Python list exercise _ verify the validity of the user name and the value of the two-dimensional list

Exercise 1: Verify that the username is valid

1. Enter the account number #input

2. If the account exists, it will prompt that it has been registered. If it does not exist, let him register #list comparison all_users[], count()

3. Cannot be empty #strip()

4. The length of the username is between 6-12 len()

5. Enter up to 3 times #loop

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Exercise 2: 2D List Values

Group students who have handed in and not handed in, and print

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