(python) functions with variable arguments

You need to return the new score from each function:

def six(score):
    print('you rolled a 6')
    print('''
 -------
| o   o |
| o   o |
| o   o |
 -------   ''')
    score += 6
    return score

But the call sites need to be like this:

player_1_score = five(player_1_score)

Compulsory link to names by Ned Batchelder.

You could update the scores separately, i.e. not in the "roll dice" functions. I think it is more readable in this phase. Later, when you advance in learning Python, you will probably solve the problem differently, perhaps with a Player class and a mapping (number 1 to 6) --> (dice face as a string).

def six():
    print('you rolled a 6')
    print('''\
 -------
| o   o |
| o   o |
| o   o |
 -------   ''')
    return 6

def five():
    print('you rolled a 5')
    print('''\
 -------
| o   o |
|   o   |
| o   o |
 -------    ''')
    return 5

player_1_score = 0
player_2_score = 0

#for player1
player_1_score += five()
player_1_score += six()

#for player2
player_2_score += five() + six()

print('scores:', player_1_score, player_2_score)

Note: I added a continuation backslash to the print('''\) statement to get rid of the blank line. I like it better but you may prefer the blank line.

You want to change player's score but you are not modifying it either. You are sending player's score but if you change score in your functions (six and five), you are not modifying original score. If you want to modify original score, you can return score from your functions and assign it to your player's scores. For example:

def five(score):
     print('you rolled a 5')
     print('''
 -------
| o   o |
|   o   |
| o   o |
 -------    ''')
     score+=5
     return score

#for player1
player_1_score=five(player_1_score)
print('p1 score:',int(player_1_score))

Passing an argument to the score parameter for five(score) and six(score) creates a score variable within the scope of each function. So when the program calls five(player_1_score), the value of player_1_score is copied to score. You'll need to

1. Modify each of your functions to return the score value

2. Assign the output of each function to each outer scope variable

player_1_score=0
player_2_score=0
def six(score):
    print('you rolled a 6')
    print('''
 -------
| o   o |
| o   o |
| o   o |
 -------   ''')
    score+=6
    return score

def five(score):
     print('you rolled a 5')
     print('''
 -------
| o   o |
|   o   |
| o   o |
 -------    ''')
     score += 5
     return score


#for player1
player_1_score = five(player_1_score)
player_1_score = six(player_1_score)
print('p1 score:',int(player_1_score))

#for player2
player_2_score = five(player_2_score)
player_2_score = six(player_2_score)
print('p2 score:',int(player_2_score))