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Re: When do default parameters get their values set?

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  To: bSneddon
Copy: python-list@python.org (python-list@python.org)

On Tue, Dec 9, 2014 at 9:10 AM, bSneddon <w.g.sneddon@gmail.com> wrote:
> I ran into an issue setting variables from a GUI module that imports a back 
end module.  My approach was wrong obviously but what is the best way to set 
values in a back end module.
>
> #module name beTest.py
>
> cfg = { 'def' : 'blue'}
>
> def printDef(argT = cfg['def']):
>         print argT

They're set when you define the function, and become attributes of the 
function.

If you want to lazily fetch the defaults, here's one common idiom:

def printDef(argT=None):
    """Print the argT value, defaults to cfg['def']"""
    if argT is None: argT = cfg['def']
    print(argT)

This depends on None not being a meaningful argument value, of course.
If you need to have any object at all able to be passed in, you'd need
to create a dedicated sentinel object, or use *args and unpack
yourself; but for a lot of cases, None works fine.

ChrisA

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Thread

When do default parameters get their values set? "bSneddon" <bsneddon@1:249/999.remove-cdt-this> - 2014-12-09 11:32 -0500
  Re: When do default parameters get their values set? "Chris Angelico" <chris.angelico@1:249/999.remove-cdt-this> - 2014-12-09 11:32 -0500
  Re: When do default parameters get their values set? "Dave Angel" <dave.angel@1:249/999.remove-cdt-this> - 2014-12-09 11:32 -0500
    Re: When do default parameters get their values set? Duncan Booth <duncan.booth@invalid.invalid> - 2014-12-09 21:18 +0000
      Re: When do default parameters get their values set? random832@fastmail.us - 2014-12-09 18:07 -0500
        Re: When do default parameters get their values set? Rustom Mody <rustompmody@gmail.com> - 2014-12-09 18:44 -0800
          Re: When do default parameters get their values set? random832@fastmail.us - 2014-12-10 13:38 -0500
            Re: When do default parameters get their values set? Rustom Mody <rustompmody@gmail.com> - 2014-12-10 18:18 -0800
              Re: When do default parameters get their values set? Chris Angelico <rosuav@gmail.com> - 2014-12-11 13:24 +1100
                Re: When do default parameters get their values set? Rustom Mody <rustompmody@gmail.com> - 2014-12-10 18:50 -0800
              Re: When do default parameters get their values set? Steven D'Aprano <steve@pearwood.info> - 2014-12-11 03:15 +0000
                Re: When do default parameters get their values set? Rustom Mody <rustompmody@gmail.com> - 2014-12-10 19:26 -0800
                Re: When do default parameters get their values set? Ben Finney <ben+python@benfinney.id.au> - 2014-12-11 14:49 +1100
                Re: When do default parameters get their values set? Chris Kaynor <ckaynor@zindagigames.com> - 2014-12-11 09:10 -0800
                Re: When do default parameters get their values set? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-12-12 09:51 +1100
              Re: When do default parameters get their values set? random832@fastmail.us - 2014-12-11 15:44 -0500
  Re: When do default parameters get their values set? "Tim Chase" <tim.chase@1:249/999.remove-cdt-this> - 2014-12-09 11:32 -0500

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