Your syntax is incorrect. When you say - if r == 0 or 1 or 2 or 3 or 4 or 5 :, it directly means if True : because you have provided numbers as condition directly which will be interpreted as True
You should do it like following - 
import random
def choose_first():                                    
    r = random.randint(0,10)
    if r == 0 or r == 1 or r == 2 or r == 3 or r == 4 or r == 5 :
        return ('player1')
    else:
        return ('player2')
Better way, you could do it in the following manner 
import random
def choose_first():                                    
    r = random.randint(0,10)
    if r in [1,2,3,4,5] :
        return ('player1')
    else:
        return ('player2')