Question

I have a program that, simplified, looks like this:

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."""
    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."""
    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."""
    for c in s[:]: # Removes extra spaces from beginning of string
        if c == ' ':
            s = s[1:]
        else:
            break
    for c in s[::-1]: # Removes spaces from end of string
        if c == ' ':
            s = s[:-1]
        else:
            break
    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 the character is a space, do nothing, but make sure that it's documented that the last character processed was a space
            b = True
        elif c == '(' or c == '[': # If the character is an opening parenthesis, add a new level of depth to the list, by appending another list to the lowest level and incrementing lvl
            l = listSurgery(l,[-1]*(lvl+1),listInsert,[])
            lvl += 1
            if c == '[': # ] (left here for overzealous parsers) If the bracket is square, also append a comma as the first element of the appended list
                l = listSurgery(l,[-1]*(lvl+1),listInsert,',')
        elif c == ')' or c == ']': # If it's a closing parenthesis, make it stop paying attention to the list it's working on
            lvl -= 1
        elif c == ',': # If it's a comma, then append it to the list as a separate element and start a new string to append characters to
            l = listSurgery(l,[-1]*(lvl+1),lambda x,y,z:listAssign(x,y,x[y]+z),[',',''])
        elif not c.alnum(): # If it's non-alphanumeric and not any of the above, then append it to string that it's working on if it's a non-alphanumeric string
            if not t: # If the string that it's working on isn't non-alphanumeric, then finish working on that string and append a new string to work on
                l = listSurgery(l,[-1]*(lvl+1),listInsert,'')
            l = listSurgery(l,[-1]*(lvl+1),lambda x,y,z:listAssign(x,y,x[y]+z),c)
            t = True
        else: # If the character is alphanumeric, append it to the string it's working on
            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+1),listInsert,'')
                b, t = False, False
            l = listSurgery(l,[-1]*(lvl+1),lambda x,y,z:listAssign(x,y,x[y]+z),c)
    return l

# ...

cmds = {
        'help':"List the available functions.",
        'quit':"Quit this application.",
        'exit':"Exit this application (exactly the same thing)."
        }

print "MAT (Michael's Analysis Tool), version 0.0.0"

op = '' # Declare string to use for input
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':
        print "The commands available in addition to free evaluation are:"
        for item in cmds:
            print ' ', item + ":", cmds[item]
    elif op == 'quit' or 'exit':
        pass
    else:
        print str(parseStringToList(op))

When I use help, quit, or exit as a command, it comes out fine. But when I give it any other kind of input like 1 + 1 = 2 (which should elicit ['1','+','1','=','2']) or This is a sentence. (which should elicit ['This','is','a','sentence','.'], it doesn’t print anything, just asks for more input. (That is, it’s functionally equivalent to the last line of this program getting replaced with continue.) And I don’t understand why. parseStringToList is defined in a very straightforward manner. It should at least return [], the original value of the variable it returns. But instead it doesn’t print anything. Why? (I’ll try more thorough debugging tomorrow, but so far it’s eluded my efforts.)

Answer 1

You need to change this:

elif op == 'quit' or 'exit':

to this

elif op == 'quit' or op == 'exit':