Question

Can I define a __repr__ for a class rather than an instance? For example, I’m trying to do this

class A(object):
    @classmethod
    def __repr__(cls):
        return 'My class %s' % cls

What I get is

In [58]: a=A()

In [59]: a
Out[59]: My class <class '__main__.A'>

In [60]: A
Out[60]: __main__.A

I’m trying to get the output of line 60 to look like “My Class A”, not for the instance a. The reason I want to do this is I’m generating a lot of classes using Python’s metaclass. And I want a more readable way to identify the class than the stock repr.

Answer 1

You need to define __repr__ on the metaclass.

class Meta(type):
    def __repr__(cls):
        return 'My class %s' % cls.__name__

class A(object):
    __metaclass__ = Meta

__repr__ returns a representation of an instance of an object. So by defining __repr__ on A, you’re specifying what you want repr(A()) to look like.

To define the representation of the class, you need to define how an instance of type is represented. In this case, replace type with a custom metaclass with __repr__ defined as you need.

>> repr(A)
My class A

If you want to define a custom __repr__ for each class, I’m not sure there’s a particularly clean way to do it. But you could do something like this.

class Meta(type):
    def __repr__(cls):
        if hasattr(cls, '_class_repr'):
            return getattr(cls, '_class_repr')()
        else:
            return super(Meta, cls).__repr__()

class A(object):
    __metaclass__ = Meta

    @classmethod
    def _class_repr(cls):
        return 'My class %s' % cls.__name__

class B(object):
    __metaclass__ = Meta

Then you can customize on a per-class basis.

>> repr(A)
My class A
>> repr(B)
<__main__.B object at 0xb772068c>