How can I tell what is invalid about my syntax?
If you Run your code and receive an error complaining about your syntax, then you may have made a simple typo somewhere. Python has no way of knowing that
grade_sum() was meant to be
grades_sum(), if you spelled it differently on accident when calling it vs when you defined it.
Also, be sure to include the word
def before a function, a colon
: after, and to
return after the loop, not inside of it.