Typically I look at the warnings and errors and fix any problems until the code will run. Then I look at the output to see if I'm getting the expected results and make changes and read the documentation for any modules or functions that aren't working like I expect. I also put in extra print statements to test my assumptions of how the code should work.

The code I posted has a missing parenthesis but otherwise does what you requested. The second warning message points you to the exact line where this occurs.