When do default parameters get their values set?

[Tim Chase]

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

On 2014-12-08 14:10, bSneddon 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']):

At this point (after the "def" has completed defining the function),
the expression is evaluated and assigned to the default argument.

> beTest.cfg['def'] = "no red"
> beTest.printDef()
>
> This prints blue.      I suppose because I am changing a local copy
> of cfg dictionary.  What is the write approach here?

Well, you can bind to a default dictionary rather than an entry in
that dictionary:


  cfg = {'def': 'blue'}
  def printDef(config=cfg):
    ...
    access(config['def'])

which will make the look-up happen at run-time rather than
definition/bind-time.

  beTest.cfg['def'] = 'Red!'
  beTest.printDef()
  # should print "Red!"

-tkc

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