object):
    def __init__(self):
        ....

class SomeClass():
    def __init__(self):
        ....

因此,解决方案是直接调用父级的init方法,如下所示:

class TextParser(HTMLParser):
    def __init__(self):
        HTMLParser.__init__(self)
        self.all_data = []

super() can be used only in the new-style classes, which means the root class needs to inherit from the ‘object’ class.

For example, the top class need to be like this:

class SomeClass(object):
    def __init__(self):
        ....

not

class SomeClass():
    def __init__(self):
        ....

So, the solution is that call the parent’s init method directly, like this way:

class TextParser(HTMLParser):
    def __init__(self):
        HTMLParser.__init__(self)
        self.all_data = []

回答 2

您也可以使用class TextParser(HTMLParser, object):。这将创建TextParser一个新样式的类,并且super()可以使用。

You can also use class TextParser(HTMLParser, object):. This makes TextParser a new-style class, and super() can be used.


回答 3

问题是super需要object一个祖先:

>>> class oldstyle:
...     def __init__(self): self.os = True

>>> class myclass(oldstyle):
...     def __init__(self): super(myclass, self).__init__()

>>> myclass()
TypeError: must be type, not classobj

经过仔细检查,发现:

>>> type(myclass)
classobj

但:

>>> class newstyle(object): pass

>>> type(newstyle)
type    

因此,解决您的问题的方法是从对象以及HTMLParser继承。但是确保对象在MRO类中排在最后:

>>> class myclass(oldstyle, object):
...     def __init__(self): super(myclass, self).__init__()

>>> myclass().os
True

The problem is that super needs an object as an ancestor:

>>> class oldstyle:
...     def __init__(self): self.os = True

>>> class myclass(oldstyle):
...     def __init__(self): super(myclass, self).__init__()

>>> myclass()
TypeError: must be type, not classobj

On closer examination one finds:

>>> type(myclass)
classobj

But:

>>> class newstyle(object): pass

>>> type(newstyle)
type    

So the solution to your problem would be to inherit from object as well as from HTMLParser. But make sure object comes last in the classes MRO:

>>> class myclass(oldstyle, object):
...     def __init__(self): super(myclass, self).__init__()

>>> myclass().os
True

回答 4

如果您查看继承树(在2.6版中),则HTMLParser继承自SGMLParser,继承自ParserBase而不继承自object。即HTMLParser是一个老式的类。

关于您的检查isinstance,我在ipython中进行了快速测试:

在[1]中:A类:
   ...:通过
   ...: 

在[2]中:isinstance(A,object)
出[2]:是

即使一个类是老式类,它仍然是的一个实例object

If you look at the inheritance tree (in version 2.6), HTMLParser inherits from SGMLParser which inherits from ParserBase which doesn’t inherits from object. I.e. HTMLParser is an old-style class.

About your checking with isinstance, I did a quick test in ipython:

In [1]: class A:
   ...:     pass
   ...: 

In [2]: isinstance(A, object)
Out[2]: True

Even if a class is old-style class, it’s still an instance of object.


回答 5

正确的方法是在不继承自’object’的旧类中执行以下操作

class A:
    def foo(self):
        return "Hi there"

class B(A):
    def foo(self, name):
        return A.foo(self) + name

the correct way to do will be as following in the old-style classes which doesn’t inherit from ‘object’

class A:
    def foo(self):
        return "Hi there"

class B(A):
    def foo(self, name):
        return A.foo(self) + name

回答 6

FWIW,尽管我不是Python专家,但我对此很满意

>>> class TextParser(HTMLParser):
...    def handle_starttag(self, tag, attrs):
...        if tag == "b":
...            self.all_data.append("bold")
...        else:
...            self.all_data.append("other")
...     
...         
>>> p = TextParser()
>>> p.all_data = []
>>> p.feed(text)
>>> print p.all_data
(...)

只是让我根据需要返回解析结果。

FWIW and though I’m no Python guru I got by with this

>>> class TextParser(HTMLParser):
...    def handle_starttag(self, tag, attrs):
...        if tag == "b":
...            self.all_data.append("bold")
...        else:
...            self.all_data.append("other")
...     
...         
>>> p = TextParser()
>>> p.all_data = []
>>> p.feed(text)
>>> print p.all_data
(...)

Just got me the parse results back as needed.


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