How do I do this? Please explain each step

Checking for A Valid PID In the file check_pid.py, write a function check_pid that takes a string and produces a boolean. It should return True if: The string is exactly 4 characters long The first character is "A" The remaining 3 characters are all digits (the strings '0', '1', and so on up to '9') It should return False if any of these conditions are not satisfied. The function should never cause an error as long as some string is given input. Examples: >>> check_pid ("A123") True >>> check_pid("A1234") False >>> check_pid ("AA23") False