[python] How to correct "TypeError: 'NoneType' object is not subscriptable" in recursive function?

def Ancestors (otu,tree):
    if tree[otu][0][0] == None:
       return []
    else:
        return [otu,tree[otu][0][0]] + Ancestors (tree[otu][0][0],tree)

The problem essentially is that at some point, the function tries to call a something which is None, this happens instead of the function returning the list that I want. I thought the if statement had accounted for that, but it would seem I was wrong. Any advice?

Traceback (most recent call last):
  File "<pyshell#41>", line 1, in <module>
    Ancestors('A',a)
  File "C:\x.py", line 129, in Ancestors
    return [otu,tree[otu][0][0]] + Ancestors (tree[otu][0][0],tree)
  File "C:\x.py", line 129, in Ancestors
    return [otu,tree[otu][0][0]] + Ancestors (tree[otu][0][0],tree)
  File "C:\x.py", line 129, in Ancestors
    return [otu,tree[otu][0][0]] + Ancestors (tree[otu][0][0],tree)
  File "C:\x.py", line 129, in Ancestors
    return [otu,tree[otu][0][0]] + Ancestors (tree[otu][0][0],tree)
  File "C:\x.py", line 126, in Ancestors
    if tree[otu][0][0] == None:
TypeError: 'NoneType' object is not subscriptable

This is what tree is

{'A': [('AD', 4.0), None, None], 'C': [('ADBFGC', 14.5), None, None], 'B': [('BF', 0.5), None, None], 'E': [('ADBFGCE', 17.0), None, None], 'D': [('AD', 4.0), None, None], 'G': [('BFG', 6.25), None, None], 'F': [('BF', 0.5), None, None], 'ADBFG': [('ADBFGC', 6.25), ('AD', 4.25), ('BFG', 2.0)], 'BF': [('BFG', 5.75), ('B', 0.5), ('F', 0.5)], 'ADBFGC': [('ADBFGCE', 2.5), ('ADBFG', 6.25), ('C', 14.5)], 'ADBFGCE': [None, ('ADBFGC', 2.5), ('E', 17.0)], 'BFG': [('ADBFG', 2.0), ('BF', 5.75), ('G', 6.25)], 'AD': [('ADBFG', 4.25), ('A', 4.0), ('D', 4.0)]}

with otu referring to any of the strings in the tree.

What is a when you call Ancestors('A',a)? If a['A'] is None, or if a['A'][0] is None, you'd receive that exception.

One of the values you pass on to Ancestors becomes None at some point, it says, so check if otu, tree, tree[otu] or tree[otu][0] are None in the beginning of the function instead of only checking tree[otu][0][0] == None. But perhaps you should reconsider your path of action and the datatype in question to see if you could improve the structure somewhat.