Python List NoneType Error - nonetype

Question

I'm having an issue with an OpenCourseWare project I'm trying to implement. I'm creating a hangman game and everything seems to be okay until I attempt to run the actual function for playing the game.
I'm receiving the following error:
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "Game of Hangman/hangman.py", line 118, in play_hangman
print(print_guessed())
File "Game of Hangman/hangman.py", line 92, in print_guessed
if letter in letters_guessed == True:
TypeError: argument of type 'NoneType' is not iterable
I can't seem to understand why the list is evaluating to a NoneType even though it is declared as an empty list. I've used the console to attempt to find the answer and says the type is NoneType. Could someone help me please? I've provided the code as a reference.
# Name:
# Section:
# 6.189 Project 1: Hangman template
# hangman_template.py
# Import statements: DO NOT delete these! DO NOT write code above this!
from random import randrange
from string import *
# -----------------------------------
# Helper code
# (you don't need to understand this helper code)
# Import hangman words
WORDLIST_FILENAME = "words.txt"
def load_words():
"""
Returns a list of valid words. Words are strings of lowercase letters.
Depending on the size of the word list, this function may
take a while to finish.
"""
print "Loading word list from file..."
# inFile: file
inFile = open(WORDLIST_FILENAME, 'r', 0)
# line: string
line = inFile.readline()
# wordlist: list of strings
wordlist = split(line)
print " ", len(wordlist), "words loaded."
print 'Enter play_hangman() to play a game of hangman!'
return wordlist
# actually load the dictionary of words and point to it with
# the words_dict variable so that it can be accessed from anywhere
# in the program
words_dict = load_words()
# Run get_word() within your program to generate a random secret word
# by using a line like this within your program:
# secret_word = get_word()
def get_word():
"""
Returns a random word from the word list
"""
word=words_dict[randrange(0,len(words_dict))]
return word
# end of helper code
# -----------------------------------
# CONSTANTS
MAX_GUESSES = 6
# GLOBAL VARIABLES
secret_word = get_word()
letters_guessed = []
# From part 3b:
def word_guessed():
'''
Returns True if the player has successfully guessed the word,
and False otherwise.
'''
global secret_word
global letters_guessed
wordGuessed = False
####### YOUR CODE HERE ######
for letter in secret_word:
if letter in letters_guessed:
wordGuessed = True
else:
wordGuessed = False
break
return wordGuessed
def print_guessed():
'''
Prints out the characters you have guessed in the secret word so far
'''
global secret_word
global letters_guessed
printedWord = []
####### YOUR CODE HERE ######
for letter in secret_word:
if letter in letters_guessed:
printedWord.append(letter)
else:
printedWord.append("-")
return printedWord
def play_hangman():
# Actually play the hangman game
global secret_word
global letters_guessed
# Put the mistakes_made variable here, since you'll only use it in this function
mistakes_made = 0
# Update secret_word. Don't uncomment this line until you get to Step 8.
secret_word = get_word()
####### YOUR CODE HERE ######
while mistakes_made < MAX_GUESSES or word_guessed() == False:
print("WORD:")
userGuess = raw_input("What is your guess?\n").lower()
if userGuess in secret_word:
letters_guessed = letters_guessed.append(userGuess)
else:
letters_guessed = letters_guessed.append(userGuess)
mistakes_made += 1
print(print_guessed())
if word_guessed() == False:
print("Sorry but you've run out of guesses...")
else:
print("You've correctly guessed the secret word!")
print("Secret Word: " + secret_word)
Just as a disclaimer, this isn't an assignment in the sense I'm not enrolled in school. I'm just a guy trying to get back in to programming and found some assignments to play with.

Answer 1

Looks like I've found my answer. The problem appeared to be with the assignment I was doing with the letters_guessed variable.
Instead of doing:
letters_guessed = letters_guessed.append(userGuess)
I should have done:
letters_guessed.append(userGuess)