Correct output:

Welcome to the game Hangman!
I am thinking of a word that is 7 letters long
-----------
You have 8 guesses left
Available Letters: abcdefghijklmnopqrstuvwxyz
Please guess a letter: A
Good guess: _ _ _ _ _ a_ 
-----------
You have 8 guesses left
Available Letters: bcdefghijklmnopqrstuvwxyz
Please guess a letter: e
Good guess: _ _ ee_ a_ 
-----------
You have 8 guesses left
Available Letters: bcdfghijklmnopqrstuvwxyz
Please guess a letter: z
Oops! That letter is not in my word: _ _ ee_ a_ 
-----------
You have 7 guesses left
Available Letters: bcdfghijklmnopqrstuvwxy
Please guess a letter: t
Good guess: _ _ eeta_ 
-----------
You have 7 guesses left
Available Letters: bcdfghijklmnopqrsuvwxy
Please guess a letter: a
Oops! You've already guessed that letter: _ _ eeta_ 
-----------
You have 7 guesses left
Available Letters: bcdfghijklmnopqrsuvwxy
Please guess a letter: z
Oops! You've already guessed that letter: _ _ eeta_ 
-----------
You have 7 guesses left
Available Letters: bcdfghijklmnopqrsuvwxy
Please guess a letter: E
Oops! You've already guessed that letter: _ _ eeta_ 
-----------
You have 7 guesses left
Available Letters: bcdfghijklmnopqrsuvwxy
Please guess a letter: r
Oops! That letter is not in my word: _ _ eeta_ 
-----------
You have 6 guesses left
Available Letters: bcdfghijklmnopqsuvwxy
Please guess a letter: t
Oops! You've already guessed that letter: _ _ eeta_ 
-----------
You have 6 guesses left
Available Letters: bcdfghijklmnopqsuvwxy
Please guess a letter: h
Good guess: _ heetah
-----------
You have 6 guesses left
Available Letters: bcdfgijklmnopqsuvwxy
Please guess a letter: c
Good guess: cheetah
-----------
Congratulations, you won!
None

hangman.py

def hangman(secretWord):
    '''
    secretWord: string, the secret word to guess.

    Starts up an interactive game of Hangman.

    * At the start of the game, let the user know how many 
      letters the secretWord contains.

    * Ask the user to supply one guess (i.e. letter) per round.

    * The user should receive feedback immediately after each guess 
      about whether their guess appears in the computers word.

    * After each round, you should also display to the user the 
      partially guessed word so far, as well as letters that the 
      user has not yet guessed.

    Follows the other limitations detailed in the problem write-up.
    '''
    # FILL IN YOUR CODE HERE...
    '''
    if(len(secretWord) < 4):
        number_guess = 8    
    else:
        number_guess = len(secretWord)
    '''
    number_guess = 8
    availableLetters = "abcdefghijklmnopqrstuvwxyz"
    print("Welcome to the game Hangman!")
    print("I am thinking of a word that is " + str(len(secretWord)) + " letters long.")
    #print(secretWord)
    print("-------------")
    
    lettersGuessed = []
    while(number_guess >= 1):
        print("You have " + str(number_guess) + " guesses left")
        print("Available letters: " + str(getAvailableLetters(lettersGuessed)))
        letter = raw_input("Please guess a letter: ").lower()

        if letter in lettersGuessed:
            print("Oops! You already guessed that letter: _")
        lettersGuessed.append(letter)

        # guess is/is not in secretWord
        if(letter in secretWord):
            print("Good guess: " + getGuessedWord(secretWord, lettersGuessed))
            print("-------------")
            availableLetters = availableLetters.replace(letter, '')
        if(letter not in secretWord):
            number_guess = number_guess - 1 
            print("Oops! That letter is not in my word: " + getGuessedWord(secretWord, lettersGuessed))
            print("-------------")

        # Win/Loose
        if(isWordGuessed(secretWord, lettersGuessed)):
            print('Congratulations, you won!')
            return
    print("Sorry, you ran out of guesses. The word was else.")
            

Your output:

Welcome to the game Hangman!
I am thinking of a word that is 7 letters long.
-------------
You have 8 guesses left
Available letters: abcdefghijklmnopqrstuvwxyz
Please guess a letter: A
Good guess: _ _ _ _ _ a_ 
-------------
You have 8 guesses left
Available letters: bcdefghijklmnopqrstuvwxyz
Please guess a letter: e
Good guess: _ _ ee_ a_ 
-------------
You have 8 guesses left
Available letters: bcdfghijklmnopqrstuvwxyz
Please guess a letter: z
Oops! That letter is not in my word: _ _ ee_ a_ 
-------------
You have 7 guesses left
Available letters: bcdfghijklmnopqrstuvwxy
Please guess a letter: t
Good guess: _ _ eeta_ 
-------------
You have 7 guesses left
Available letters: bcdfghijklmnopqrsuvwxy
Please guess a letter: a
Oops! You already guessed that letter: _
Good guess: _ _ eeta_ 
-------------
You have 7 guesses left
Available letters: bcdfghijklmnopqrsuvwxy
Please guess a letter: z
Oops! You already guessed that letter: _
Oops! That letter is not in my word: _ _ eeta_ 
-------------
You have 6 guesses left
Available letters: bcdfghijklmnopqrsuvwxy
Please guess a letter: E
Oops! You already guessed that letter: _
Good guess: _ _ eeta_ 
-------------
You have 6 guesses left
Available letters: bcdfghijklmnopqrsuvwxy
Please guess a letter: r
Oops! That letter is not in my word: _ _ eeta_ 
-------------
You have 5 guesses left
Available letters: bcdfghijklmnopqsuvwxy
Please guess a letter: t
Oops! You already guessed that letter: _
Good guess: _ _ eeta_ 
-------------
You have 5 guesses left
Available letters: bcdfghijklmnopqsuvwxy
Please guess a letter: h
Good guess: _ heetah
-------------
You have 5 guesses left
Available letters: bcdfgijklmnopqsuvwxy
Please guess a letter: c
Good guess: cheetah
-------------
Congratulations, you won!
None