Which of the 4 ways to call super() in Python 3 to use?

问题

I wonder when to use what flavour of Python 3 super().

Help on class super in module builtins:

class super(object)
 |  super() -> same as super(__class__, <first argument>)
 |  super(type) -> unbound super object
 |  super(type, obj) -> bound super object; requires isinstance(obj, type)
 |  super(type, type2) -> bound super object; requires issubclass(type2, type)

Until now I've used super() only without arguments and it worked as expected (by a Java developer).

Questions:

What does "bound" mean in this context?
What is the difference between bound and unbound super object?
When to use super(type, obj) and when super(type, type2)?
Would it be better to name the super class like in Mother.__init__(...)?

回答1:

Let's use the following classes for demonstration:

class A(object):
    def m(self):
        print('m')

class B(A): pass

Unbound super object doesn't dispatch attribute access to class, you have to use descriptor protocol:

>>> super(B).m
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'super' object has no attribute 'm'
>>> super(B).__get__(B(), B)
<super: <class 'B'>, <B object>>

super object bound to instance gives bound methods:

>>> super(B, B()).m
<bound method B.m of <__main__.B object at 0xb765dacc>>
>>> super(B, B()).m()
m

super object bound to class gives function (unbound methods in terms of Python 2):

>>> super(B, B).m
<function m at 0xb761482c>
>>> super(B, B).m()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: m() takes exactly 1 positional argument (0 given)
>>> super(B, B).m(B())
m

See Michele Simionato's "Things to Know About Python Super" blog posts series (1, 2, 3) for more information