Functions can call other functions¶
It is important to understand that each of the functions we write can be used and called from other functions we write. This is one of the most important ways that computer programmers take a large problem and break it down into a group of smaller problems. This process of breaking a problem into smaller subproblems is called functional decomposition.
Here’s a simple example of functional decomposition using two functions. The
first function called square
simply computes the square of a given number.
The second function called sum_of_squares
makes use of square to compute
the sum of three numbers that have been squared.
Even though this is a pretty simple idea, in practice this example
illustrates many very important Python concepts, including local and global
variables along with parameter passing. Note that the body of square
is not
executed until it is called from inside the sum_of_squares
function for the first time on line 6.
Also notice that when square
is
called (at Step 8, for example), there are two groups of local variables, one for square
and one
for sum_of_squares
. Each group of local variables is called a stack
frame. The variables x
, and y
are local variables in both functions. These are completely differenet variables, even
though they have the same name. Each function invocation creates a new frame, and
variables are looked up in that frame. Notice that at step 9, y has the value 25 is one frame
and 2 in the other.
What happens when you to refer to variable y on line 3? Python looks up the value of y
in the stack frame for the square
function. If it didn’t find it there, it
would go look in the global frame.