When do default parameters get their values set?

[Chris Angelico]

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

On Tue, Dec 9, 2014 at 9:10 AM, bSneddon <w.g.sneddon at 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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