list returned from recursive function always empty

Question

Im trying to return a list from a recursive function, appending to this list for each depth, but the final returned list is always empty. I'm not very experienced pythonian so it might be a trivial mistake.

Here's the code:

import pymel.core as pm

def getParent(jnt):
    something = pm.listRelatives(jnt, parent=True);
    if something:
        print 'got parent: ' + something[0]
        return something[0]
    else:
        return None

def getAllParents(jnt):
    parents = []
    parents.append(jnt)
    prnt = getParent(jnt)
    if prnt == None:
        return parents
    else:
        prnts = parents.insert(0, getAllParents(prnt))
        return prnts


selection = pm.ls(sl=True)[0]

parents = getAllParents(selection)

print '\n'
print parents

pm.listRelatives(jnt, parent=True); returns a list of strings, of which i grab the first if it is not empty.

Here's the output:

got parent: joint3
got parent: joint2
got parent: joint1


None

Any help appreciated.

@Tanmayjain no difference. no, `parents[] = []` is incorrect, typo. (it's not the issue) — Elias Finoli, Aug 17 '18 at 07:30
@MarcinOrlowski `prnts = parents.insert(0, getAllParents(prnt))` — Elias Finoli, Aug 17 '18 at 07:32
`parents [] = []` is an issue. This code does not return an empty list, because it has a syntax error. So what does your code actually look like? Please fix the question. — zvone, Aug 17 '18 at 07:32
please, see the comment from @MarcinOrlowski. is it a typo or not ? — Massimo Costa, Aug 17 '18 at 07:34
Please see [Why is “Can someone help me?” not an actual question?](https://meta.stackoverflow.com/questions/284236/why-is-can-someone-help-me-not-an-actual-question) — glennsl, Aug 17 '18 at 10:48

score 2 · Accepted Answer · answered Aug 17 '18 at 07:35

2

Here:

def getAllParents(jnt):
    # ...
    prnts = parents.insert(0, getAllParents(prnt))
    return prnts

list.insert() (as well as all methods that modify a list in place - sort() etc) returns None. You want:

    parents.insert(0, getAllParents(prnt))
    return parents

instead.

answered Aug 17 '18 at 07:35

bruno desthuilliers

75,974
6
88
118

This was indeed the issue! Thank you. – Elias Finoli Aug 17 '18 at 07:41

score 1 · Answer 2 · answered Aug 17 '18 at 07:40

Your getAllParents method takes a string and returns a list. Then you insert this list into another list which makes it a list of strings and lists of strings and lists...

To concatenate two lists you can simply use + operator, so:

prnts = getAllParents(prnt) + parents

score 0 · Answer 3 · answered Aug 17 '18 at 07:33

0

Keep in mind that you're using list functions like append, insert passing list. In this case you'll end-up to weird results. For instance

a = [1, 2, 3]
b = [4, 5]
print(a.append(b))

>> [1, 2, 3, [4, 5]]

answered Aug 17 '18 at 07:33

Massimo Costa

1,864
15
23

score 0 · Answer 4 · answered Aug 17 '18 at 07:55

I don't know python but I guess the following code works fine

listPrts = ["","toto","idiot","happy"]

def getParent(jnt):
    something = listPrts
    del listPrts[0]
    if something:
        print 'got parent: ' + something[0]
        return something[0]
    else:
        return None

def getAllParents(jnt):
    parents = []
    parents.append(jnt)
    prnt = getParent(jnt)
        if prnt == None:
            return parents
        else:
            prnts = parents.insert(0, getAllParents(prnt))
            return parents


selection = "childof"

parents = getAllParents(selection)

print '\n'
print parents

list returned from recursive function always empty

4 Answers4