recursive function: use a global or pass a parameter?

Chris Angelico rosuav at
Fri Jan 16 18:56:08 CET 2015

On Sat, Jan 17, 2015 at 4:49 AM, Tim <jtim.arnold at> wrote:
> I want to get a union of all the values that any 'things' key may have, even in a nested dictionary (and I do not know beforehand how deep the nesting might go):
> d = {'things':1, 'two':{'things':2}}
> def walk(obj, res):
>     if not hasattr(obj, 'keys'):
>         return set(), set()
>     if 'things' in obj:
>         res.add(obj['things'])
>     for k in obj:
>         walk(obj[k], res)
>     return res
> walk(d, set()) # returns {1, 2}
> Is it better to use a global to keep track of the values or does it even matter?

I would use a parameter rather than a global, but I'd make the
parameter optional:

def all_keys(obj, accum=None):
    if accum is None: accum=set()
    if 'things' in obj:
    for val in obj.values():
        all_keys(val, accum)
    return all_keys


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