To keep track of which variables can be used where, it is sometimes useful to draw a stack diagram. Like state diagrams, stack diagrams show the value of each variable, but they also show the function each variable belongs to.
Each function is represented by a frame. A frame is a box with the name of a function beside it and the parameters and variables of the function inside it. The stack diagram for the previous example is shown in Figure 3.10.1.
The frames are arranged in a stack that indicates which function called which, and so on. In this example,
print_twice was called by
cat_twice was called by
__main__, which is a special name for the topmost frame. When you create a variable outside of any function, it belongs to
Each parameter refers to the same value as its corresponding argument. So,
part1 has the same value as
part2 has the same value as
bruce has the same value as
If an error occurs during a function call, Python prints the name of the function, and the name of the function that called it, and the name of the function that called that, all the way back to
For example, if you try to access cat from within
print_twice, you get a
Traceback (innermost last): File "test.py", line 13, in __main__ cat_twice(line1, line2) File "test.py", line 5, in cat_twice print_twice(cat) File "test.py", line 9, in print_twice print cat NameError: name 'cat' is not defined
This list of functions is called a traceback. It tells you what program file the error occurred in, and what line, and what functions were executing at the time. It also shows the line of code that caused the error.
The order of the functions in the traceback is the same as the order of the frames in the stack diagram. The function that is currently running is at the bottom.