Mastering Python lesson 4_functions_parameters_arguments

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Mastering Programming in Python
Lesson 4
Functions, parameters and local/global variables

In this lesson you will …
 Learn about Functions in Python
 The use (advantages) of using Functions
 Robotics – what do you know?
 Introduction to Modular Design
 Local and Global Variables
 Passing parameters & What are ‘arguments’?
 Built in Functions (Python 3)
 Big Questions: Evolution vs Intelligent Design in
light of functions (modular design)
 Challenges, including Q and A (with solutions)
 Suggested Research/HW and Youtube Videos
*For this series we
assume students know
how to open, save and
run a python module.
Version: Python 3

• Introduction to the language, SEQUENCE variables, create a Chat bot
• Introduction SELECTION (if else statements)
• Introducing ITERATION (While loops)
• Introducing For Loops
• Use of Functions/Modular Programming
• Introducing Lists /Operations/List comprehension
• Use of Dictionaries
• String Manipulation
• File Handling – Reading and writing to CSV Files
• Importing and Exporting Files
• Transversing, Enumeration, Zip Merging
• Recursion
• Practical Programming
• Consolidation of all your skills – useful resources
• Includes Computer Science theory and Exciting themes for every
lesson including: Quantum Computing, History of Computing,
Future of storage, Brain Processing, and more …
Series Overview
*Please note that each lesson is not bound to a specific time (so it can be taken at your own pace)
Information/Theory/Discuss
Task (Code provided)
Challenge (DIY!)
Suggested Project/HW

Robotics
Robotics is a branch of mechanical engineering and
computer science and it deals with the design, creation
and application of robots.
You may have watched I, ROBOT and considered the
impact that robots will have on society in the future.
Imagine a world in which robots brought us tea and ran
errands for us! I, ROBOT was based on a popular book.
The author Isaac Asimov wrote science fiction and was
famous for his “Three Laws of Robotics”. He imagined
things way back then that are now a reality!
I, Robot – a book by Isaac Asimov
If you’re interested in this field, you may want to check out the WIKIPEDIA page on the
topic and look at the timeline of robotics through the ages. Where will it all lead?!

Introducing the concept of Modular Design
Imagine a company with one hundred robotics specialists that was planning to build a
robot. It wouldn’t make much sense for all one hundred people to work on the arm,
and then the leg and then the eye.
Modular design, or "modularity in design", is a design
approach that subdivides a system into smaller parts
called modules. These modules can be independently
created and then used in different systems. A modular
system can be characterized by functional partitioning
into discrete scalable, reusable modules.
Instead groups could be formed and each group (team) could work on a specific
module. Advantages: The module for each body part could then potentially be re-used
in future robots as well! This would save both time and money in development and
testing. Each module would however, have to be integrated into the whole system.

More on Modular Design
Modular programming is basically a design technique to
split your code into separate parts – or modules. One of
the goals of this separation should be to have modules
with no or just few dependencies upon other modules.
Or to put it another way: Minimization of dependencies is
the goal.
When creating a modular system, several modules are
built separately and more or less independently.
The executable application will be created by putting
them together.
Modules can contain functions, but there are statements in them as well. Statements
are typically used to initiate the module, only executed when the module is imported.
readable reliable
maintainable Easy to test
Modular design helps produce
programs that are:

What are functions?
Python has a number of built in functions (like “Print()”, but we can also
code our own functions and these are called ‘user defined’ functions.
A function is a block of organized and
generally reusable code that is used to
perform a single, related action. A function
typically returns a single value (like a
number, or ‘true’ or ‘false’. Functions
provide a better platform for modularity
and a high degree of code reusing.
def functionname( parameters ):
"function_docstring"
function_suite
return [expression]
Syntax of a Function in Python
By default, parameters have a positional behaviour and you need to
inform them in the same order that they were defined.

All you need to know about Functions
 A Function block will start with the keyword “def” followed by the function name
and parentheses ( ( ) ). A function (unlike a procedure) returns a single value.
Input “parameters” or “arguments” should be placed within these parentheses. You
can also define parameters inside these parentheses.
The first statement of a function can be an optional statement - the documentation
string of the function or docstring.
The code block within every function starts with a colon (:) and is indented.
The statement “return” [expression] exits a function. It can pass back an
expression to the caller. A return statement with no arguments is the same as
“return None”.

The anatomy of a function
def function_name(parameter_1,parameter_2):
{this bit is your code which is in the function}
{more code}
{and still more code}
return {value to return to the main program}
{now, this bit of code is NOT in the function}
{Why? because it isn't indented}
#don’t forget to put a colon ":" at the end …
#... of the line that starts with 'def'

Video demonstration
(function and passing parameters)

Example of the use of a Function #1
First, type this in. Note
you have a function
called “functionprint”
and you need to pass
a string parameter
into it.
Next, press F5 to run
the code. Type in
“functionprint(“boo”).
The parameter “boo”
(which is a string) is
passed to the function
and printed!

Example of the use of a Function #2
In this example we are
calling the
functionprint function
twice (and passing it
different string
parameters each time)
Once you have the
basic structure of a
function, you can
execute it by calling it
from another function
or directly from the
prompt.

Parameters / Arguments
 Passing parameters by reference or passing parameters by value
All parameters
(arguments) in the
Python language are
passed by reference.
It means if you
change what a
parameter refers to
within a function, the
change also reflects
back in the calling
function. For example
−
#Example of passing parameters
def values_change ( mylist ):
"This changes a passed list into this function"
mylist.append([1,2,3,4]);
print "Values inside the function: ", mylist
return
# Now you can call values_change function
mylist = [10,20,30];
values_change( mylist );
print "Values outside the function: ", mylist
Here, we are maintaining
reference of the passed
object and appending
values in the same
object. So, this would
produce the following
result −
Values inside the function: [10, 20, 30, [1, 2, 3, 4]]
Values outside the function: [10, 20, 30, [1, 2, 3, 4]]

Passing multiple parameters to a function
Function “printdetails”
which requires the
input (on calling the
function) of two
parameters.
Here we are calling
the function with two
parameters. These are
then printed.

Arguments ….
# Function ‘sum’
def sum( arg1, arg2 ):
# Add both the parameters and return them."
total = arg1 + arg2
print "Inside the function : ", total
return total;
In essence, the words ‘parameter’ and ‘argument’ are used interchangeably.
You can call a function by using the following types of formal arguments:
required
keyword
default
variable
We won’t get in to
these in detail in this
lesson, but feel free to
look them up!

Understanding: Local and Global Variables
Imagine a big school building. Reception has a big computer screen in it, and
there are individual offices and classrooms with computer screens too.
Good morning
Employees!
Reception Computer Screen
“Today I am
Going to expel
Joe Bloggs”
Headmasters office – private email
Test scores:
Tom = A*
Jane = F
Hal = B
Head of History – teacher’s computer screen

Understanding: Local and Global Variables
Good morning
Employees!
Reception Computer Screen
“Today I am
Going to expel
Joe Bloggs”
Headmasters office – private email
Test scores:
Tom = A*
Jane = F
Hal = B
Head of History – teacher’s computer screen
This message is
available for
EVERYONE in the
school to see. It is like
a GLOBAL variable
(accessible to the
whole program)
Only people in the
head’s office can see
this crazy message! It’s
like a local variable!
Only accessible inside
the function it’s
declared in.
Only those in this
classroom would be able
to ‘see’ these scores.
The scope of a variable
determines the portion of
the program where you
can access a particular
identifier

*Answers on the next slide
These are variables with GLOBAL scope
Num3 is only declared inside the test1 function
So it is a variable with LOCAL scope.
Analyse the following
code carefully and see if
you can predict the
output when both
functions are called.
What will TEST1() output?
What about TEST2()?
Task: Local and Global variables

Num3 is not a GLOBAL
variable so cannot be
accessed by the function
test2. It results in an
error!
Answers:

See if you can fill in the blanks …
#This is a comment
total = 0; #This is a global variable.
# ‘sum’ is the name of a function
def sum( arg1, arg2 ):
# Add both the parameters and return them."
total = arg1 + arg2; # Here, total is a local variable.
print ("Inside the function local total : ", total)
return total;
# Here we are calling the sum function with two parameters.
sum( 10, 20 );
print ("Outside the function global total : ", total)
#the output will be Global total = 0 and Local total = 30
?
?
?
?
?
?
? ?
Run the code and try it
for yourself.
Note how the GLOBAL
variable is accessible by
the whole program.
The local variable is only
accessible inside the
function it is declared in.
Note: It is also possible to store the output of a function in a
variable. To do so, we use the keyword return.

Challenge 1:
Discuss and answer the following questions. Be prepared to share!
1. Analyse the code on the left. What does this
program do? Be specific and mention example
inputs and outputs.
2. Describe, using specialist terminology, what is
happening where the blue square is placed. What
is happening to variables w and h?
Your answer here
Your answer here

Challenge 2: Answer the following questions
1. Name all the functions in the program.
2. Name all the numeric parameters
3. Name any parameters that require string input.
4. Name any variables.
5. If the input is: “Moose”, 3, 10 – what’s the output?
Your answer here
Your answer here
Your answer here
Your answer here
Your answer here

Watch the following video demo:
Challenge coming up on the following slide! This demo shows a little a program
that prints a menu and converts from Celsius to Fahrenheit (code below)
Note the use of
functions for the menu
and one function for
each conversion.

Challenge 3: The Ultimate converter!
On the previous slide you analysed the code that uses functions to print a menu
and convert, depending on user selection. Ready for your challenge? Read on…
1. Use the code in the previous slide (copy
and paste from the right although it is
preferred you type it out yourself)
2. Run it to see what it does.
3. Add a menu option and corresponding
function for
a) converting pounds to kilograms
(you’ll have to research this) and
b) converting £ into Euro (or any other
currency)
# This is a little program that converts temperature to Fahrenheit or Celsius
def print_options():
print("Make a selection:")
print(" 'p' Print this menu again")
print(" 'c' Convert from Celsius")
print(" 'f' convert from Fahrenheit")
print(" 'q' Exit this program")
def celsius_to_fahrenheit(c_temp):
return 9.0 / 5.0 * c_temp + 32
def fahrenheit_to_celsius(f_temp):
return (f_temp - 32.0) * 5.0 / 9.0
choice = "p"
while choice != "q":
if choice == "c":
c_temp = float(input("Celsius temperature: "))
print("Fahrenheit:", celsius_to_fahrenheit(c_temp))
choice = input("option: ")
elif choice == "f":
f_temp = float(input("Fahrenheit temperature: "))
print("Celsius:", fahrenheit_to_celsius(f_temp))
else:
choice = "p" #Alternatively choice != "q": so that print
#when anything unexpected inputed
print_options()

Challenge 4: Spot and change the mistake!
The following code (once you fix it) will create a simple but nifty calculator!
1. Analyse the code on the right – you can
cut and paste it into Python to look at it
more closely. It doesn’t work – can you
figure out why?
2. Hint: the word ‘function’ in the code needs
to be replaced. What do you need to
replace it with?
3. Can you add any additional functionality to
this calculator – things like square root
and MOD? Research MOD if you don’t
know what it is – it’ll come in handy when
you’re programming!
# Program make a simple calculator that can add, subtract, multiply and divide using functions
# define functions
def add(x, y):
"""This function adds two numbers"""
return x + y
def subtract(x, y):
"""This function subtracts two numbers"""
return x - y
def multiply(x, y):
"""This function multiplies two numbers"""
return x * y
def divide(x, y):
"""This function divides two numbers"""
return x / y
# take input from the user
print("Select operation.")
print("1.Add")
print("2.Subtract")
print("3.Multiply")
print("4.Divide")
choice = input("Enter choice(1/2/3/4):")
num1 = int(input("Enter first number: "))
Solution in Speaker Notes (teachers – delete before giving to students!)

Python has built in functions
 Remember, at any
given time you can
always look up the
official python
documentation website
which will give you a
wealth of information.
 Python also has a
range of built in
functions:
https://docs.python.org/
3.2/library/functions.ht
ml

Discussion:
The Big Questions:
Do you think the fact that we see re-used
code and modules in DNA points to
Intelligent design? Why or why not?
Read the quote from author Bill Haines
(below) and discuss. What do you think?
“Darwin's time was not only before the advent of computers and computer programming but also before the
discovery of the electron itself. We now know that everything is not the result of natural causes, with the
proliferation of computer programming and apps being a good example and that the genome, is the DNA
software of life- with many reusable code objects (genes) accepting many different design parameters (e.g.. short
beaks / long beaks.) “

Useful Videos to watch on covered topics
https://youtu.be/37VwgGRRt5g https://youtu.be/CGRKqnoQGgM
Advances in Robotics – advanced robots. Recommended video on Python: Functions

1. Create a research information point on the types of software development.
Research and include information on the following three.
 Modular Design
 Rapid Application development
 Waterfall Method
2. Write an essay (or create an informational power point) on one of the following:
 Recent developments in Robotics (what countries are at the forefront and where
will we be in 5 years?)
 A history of robotics – where did it all start, and where are we now?
 The creation of a robot. What is involved? Give examples of robots that have been
created and are currently in use.
Extension task: What is MOD (Modulus). How does it work and can you find examples of how it is used in
programming or game design?
Suggested Project / HW / Research

Useful links and additional reading
https://docs.python.org/3/tutorial/interpreter.html#argument-passing
https://en.wikipedia.org/wiki/Robot
http://www.afterhoursprogramming.com/tutorial/Python/Functions/
https://en.wikibooks.org/wiki/Python_Programming/Functions
https://youtu.be/CGRKqnoQGgM

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