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Message from discussion Default arguments in Python - the return - running out of ideas but...
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Terry Reedy  
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 More options May 13 2009, 7:02 pm
From: Terry Reedy <tjre...@udel.edu>
Date: Wed, 13 May 2009 19:02:08 -0400
Local: Wed, May 13 2009 7:02 pm
Subject: Re: [Python-ideas] Default arguments in Python - the return - running out of ideas but...
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Steven D'Aprano wrote:
> On Thu, 14 May 2009 05:18:37 am CTO wrote:

>> If you thought not reevaluating function expressions
>> was confusing for newbies, wait until you see what making up a new
>> kind of yield will do for them.

>> Why not just push for some decorators that do this to be included in
>> stdlib? I see the utility, but not the point of adding extra syntax.

> Even if a decorator solution can be made to work, it seems to me that
> the difficulty with a decorator solution is that it is
> all-or-nothing -- you can decorate the entire parameter list, or none
> of the parameters, but not some of the parameters. You can bet that
> people will say they want delayed evaluation of some default arguments
> and compile-time evaluation of others, in the same function definition.

Not all or nothing, and selection is easy.  A decorator could only call
callable objects, and could/should be limited to calling function
objects or even function objects named '<lambda>'.  And if one wanted
the resulting value to such a function, escape the default lambda
expression with lambda.

x=[1,2]
@call_lambdas
def f(a=len(x), lst = lambda:[], func = lambda: lambda x: 2*x):
   # a is int 2, lst is a fresh list, func is a one-parameter function

Terry Jan Reedy

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