下面使用super()会引发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上也有类似的问题:Python super()引发TypeError，其中错误是由用户类不是新样式类这一事实解释的。然而，上面的类是一个新型的类，因为它继承自object:
>>> isinstance(HTMLParser(), object)
True
我错过了什么?我如何使用super()，在这里?
使用HTMLParser.__init__(self)而不是super(TextParser, self).__init__()将工作，但我想了解TypeError。
PS: Joachim指出，作为一个新型的类实例并不等同于一个对象。我读过很多次相反的文章，因此感到困惑(基于对象实例测试的新型类实例测试示例:https://stackoverflow.com/revisions/2655651/3)。
好吧，这是常见的“super()不能用于旧式类”。
然而，重要的一点是，正确的测试“这是一个新风格的实例(即对象)吗?
>>> class OldStyle: pass
>>> instance = OldStyle()
>>> issubclass(instance.__class__, object)
False
而不是(在问题中):
>>> isinstance(instance, object)
True
对于类，正确的“这是一种新型类吗”测试是:
>>> 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:新型类和老式类之间的区别还可以从以下方面看到:
>>> 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
(老式的类不是类型，所以它们不能是它们实例的类型)。
问题是super需要一个对象作为祖先:
>>> 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
仔细观察就会发现:
>>> type(myclass)
classobj
But:
>>> class newstyle(object): pass
>>> type(newstyle)
type
所以你的问题的解决方案是继承对象以及从HTMLParser。
但是要确保object在MRO类中排在最后:
>>> class myclass(oldstyle, object):
... def __init__(self): super(myclass, self).__init__()
>>> myclass().os
True
Super()只能在新型类中使用，这意味着根类需要从'object'类继承。
例如，顶级类需要是这样的:
class SomeClass(object):
def __init__(self):
....
not
class SomeClass():
def __init__(self):
....
所以，解决方案是直接调用父类的init方法，像这样:
class TextParser(HTMLParser):
def __init__(self):
HTMLParser.__init__(self)
self.all_data = []
