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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _generate_random_string(self, container, length=20):
 """Generate a random cookie or token string not in container. The cookie or token should be secure in the sense that it should not be likely to be able guess a value. Because it is not derived from anything else, there is no vulnerability of the token from computation, or possible leakage of information from the token. """

        while True:
            s = ''.join([random.SystemRandom().choice(string.digits + string.ascii_letters)
                         for n in range(length)])
            if (s not in container):
                break
        return s