How to correct TypeError NoneType object is not subscriptable in recursive function

Question

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   lt pyshell 41 gt    line 1  in  lt module gt      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

User · Answer

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

User · Answer

This simply means that either tree  tree otu   or tree otu  0  evaluates to None  and as such is not subscriptable  Most likely tree otu  or tree otu  0   Track it down with some simple debugging like this   def Ancestors  otu tree       try          tree otu  0  0      except TypeError          print otu  tre otu          raise      etc      or pdb

User · Answer

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

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

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