recursive function: use a global or pass a parameter?

[Peter Otten]

Tim wrote:

> I have this type of situation and wonder if I should use a global variable
> outside the recursive function instead of passing the updated parameter
> through.
> 
> 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?

Globals are generally bad as they make code non-reentrant; when two calls of 
the function run simultaneously the data will be messed up.

I recommend that you use a generator:

>>> def walk(obj):
...     if not hasattr(obj, "keys"):
...         return
...     if "things" in obj:
...         yield obj["things"]
...     for v in obj.values():
...         yield from walk(v)
... 
>>> d = {'things':1, 'two':{'things':2}}
>>> set(walk(d))
{1, 2}

In Python before 3.3 you have to replace

yield from walk(v)

with a loop:

for t in walk(v):
    yield t