issubclass(dict, Mapping)

[Terry Reedy]

On 12/22/2010 9:20 AM, kj wrote:

>>>> from collections import Mapping

Documented as an *ABSTRACT* base class. ABCs were added in 3.0 and 
backparted to 2.7. One can be quite competant in Python completely 
ignoring ABCs.

>>>> issubclass(dict, Mapping)
> True

Yes, dict is a concrete Mapping class. I suppose we could have instead 
added a new builtin function 'isconcretetizationof' but is seemed easier 
to reuse issubclass for the test. Most people have no problem with that.

>>>> dict.__bases__
> (<type 'object'>,)

The one and only *CONCRETE* base class. In 3.x, all classes are 
subclasses of object, which simplifies the class model a bit.

>>>> [issubclass(b, Mapping) for b in dict.__bases__]
> [False]

> So dict is a subclass of Mapping, even though none of the bases of
> dict is either Mapping or a subclass of Mapping.  Great.

Right. dict is direct concrete Mapping implementation and not subclassed 
from one.

The main reason for introducing ABCs was to make it easier to test 
whether an object passed to a function is an instance of a possibly 
unknown or yet-to-be-written class in an abstract category, which has a 
certain api or interface. The actual usage of an ABC would be more like 
this:

from collections import Mapping
def f(m):
	if not isinstance(m, Mapping):
		raise ValueError('input is not a mapping')
	else: return True

f(dict())
# True
f([])
# produces
Traceback (most recent call last):
   File "<pyshell#8>", line 1, in <module>
     f([])
   File "<pyshell#6>", line 3, in f
     raise ValueError('input is not a mapping')
ValueError: input is not a mapping

-- 
Terry Jan Reedy