Python：__str__，但对于一个类，而不是一个实例？

Question

I understand the following Python code:

>>> class A(object):
...     def __str__(self):
...         return "An instance of the class A"
... 
>>> 
>>> a = A()
>>> print a
An instance of the class A

Now, I would like to change the output of

>>> print A
<class '__main__.A'>

Which function do I need to overload to be able to do that? The solution has to work even if the class is never instantiated. Is the situation different in Python 2.x and 3?

Answer 1

Define __str__() on the metaclass:

class A(object):
    class __metaclass__(type):
        def __str__(self):
            return "plonk"

Now, print A will print plonk .

Edit : As noted by jsbueno in the comments, in Python 3.x you would need to do the following:

class Meta(type):
    def __str__(self):
        return "plonk"
class A(metaclass=Meta):
    pass

Even in Python 2.x it might be a better idea to define the metaclass outside the class body -- I chose the nested form above to save some typing.

Answer 2

Define the __repr__ method on your meta class:

class MetaClass(type):

    def __repr__(self):
          return "Customized string"

class TestClass(object):
   __metaclass__  = MetaClass


print TestClass # Customized string