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Date: Mon, 13 Jun 2011 16:43:11 +0100
From: MRAB <python@mrabarnett.plus.com>
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To: python-list@python.org
Subject: Re: [Python-ideas] 'Injecting' objects as function-local constants
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On 13/06/2011 15:33, Steven D'Aprano wrote:
> Nick Coghlan wrote:
>> On Mon, Jun 13, 2011 at 5:11 PM, Steven D'Aprano <steve@pearwood.info>
>> wrote:
>>> Function parameters should be kept for actual arguments, not for
>>> optimizing
>>> name look-ups.
>>
>> Still, the post-** shared state (Jan's option 2) is likely the most
>> obvious way to get early binding for *any* purpose without polluting
>> the externally visible parameter list.
>
> I wouldn't call adding even more complexity to function signatures
> "obvious", although I grant that it depends on whether you're Dutch :)
>
> Another disadvantage is that it uses a symbol instead of a word. Too
> many symbols, and your code looks like Perl (or APL). It's hard to
> google for ** to find out what it means. It's harder to talk about a
> symbol than a word. (In written text you can just write ** but in speech
> you have to use circumlocutions or made-up names like double-splat.)
>
> [...]
>> It seems like the path of least resistance to me - the prevalence of
>> the default argument hack means there's an existing, widespread
>> practice that solves real programming issues, but is flawed in some
>> ways (specifically, messing with the function's signature). Allowing
>> declarations of shared state after the keyword-only arguments seems
>> like a fairly obvious answer.
>
> The problem with injecting locals in the parameter list is that it can
> only happen at write-time. That's useful, but there's a major
> opportunity being missed: to be able to inject at runtime. You could add
> test mocks, optimized functions, logging, turn global variables into
> local constants, and probably things I've never thought of.
>
> Here's one use-case to give a flavour of what I have in mind: if you're
> writing Unix-like scripts, one piece of useful functionality is "verbose
> mode". Here's one way of doing so:
>
> def do_work(args, verbose=False):
>     if verbose:
>         pr = print
>     else:
>         pr = lambda *args: None
>     pr("doing spam")
>     spam()
>     pr("doing ham")
>     ham()
>     # and so on
>
> if __name__ == '__main__':
>     verbose = '--verbose' in sys.argv
>     do_work(my_arguments, verbose)
>
[snip]
Here's another way:

def do_work(args):
     do_work.pr("doing spam")
     spam()
     do_work.pr("doing ham")
     ham()
     # and so on

if __name__ == '__main__':
     verbose = '--verbose' in sys.argv
     if '--verbose' in sys.argv:
         do_work.pr = print
     else:
         do_work.pr = lambda func: func  # do nothing
     do_work(my_arguments)