i = self._randbelow(len(seq)) TypeError: object of type 'NoneType' has no len()

Question

I have this error when I'm running my code for building a perfect maze. Here is the code:

   def walk(self, s, x, y):

        neighboor = [(x + 1, y), (x - 1, y), (x, y + 1), (x, y - 1)]

        if s.size() == self.size**2: return

        else:

            while True:
                new = choice(neighboor)
                if self.is_valid(new[0], new[1]): break

            while self.maze[new[0]][new[1]].getVisit():

                if len(neighboor) != 0: new = choice(neighboor.remove(new))
                else:
                    temp = s.pop(s)
                    self.walk(s, temp[0], temp[1])

            #print(new)

            self.maze[new[0]][new[1]].changeVisit()
            if new == neighboor[1]:
                self.maze[x][y].changeNorth()
                self.maze[new[0]][new[1]].changeSouth()
            elif new == neighboor[0]:
                self.maze[x][y].changeSouth()
                self.maze[new[0]][new[1]].changeNorth()
            elif new == neighboor[2]:
                self.maze[x][y].changeEast()
                self.maze[new[0]][new[1]].changeWest()
            elif new == neighboor[3]:
                self.maze[x][y].changeWest()
                self.maze[new[0]][new[1]].changeEast()


            s.push(new)

            self.walk(s, new[0], new[1])

Here is the Error I got:

Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    a.search()
  File "C:\Users\Serena\Desktop\XAproject\Stack.py", line 93, in search
    self.walk(s, startX, startY)
  File "C:\Users\Serena\Desktop\XAproject\Stack.py", line 76, in walk
    self.walk(s, new[0], new[1])
  File "C:\Users\Serena\Desktop\XAproject\Stack.py", line 76, in walk
    self.walk(s, new[0], new[1])
  File "C:\Users\Serena\Desktop\XAproject\Stack.py", line 76, in walk
    self.walk(s, new[0], new[1])
  File "C:\Users\Serena\Desktop\XAproject\Stack.py", line 76, in walk
    self.walk(s, new[0], new[1])
  File "C:\Users\Serena\Desktop\XAproject\Stack.py", line 76, in walk
    self.walk(s, new[0], new[1])
  File "C:\Users\Serena\Desktop\XAproject\Stack.py", line 76, in walk
    self.walk(s, new[0], new[1])
  File "C:\Users\Serena\Desktop\XAproject\Stack.py", line 52, in walk
    if len(neighboor) != 0: new = choice(neighboor.remove(new))
  File "C:\Python34\lib\random.py", line 253, in choice
    i = self._randbelow(len(seq))
TypeError: object of type 'NoneType' has no len()

I think 'neighboor' is a list contains four sets of number, it should have len().

I'm new to programming, any help would be appreciate.

Aside - The spelling of neighboor here is actually [neighbor or neighbour](http://simple.wikipedia.org/wiki/Neighbour) — Bhargav Rao, Apr 05 '15 at 04:38

score 5 · Accepted Answer · edited May 23 '17 at 10:27

Couple of things. First, if you're using something like choice() that's imported from a wildcard import (from random import *), it would be useful to include that, because otherwise we're just guessing where you got that function from. Also, wildcard imports are considered bad practice anyway.

The problem is that the remove() method of a list returns None. You cannot choose something from None, because None is not an iterable. That is to say that the remove() method does not support chaining. Try changing:

if len(neighboor) != 0: new = choice(neighboor.remove(new)) # Passes None to choice() which raises an error

to

if len(neighboor) != 0:
    neighboor.remove(new) # removes the element from neighboor
    new = choice(neighboor) # Chooses from the list as you intended

You could have other errors, but that's the one that came from the traceback you posted.

For the future, I recommend familiarizing yourself with Python's traceback, because it is telling you exactly what is going wrong. Here's a resource.

As an aside, do you mean neighbor instead of neighboor?

Being picky, but it `choice` doesn't have to be from a wildcard import, it could have been imported with `from random import choice`. — mhawke, Apr 05 '15 at 04:45
Fair enough. I don't intend to edit it though, as I like to share that wildcard imports should generally be avoided and I still think the point about it being more difficult for answers to see what's going on is valid. — Gustav, Apr 05 '15 at 04:52
I also use other function from random, not only choice. and it's my first year in CS, I have no idea that wildcard import should be avoid. Thx for telling me and will improve it. — Serena Qi, Apr 05 '15 at 05:06

i = self._randbelow(len(seq)) TypeError: object of type 'NoneType' has no len()

1 Answers1