[Python-ideas] Default arguments in Python - the return - running out of ideas but...

[Stephen J. Turnbull]

Chris Rebert writes:

 > +1 on throwing a ValueError for non-hash()-able (and thus probably
 > mutable) default argument values. It's by no means perfect since
 > objects are hash()-able by default using their ID, but it would at
 > least help in the frequent "well-behaved mutable container object"
 > cases.

-1

nonet_default_options = { 'dryrun': False, 'verbose': False }
net_default_options = { 'dryrun': True, 'verbose': True }

def command1(options=nonet_default_options):
    pass

def command2(options=net_default_options):
    pass

def command3(options=net_default_options):
    pass

>>> from mystuff import command1, command2, net_default_options
>>> command1()
>>> command2()
>>> command3()
>>> net_default_options['dryrun'] = False
>>> command2()
>>> command3()

is a common use-case for me, both during development and in scripts
for occasional personal use.  AFAICS this would break under your
suggestion.

Really, the only commonly-encountered problematic cases I can think of
are the anonymous empty objects, because they're typically used not
for their contents (d'oh), but rather as containers.  pylint and
friends can easily detect [] and {} as default values for arguments.

 > The barrier to this idea would be the code breakage involved; IMHO,
 > code exploiting mutable defaults as static variables is in poor style
 > anyway,

Sure, but it also breaks "globals" as above.