为什么 Python 认为 return_value 是一个字符串？

Question

What is wrong with the following bit of code?

class Dice:
    def roll_dice(x):
        import random
        return_value = '('
        try:
            y = int(x)
        except ValueError:
            return None
        for char in range(y):
            return_value += f'{random.randint(1, 6)}, '
        return_value = list(return_value).remove[-1 and -2]
        return_value = ''.join(str(e) for e in return_value)
        return return_value + ')'

parameter = input('Number of dice rolls: ')
print(Dice.roll_dice(parameter))

The error is

Traceback (most recent call last):
  File "C:/Users/MyName/PycharmProjects/ProjectName/filename.py", line 17, in <module>
    print(Dice.roll_dice(parameter))
  File "C:/Users/MyName/PycharmProjects/ProjectName/filename.py", line 11, in roll_dice
    return_value = list(return_value).remove[-1 and -2]
TypeError: 'builtin_function_or_method' object is not subscriptable

If I understand Python correctly, it thinks return_value is a string. But didn't I take the list of return_value on line 11 where the problem is? How does that work?

Answer 1

The problem is remove[-1 and -2] . remove is a method, so it must be followed by parentheses, not brackets. That's what not subscriptable means.

Also, -1 and -2 does not do what you think it does, apply remove(-1) and remove(-2) separately.

Answer 2

The first argument of a class method should be named self , but the class is not a real class, because this missing self is never used. Write it as a normal function. import belongs to the beginning of the file, not inside functions. Instead of building a string with extra , and removing it by converting it to a list and back again to a string, you could simply use

return_value = return_value[:-2]

but it would be better to not create a string in first place, but a list:

import random

def roll_dice(x):
    try:
        y = int(x)
    except ValueError:
        return None
    values = []
    for _ in range(y):
        values.append(str(random.randint(1, 6)))
    return f'({", ".join(values)})'