TypedAttributes

[Peter Inglesby]

At this evening's meeting I gave a talk about descriptors which included a way of ensuring that an object's attributes are of a certain type.  Tom objected that my original solution was too verbose, with code that looked like this:

>>> class Person(object):

...     name = TypedAttribute("name", str)

...     age = TypedAttribute("age", int)

It was suggested that this could be shortened with the help of a metaclass, and this turned out to be easier than I'd expected:

>>> class _TypedAttribute(object):

...     def __init__(self, name, typ):

...         self.name = name

...         self.typ = typ

...     def __set__(self, inst, val):

...         if isinstance(val, self.typ):

...             inst.__dict__[self.name] = val

...         else:

...             raise TypeError

... 

>>> class TypedAttribute(object):

...     def __init__(self, typ):

...         self.typ = typ

... 

>>> class TypedClass(type):

...     def __new__(cls, name, bases, attrs):

...         for k, v in attrs.items():

...             if isinstance(v, TypedAttribute):

...                 attrs[k] = _TypedAttribute(k, v.typ)

...         return type.__new__(cls, name, bases, attrs)

... 

>>> class Person(object):

...     __metaclass__ = TypedClass

...     name = TypedAttribute(str)

...     age = TypedAttribute(int)

... 

>>> p = Person()

>>> p.name = 'Bill'

>>> p.name

'Bill'

>>> p.age = 25

>>> p.age

25

>>> p.age = '25'

Traceback (most recent call last):

  File "<console>", line 1, in <module>

  File "<console>", line 9, in __set__

TypeError

>>> p.age

25

[Tibs]

That's really cool.

The other day I was asked if I was an expert Python programmer, and I said that this was difficult to answer in the way they wanted, because I know several EXPERT Python programmers. I stand by that opinion.

Tibs