[Python-ideas] Adding`Unpicklable` to the `collections` module

[Éric Araujo]

> For instance, as of Python 2.7 these blocks of code are roughly
> equivalent (the first two are in fact identical in effect):
> 
>     if isinstance(obj, collections.Sized):
>         doSomethingWith(len(obj))
> 
>     if hasattr(obj, '__len__'):
>         doSomethingWith(len(obj))
> 
>     try:
>         doSomethingWith(len(obj))
>     except TypeError:
>         pass
> 
> They all test for the same thing: that you can call len() on the object.

Not exactly: magic methods are looked up on the class, not on the
instance.  That’s why isintance+ABCs is the right test to use here.  See
http://docs.python.org/dev/reference/datamodel#special-method-names

(The third block also suffers from an over-catching except clause.  Put
as little code as possible in the try block and use an else clause.)


ABCs are not contradictory to duck typing at all:

>>> import collections
>>> class Demo:
...     def __len__(self):
...         return 42
...
>>> isinstance(Demo(), collections.Sized)
True

Demo is not required to subclass Sized or register.  ABCs are an
optional mechanism that implements duck typing, so to speak.
http://docs.python.org/dev/glossary
http://docs.python.org/whatsnew/2.6#pep-3119-abstract-base-classes
http://www.python.org/dev/peps/pep-3119/#abcs-vs-duck-typing

Regards