area.md

Simple statements and functions

Goals

• use assignment statements
• make a simple function and call it

The end result is file area.py.

Description

Here are some simple Python statements to compute the area from a width and height:

w = 10
h = 30
area = w * h
print area

Something super important: Programming languages (most but not all) execute one statement after the other, in order, and one statement finishes before the next is started. They are executed synchronously.

That is a very simple formula but we should get used to the practice of creating functions to encapsulate common bits of functionality like recipes.

def area(w,h): # w, h are called args/parameters
    a = w * h  # create local variable 'a'
    return a   # exit the function, return to caller

print area(10,30)

Add comment to area.

Confusion point: A return statement (all paths that can reach end of function) is necessary otherwise it returns None.

>>> def hi(): print "hi"
... 
>>> x = hi()
hi
>>> print x
None
>>> 

Printing something is not returning a value! It generates a function side-effect, naming printing a value to the output window.

Local variables, arguments

Notice that w and h are not visible before or after execution of the function:

print w # "NameError: name 'w' is not defined"

Behavior of variables

• globals assigned to outside of a function
• global variables can be accessed by any statement in your program file (but not other files)
• locals assigned to inside of a function, not visible outside of that function; locals get created each time we call the function and they disappear when the function finishes.

Python program template

Note that the general form of a Python program is:

    imports

    global variable / constant definitions

    function definitions

    script code

Student exercise

do volume:

def volume(w,h,l): # width, height, length
    ...

print volume(10,30,5)