如果我运行下面的代码，我得到i = 42598。

import numpy as np
all_sets = []
working_sets = []

numbers = np.arange(10)
operators = ['+', '-', '*', '/']

i = 0
expression = ['', '', '', '', '', '', '']
for n1 in numbers:
    expression[0] = str(n1)
    for o1 in operators:
        expression[1] = o1
        for n2 in numbers:
            expression[2] = str(n2)
            for o2 in operators:
                expression[3] = o2
                for n3 in numbers:
                    expression[4] = str(n3)
                    for o3 in operators:
                        expression[5] = o3
                        for n4 in numbers:
                            expression[6] = str(n4)
                            i += 1

                            # get all sets for comparison
                            numbers = sorted([expression[0], expression[2], expression[4], expression[6]])
                            if not (numbers in all_sets):
                                all_sets.append(numbers)

print(i)

但是如果我注解掉这一点

# get all sets for comparison
numbers = sorted([expression[0], expression[2], expression[4], expression[6]])
if not (numbers in all_sets):
    all_sets.append(numbers)

我得到i = 640000，为什么？是什么破坏了我的for循环？