On the left margin of the source window on the current line you should be able to see mysterious capital letters. These letters represent the state of the Python interpreter and are explained here:
C - CALL - A function is called (or some other code block entered).
L - LINE - The interpreter is about to execute a new line of code (sometimes multiple line events on one line exist).
R - RETURN - A function (or other code block) is about to return.
E - EXCEPTION - An exception has occurred.
* - RUNNING - The thread is still running (probably blocked in C code)
You can read more about the E letter in the exceptions page.