How to Think Like a Computer Scientist - Learning with Python, 2008a

3.12 Functions with results 33
The order of the stack shows the flow of execution. printTwice was called by
catTwice, andcatTwice was called by main , which is a special name for the
topmost function. When you create a variable outside of any function, it belongs
to main .
Each parameter refers to the same value as its corresponding argument. So, part1
has the same value as chant1, part2 has the same value as chant2, andbruce
has the same value as cat.
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 main .
For example, if we try to access cat from within printTwice, we get a NameError:
Traceback (innermost last):
File "test.py", line 13, in __main__
catTwice(chant1, chant2)
File "test.py", line 5, in catTwice
printTwice(cat)
File "test.py", line 9, in printTwice
print cat
NameError: cat
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.
Notice the similarity between the traceback and the stack diagram.
coincidence.
It’s not a
3.12 Functions with results
You might have noticed by now that some of the functions we are using, such
as the math functions, yield results. Other functions, like newLine, perform an
action but don’t return a value. That raises some questions:
1. What happens if you call a function and you don’t do anything with the
result (i.e., you don’t assign it to a variable or use it as part of a larger
expression)?
2. What happens if you use a function without a result as part of an expression,
such as newLine() + 7?