duck typing assert‏

Steven D'Aprano steve+comp.lang.python at
Fri Nov 9 00:33:01 CET 2012

On Thu, 08 Nov 2012 20:34:58 +0300, Andriy Kornatskyy 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)

How very cute. And I don't mean that in a good way.

Why is this a class with a method, instead of a function that takes two 
class arguments (plus any optional arguments needed)?

looks_like(Foo, IFoo)

is less "cute", reads better to English speakers, and much more Pythonic. 
This isn't Java, not everything needs to be a class.

> The post below shows how programmer can assert duck typing between two
> Python classes:

I don't understand the emphasis on assert for this code. It is enough 
that looks() return a flag. The caller can then use that as an assertion:

assert looks(Spam).like(Ham)

or as a conditional:

if looks(food).like(Meat):

Assertions are only one use for this check, and in my opinion, the least 
useful one.

And why the warnings? In my opinion, using the warning mechanism as a way 
to communicate the differences between the classes is an abuse of 
warnings: they're not *warnings*, they are *diagnostic information*.

It is also fragile: the caller may have filtered warnings, and will not 
see the messages you generate.

Lastly, I do not understand the purpose of this "wheezy.core" package. 
Does it have something to do with Ubuntu Wheezy? The documentation is 
unclear -- it refers to it as a "Python package" that "provides core 
features", but doesn't say what the purpose of the package is: core 
features of *what*? And the examples.rst file doesn't show any examples.


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