Question

I’m writing a function that parses strings into lists that get used by another function. One of the operations that it performs is that it attaches a character to a string inside the (sometimes deeply recursive) list at a particular depth of recursion (the depth defined by a variable named lvl). This operation, a function named listSurgery that is supposed to get called with a list of indices that indicate where the next list is inside the previous list, with the final index telling what index within the deep list to perform an operation at, is getting called with a blank list of indices, and I don’t know why. The list that it should be getting called with is [-1], but debugging shows that it’s getting called with []. Here is the code, abbreviated:

def listAssign(lst,index,item):
    """Assigns item item to list lst at index index, returns modified list."""
    lst[index] = item
    return lst

def listInsert(lst,index,item):
    """Inserts item item to list lst at index index, returns modified list."""
    print "listInsert just got called with these arguments:",lst,index,item
    if index == 'end':
        index = len(lst)
    lst.insert(index,item)
    return lst

def listSurgery(lst,indices,f,*extraArgs):
    """Performs operation f on list lst at depth at indices indices, returns modified list."""
    print "listSurgery just got called with these arguments:",lst,indices,f,extraArgs
    parent = lst
    for index in indices[:-1]:
        parent = parent[index]
    parent = f(parent,indices[-1],*extraArgs)
    return listSurgery(lst,indices[:-1],listAssign,parent)

def parseStringToList(s):
    """Takes in a user-input string, and converts it into a list to be passed into parseListToExpr."""
    # ...
    l = [] # List to build from string; built by serially appending stuff as it comes up
    b = True # Bool for whether the parser is experiencing spaces (supposed to be True if last character processed was a space)
    t = False # Bool for whether the parser is experiencing a string of non-alphanumeric characters (supposed to be True if last character was a non-alphanumeric character)
    lvl = 0 # Keeps track of depth at which operations are supposed to be occurring
    for c in s:
        if c == ' ': # If c is a space, ignore it but send signal to break off any strings currently being written to
            b = True
        # Some elifs for c being non alphanumeric
        else: # If c is alphanumeric, append it to the string it's working on
            print c,"got passed as an alphanumeric; lvl is",lvl
            assert c.isalnum()
            if b or t: # If the string it's working on isn't alphanumeric or doesn't exist, append a new string
                l = listSurgery(l,[-1]*lvl + ['end'],listInsert,'')
                b, t = False, False
            l = listSurgery(l,[-1]*(lvl+1),lambda x,y,z:listAssign(x,y,x[y]+z),c)
        print l
    return l

while op != 'exit' and op != 'quit': # Keep a REPL unless the user types "exit" or "quit", in which case exit
    op = raw_input("> ")
    if op == 'help':
        pass # Print help stuff
    elif op in {'quit','exit'}:
        pass
    else:
        print str(parseStringToList(op))

I called the code with python -tt code.py and typed 1+1=2, and this is what I got:

> 1+1=2
1 got passed as an alphanumeric; lvl is 0
listSurgery just got called with these arguments: [] ['end'] <function listInsert at 0x10e9d16e0> ('',)
listInsert just got called with these arguments: [] end 
listSurgery just got called with these arguments: [''] [] <function listAssign at 0x10e9d10c8> ([''],)
Traceback (most recent call last):
  File "analysis.py", line 276, in <module>
    print str(parseStringToList(op))
  File "analysis.py", line 218, in parseStringToList
    l = listSurgery(l,[-1]*lvl + ['end'],listInsert,'')
  File "analysis.py", line 63, in listSurgery
    return listSurgery(lst,indices[:-1],listAssign,parent)
  File "analysis.py", line 62, in listSurgery
    parent = f(parent,indices[-1],*extraArgs)
IndexError: list index out of range

Can anyone explain this? Why is listSurgery getting [] instead of [-1]? lvl is 0, and the argument that’s supposed to be passed at that point is [-1]*(lvl+1). Never even mind why it’s getting called with [''] instead of '1'.

Answer 1

Your lvl is 0, so [-1]*lvl + ['end'] is ['end'], and indices[:-1] is ['end'][:-1]. Now [‘end’] is a list of length 1, so ['end'][:-1] is the same thing as ['end'][:1-1], which is the same thing as ['end'][:0]. This evaluates to empty list.