Python - does not allow assigning value to the variable in if conditional, why?

Question

I did something in Python that is common to do in C and javascript, ie, give a value to a variable when an if condition is met. I’m sure that in Python it’s also possible to program in this way, but, I don’t know why, it hasn’t worked in the traditional way. The variable is not accepted. The code in question is as follows:

#variáveis globais
N = ''
nao = 1
def jogador(N):
    if N == '':
        return '', 1
    elif N == 'X' or N == 'x' or N == 'O' or N == 'o':
        if N == 'X' or N == 'x':
            nao = 0  <- aqui deveria haver esta variável, mas ela não é aceita, fica cinza no PyCharm
            return 'X', 0 <- solução usada para dar valor 0 à variável nao
        else:
            nao = 0 <- idem
            return 'O', 0 < solução usada para dar valor 0 à variável nao
    else:
        print('Letra errada!')
        print('Tente outra vez!')
        return '', 1

while nao: <- quando fica igual a 0 o while para, bom vcs sabem disso
    print('Qual você escolhe para jogar: X ou O? ')
    N = input()
    E, nao = jogador(N) <- solução encontrada para dar o valor 0 ao nao

The whole program comes down to this for now. I’m learning Python and I’m playing the old-fashioned game. Another thing, if does not accept any other type of variable. I can by var = 0 and it is not accepted. Does anyone have any explanation? Thank you.

Answer 1

In Python the external variables referenced within a function are read-only global. If a variable that is global has a value assigned anywhere in the body of the function, it will be automatically recreated as local, unless which has been explicitly stated as global.

If you want to assign value to a global variable within a local scope use the keyword global before using the variable.

#variáveis globais
nao = 1

def jogador(N):
    global nao #Declarando a variável global para uso nese escopo
    N = N.strip().upper()
    if len(N) == 1:
      if N in 'XO':
        nao = 0  
        return N      
      print('Letra errada!')
      print('Tente outra vez!')
    return ''

while nao: 
    print('Qual você escolhe para jogar: X ou O? ')
    E = jogador(input())

Test the code in the Repl.it: https://repl.it/repls/BustlingCylindricalLinkedlist

But I personally am not in favor of global use within functions and methods:

def jogador(N):
    N = N.strip().upper() 
    if len(N) == 1:
      if N in 'XO':
        return N       
      print('Letra errada!')
      print('Tente outra vez!')
    return ''

while True: 
    print('Qual você escolhe para jogar: X ou O? ')
    E = jogador(input())
    if len(E) == 1: break

print(f'O jogador escolheu {E}')

Test the code in the Repl.it: https://repl.it/repls/WellwornMintyDatasets

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References:

• Python Programmer FAQ: What are the rules for local and global variables in Python?
• Python Documentation: The Global Declaration