[Python-3000] Draft pre-PEP: function annotations

Phillip J. Eby pje at telecommunity.com
Sun Aug 13 08:06:50 CEST 2006

At 01:29 AM 8/13/2006 -0400, Jim Jewett wrote:
>On 8/13/06, Josiah Carlson <jcarlson at uci.edu> wrote:
>>"Phillip J. Eby" <pje at telecommunity.com> wrote:
>> > However, if you have:
>> >     def myfunc( x : doc("The x coordinate"), y : doc("The y coordinate") )
>> > There is no ambiguity.
>Sure there is.  There will probably be several frameworks using the
>magic name "doc".
>This isn't a problem for the person writing myfunc, and therefore
>isn't a problem for immediate decorators.  It is a problem for
>inspection code that wants to present information about arbitrary
>3rd-party libraries.

By this argument, we shouldn't have metaclasses or function attributes, 
because they have the same "problem".

However, it's only a problem if you insist on writing brain-damaged 
code.  If you want interoperability here, you must write tell-don't-ask 
code.  This is true for *any* use case where frameworks might share 
objects; there is absolutely *nothing* special about annotations in this 

I'm really baffled by the controversy over this; is it really the case that 
so many people don't know what tell-don't-ask code is or why you want 
it?  I guess maybe it's something that's only grasped by people who have 
experience writing code intended for interoperability.

After you run into the issue a few times, you look for a solution, and end 
up with either duck typing, interfaces/adaptation, overloaded functions, or 
ad hoc registries.  ALL of these solutions are *more* than adequate to 
handle a simple thing like argument annotations.  That's why I keep 
describing this as a trivial thing: even *pickling* is more complicated 
than this is.  This is no more complex than len() or iter() or filter()!

However, it appears that mine is a minority opinion.  Unfortunately, I'm at 
a bit of a communication disadvantage, because if somebody wants to believe 
something is complicated, there is nothing that anybody can do to change 
their mind.  If you don't consider the possibility that it is way simpler 
than you think, you will never be able to see it.

The other possibility, of course, is that all of you have some horrendously 
complex use case in mind that I just don't "get".  But so far all the 
examples that anybody else has put forth have been practically whimsical in 
their triviality -- while I've been explaining how the same principles will 
even work for complex things like type-checking code generation, let alone 
the trivial examples.  So I don't think that's it.  And at least Paul and 
Josiah have shown that they "get" what I'm saying, so I don't think that 
the answer is simply that I'm crazy, either.

[Meanwhile, I'm not going to respond to the rest of your message, since it 
contained some things that appeared to me to be a mixture of ad hominem 
attack and straw man argument.  I hope that was not actually your intent.]

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