How to declare a global variable in Python correctly

Question

I have the following piece of code that presented below with tic-tac-toe implementation. As far as I understand variable "player" is global one and it's value 'X' should be accessed from any place of the program.

board = [['_' for x in range(3)] for y in range(3)]
player = 'X'

def print_board():
    for row in board:
        print(row)

def check_win():
    for i in range(3):
        if board[i][0] == board[i][1] == board[i][2] != '_':
            return board[i][0]
        if board[0][i] == board[1][i] == board[2][i] != '_':
            return board[0][i]
    if board[0][0] == board[1][1] == board[2][2] != '_':
        return board[0][0]
    if board[0][2] == board[1][1] == board[2][0] != '_':
        return board[0][2]
    return None

def greet():
    while True:
        print_board()
        print(f"Player {player}, make your move (row column): ")
        row, col = map(int, input().split())
        if board[row][col] != '_':
            print("Invalid move, try again.")
            continue
        board[row][col] = player
        winner = check_win()
        if winner:
            print_board()
            print(f"Player {winner} wins!")
            break
        if "_" not in [cell for row in board for cell in row]:
            print_board()
            print("Tie!")
            break
        player = 'X' if player == 'O' else 'O'

greet()

But as a result I got the error: print(f"Player {player}, make your move (row column): ") UnboundLocalError: local variable 'player' referenced before assignment. How to declare global variables in a correct way in Python?

What I'm doing wrong?

I expected the code of tic-tac-toe working correctly but it didn't. To resolve the issue I tried to change "player" scope and moved an assignment inside greet() method before infinite loop. It works perfect, but I have no clue why it didn't work before, as a global variable.

Answer 1

You "declare" it outside the the functions as you did here. But you're missing the global keyword.

Then:

• If you only read its value inside a function, it's treated as a global variable.

  variable_name = "old value"
  
  def function():
      print(variable_name)
  
  function()  # old value
  print(variable_name)  # old value
  

• Normally, if you write to it, it's treated as a local variable.

  variable_name = "old value"
  
  def function():
      variable_name = "new value"
      print(variable_name)
  
  function()  # new value
  print(variable_name)  # old value
  

• To write to it as a global variable, add the global keyword in the function definition before using the variable.

  variable_name = "old value"
  
  def function():
      global variable_name
      variable_name = "new value"
      print(variable_name)
  
  function()  # new value
  print(variable_name)  # new value
  

Source: https://docs.python.org/faq/programming.html#what-are-the-rules-for-local-and-global-variables-in-python