1. Elements of a Python Program
Professor Hugh C. Lauer CS-1004 — Introduction to Programming for Non-Majors
(Slides include materials from Python Programming: An Introduction to Computer Science, 2nd edition, by John Zelle and copyright notes by Prof. George Heineman of Worcester Polytechnic Institute)
CS-1004, A-Term 2014
Elements of a Python Program

2. Elements of a Python Program
Python Language
Every input line (plus its continuations, if any) defines some action by Python interpreter
I.e., line causes interpreter to DO something
I.e., effect some change in state
Examples already encountered:–
Expression:– evaluate the expression and print result on output
Assignment associate a value or object with a name
Three more this week:–
Function definitions
Function calls
Definite loops
CS-1004, A-Term 2014
Elements of a Python Program

3. Elements of a Python Program
Function
A sequence of one or more lines of Python code that can be invoked as a group under a single name
A way of encapsulating a subset of a program that is used repeatedly …
… so that the user (or using program)
doesn‘t have to write it multiple times
doesn‘t have to think about it works again
doesn’t have to even know how it works at all
It is not humanly possible for an ordinary person to hold an entire program in his/her head at one time!
Therefore, we partition it into functions — many small functions!
CS-1004, A-Term 2014
Elements of a Python Program

4. Elements of a Python Program
Defining functions
Colon denotes next line is continuation
def functionName( ):
python statement python statement python statement return <optional expression>
Note: continuation lines may, themselves, have continuations
Indented two units of continuation
In general, continuation of n-level indentation is (n+1) units of indentation
Each continuation line is indented one “unit”
End of function definition denoted by reversion to previous indentation level
CS-1004, A-Term 2014
Elements of a Python Program

5. Elements of a Python Program
“Calling” a function
a.k.a., “invoking” a function
Function_name followed by parentheses
Containing zero or more “arguments”
Meaning:–
“DO the function”
I.e., execute its body as if you had typed it into a Python “shell”
Or in an expression
Definition: BODY (of a function)
The statements following the “:”
Ending with the last indented line
Examples:–
Area of disk
Greet function (p. 11 of textbook)
CS-1004, A-Term 2014
Elements of a Python Program

6. So what’s with the parentheses?
Any function may be designed to take arguments
I.e., a means of conveying information into a function by caller
Examples:
sin(radians) # Sine of that angle
sqrt(nonNegativeNumber) # square root
atan(number) # arctangent
CS-1004, A-Term 2014
Elements of a Python Program

7. So what’s with the parentheses? (continued)
Every function definition must include zero or more parameters between the parentheses
I.e., identifiers separated by commas
Place to receive the information provided by caller
Example
Area of disk
Most functions should have one or more return statements
Means of communicating result back to caller
Needed if function call is part of expression
More about parameters and returns later in course
CS-1004, A-Term 2014
Elements of a Python Program

8. So what’s with the parentheses? (continued)
Parentheses on function definition:–
Place to put the parameters
Error to leave off parameters
Parentheses on function call:–
Place to put arguments
Each argument is expression
Separated by commas
Tells Python:–
Evaluate arguments in order
Assign to corresponding parameter
Suspend whatever it was doing
Interpret body of function (using argument values)
Deliver (optional) result back to caller
Resume whatever it was previously doing
Example
CS-1004, A-Term 2014
Elements of a Python Program

9. Elements of a Python Program
Questions?
CS-1004, A-Term 2014
Elements of a Python Program

10. Elements of a Python Program
Digression
Homework #2 (and all subsequent assignments) are to be completed by two-person teams
Please choose a partner
Send partner information to
If you do not have a partner by the time HW2 is assigned (on Thursday), …
… a partner will be assigned to you!
CS-1004, A-Term 2014
Elements of a Python Program

11. Elements of a Python Program
Module
A separate file with .py extension in name
Contains function definitions
And possibly other stuff
Packaged for convenient organization
And sanity on part of programmer
And programmer’s boss or advisor!
Lauer’s advice to rising programmers:–
Any program that is more than a couple of pages in length is too long to wrap your head around!
Divide it into smaller “modules” organized around related groups of functions
CS-1004, A-Term 2014
Elements of a Python Program

12. Elements of a Python Program
Reading Assignments
Study §1.7
Especially range, for-loop
Note specification of a module
Chapter 2, §2.1–2.5
Need to complete Chapter 2 by end of week!
CS-1004, A-Term 2014
Elements of a Python Program

13. Elements of a Python Program
Questions?
CS-1004, A-Term 2014
Elements of a Python Program

14. Laboratory assignment tomorrow
Define and use functions
Including with parameters
Will also use Definite loops
I.e., a for-loop with a fixed number of iterations
Read §1.7 in textbook
CS-1004, A-Term 2014
Elements of a Python Program