Почему происходит только одна итерация цикла?

Код:

import random
def dividers(n):
    zero = n
    i = 2
    dividers = set()
    while i**2 <= n:
        while n % i == 0:
            dividers.add(i)
            n = n / i
        i = i + 1
    if n > 1:
        dividers.add(int(n))
    dividers.discard(zero)
    return dividers
def newexpression(length, eoperators=None, extra=None):
    nums = []
    expression = ""
    while len(expression) <= length:
        if expression != "" and expression[-1] == "/":
            num = random.choice(numdividers)
            expression += num
        else:
            num = random.choice("123456789")
            expression += num
            if (random.choice([True, False]) == True or len(expression) == length - 1) and length != 1:
                num = num + random.choice("0123456789")
                expression += num[1:]
        numdividers = dividers(int(num))
        if len(expression) < length and eoperators != None:
            if "/" in eoperators and numdividers != set():
                expression = expression + random.choice(eoperators)
            elif "/" not in eoperators:
                expression = expression + random.choice(eoperators)
            else:
                expression = expression + random.choice(eoperators[:-1])
            print(expression)
        return expression
print(newexpression(6, "+-"))

Цикл, о котором идёт речь:

while len(expression) <= length:
            if expression != "" and expression[-1] == "/":
                num = random.choice(numdividers)
                expression += num
            else:
                num = random.choice("123456789")
                expression += num
                if (random.choice([True, False]) == True or len(expression) == length - 1) and length != 1:
                    num = num + random.choice("0123456789")
                    expression += num[1:]
            numdividers = dividers(int(num))
            if len(expression) < length and eoperators != None:
                if "/" in eoperators and numdividers != set():
                    expression = expression + random.choice(eoperators)
                elif "/" not in eoperators:
                    expression = expression + random.choice(eoperators)
                else:
                    expression = expression + random.choice(eoperators[:-1])
                print(expression)

Я не понимаю