duck typing assert

Ian Kelly ian.g.kelly at
Thu Nov 8 19:34:45 CET 2012

On Thu, Nov 8, 2012 at 10:30 AM, Andriy Kornatskyy
<andriy.kornatskyy at> wrote:
> People who come from strongly typed languages that offer interfaces often are confused by lack of one in Python. Python, being dynamic typing programming language, follows duck typing principal. It can as simple as this:
> assert looks(Foo).like(IFoo)
> The post below shows how programmer can assert duck typing between two Python classes:
> Comments or suggestions are welcome.

Overall, it looks potentially useful to me.  Looking over the
wheezy.core.introspection source, it appears to ignore special method
names.  For example, if you do:

class IFoo(object):
    def __len__(self):

class Foo(object):

assert looks(Foo).like(IFoo)

I would expect this to fail, but judging from the code it would not,
as it ignores all names starting with '_'.  That makes some sense for
private names (but on the other hand, why would you declare private
names in an interface unless you want to enforce their presence?), but
names like __len__ should not be considered private.

Another comment I have is on properties, and descriptors in general.
Does this allow something declared as a property to be implemented as
a simple class attribute, and vice versa?  Or can something declared
as a property be implemented with some custom descriptor class?  I
think the answer is "no" to both, as the check it performs is
"t2.__class__ is not t.__class__".  I assert though that in general
the type of a descriptor (that is not a simple class attribute) is not
as important in duck testing as its API -- and all descriptors have
basically the same API of __get__, __set__, and __delete__.

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