What value should be passed to make a function use the default argument value?

Nick Craig-Wood nick at craig-wood.com
Wed Oct 4 12:30:04 CEST 2006

LaundroMat <Laundro at gmail.com> wrote:
>  I have in fact a bunch of functions that all pass similar information
>  to one main function. That function takes (amongst others) a template
>  variable. If it's not being passed, it is set to a default value by the
>  function called upon.
>  For the moment, whenever a function calls the main function, I check
>  whether the calling function has the template variable set:
> >>> if template:
> >>>    return mainFunction(var, template)
> >>> else:
> >>>    return mainFunction(var)
>  Now, I thought this isn't the cleanest way to do things; so I was
>  looking for ways to initialize the template variable, so that I could
>  always return mainFunction(var, template). mainFunction() would then
>  assign the default value to template.
> From your answers, this seems to be impossible. The minute my variable
>  is initialised, there's no way I can have mainFunction() assign a value
>  without explicitly asking it to do so.
>  I guess the best way would then be to change mainFunction from:
> >>> def mainFunction(var, template='base'):
>  to
> >>> def mainFunction(var, template):
> >>> if len(template)=0:
> >>>    template = 'base'
>  and have the calling functions call mainFunction (var, template) and
>  initialise template to ''.

None is the traditional value to use for value not present, then you'd
get this for the function

  def mainFunction(var, template=None):
    if template is None:
        template = 'base'

And this for the calling bit

  if not_set_properly(template):
     template = None
  return mainFunction(var, template)

Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick

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