This is a simple while True: loop that takes input with input(), then determines to break only if the input is 4 digits. Of course, there are two tests necessary to determine whether a string is 4 digits: whether its len() is 4, and whether it is all digits (which can be determined using .isdigit().

while True:
    if year has len() of 4:
        if year is all digits:
            print thanks
            break

while True:
    take input for year
    if year has len() of 4 and year is all digits:
        print thanks
        break

while True:
    take input for year
    if year is not len() of 4 or year is not all digits:
        print try again
    else
        print thanks
        break

Keep in mind that you can put 'not' in front of any test to negate it -- with a compound test, however, each test must stand alone -- so if you're negating a test with 'not', it will apply only to the individual test (i.e., not both tests in a compound test). I am asking you to choose a compound test because it's inherently simpler, and because it allows you to practice compound tests.

Start with a 4-digit string year (for example, '1990') assigned to a string variable:

Then looping through the FF_data.txt file, calculate the average MktRF value (the 1st column of floating-point values, i.e. the 2nd column in the file) for that year. I recommend an incremental approach:

• First, loop through FF_abbreviated.txt line-by-line, printing each line. Note the type and value of the first such value at the right margin.
• Next, still inside the loop, split each line. Note the type and value of the first value at the right margin.
• Still inside the loop, subscript the list that came from split(). Note the type and value of the resulting value at the right margin.
• Still inside the loop, convert the value to a float.
• Next, consider how you might sum up the values that are presented at each iteration of the loop. (Hint: if you initialize a float value before the loop begins, you can add to that float inside the loop to sum up the values.)
• Lastly, consider how you might count the number of iterations of the loop (Hint: if you initialize an int value before the loop begins, you can add 1 to that int inside the loop to count the iterations of the loop.)

This is a simple division. It's been made a separate program so you'll understand that it does not belong inside a loop. It needs to be calculated only once.

PASTE IN THE CODE, ONE AFTER THE OTHER for the previous three assignments into one complete program, remove hard-coded values, and alter variable names so that they work together. (However please don't indent any one of the pasted solutions inside any of the others' blocks!) This is a complete program. You must test this entire program as shown below. If your previous three programs work as expected, it should be possible to combine them into one program without rewriting any code or "nesting" part of the code in another part. However, you will need to remove the hard-coded year, count and sum, any print statements from the first two solutions, and will probably also need to change variable names in one or more of the programs so that the three programs will be connected. Please do not "nest" one program into the code of another. You only need to paste the three solutions, in order, and "hook them up" by changing the variable names so that they match between parts.

the input-taking program produces a 4-digit year

the file parsing program takes a 4-digit year to calculate a count and sum

the calculating program takes a count and sum to produce an average

Of course you must test the overall program so that works as expected -- if it doesn't, you'll need to make sure that each program is producing the value as expected, and the the next program is using the value produced by the prior program.