Introduction to Python Module #4 - PowerPoint Presentation

Slide1

Introduction to Python Module #4Comments, Programming concepts, Conditional statements

Lois

Delcambre

a

little discussion

a little more Python

encryption and decryption programsSlide2

Planmaking your programs more readable:

comments

coding style

booleans – if elif else while in Pythonadditional Python instruction:Python console (aka Python interpreter)subscripts and decryption/encryptionSlide3

Who is going to read code?When?How often?Slide4

Food for thought …

code

is read much more often than it is

written1 The visual appearance of source code is important. We want to

require

less human cognitive effort

to

understand a

program

.

2

A program is a human-readable essay on problem solving

that also happens to execute on a computer.

3

1 Python Style guide from Guido van

Rossum

, the developer of Python

2

http://en.wikipedia.org/wiki/Programming_style

3 Punch and

Enbody

,

The Practice of Computing using Python

, p. 10, 2013, Pearson.Slide5

You can add commentsto your program

use the “#”

symbol for simple comments

You put comments in to your program so that the next programmer can read your code more easily.Python interpreter ignores all comments.Slide6

Comments are ignored by Python

an example program with comments (

guess_my_number3

)Comments at the top indicate who wrote

the program and when. Provides a brief

summary.

Comment here explains one line of code.Slide7

the way you type your program mattersexcerpt from Google Python Style Rules

Line length

Maximum

line length is 80 characters. Parentheses Use parentheses sparingly. Indentation Indent

your code blocks with

4 spaces

.

Blank

Lines

Two

blank lines between top-level definitions, one blank line between method definitions.

Imports formatting

Imports should be on separate lines.

Statements

Generally only one statement per line. Slide8

Programming concept #1: SimplicityIf something is less complicated, it’s less likely to have problems and easier to troubleshoot and fix.

easier to read!

easier to maintain!

easier to test!therefore it is more likely to work as intendedand it is much more likely to be secure.Slide9

note: functions/methods are like limousines with tinted windows

the outside program CAN’T see any of the variables in the function/method

and ...

if you put functions/methods in a separate file (and then import them when you want them), even the programmer can’t see the code (or variables)Slide10

let’s try it out:

variable inside a function

The calling program

can’t see this variable

called y.

10

add_one_function

Slide11

example showing that y (inside function)is not seen from outside program (try this)

y is defined only

inside the function

code block

The outer program

tries to print y

but Python tells us

that y is not

defined.

add_one_function

Slide12

Programming concept #2: Abstraction

Each function/method introduces an abstraction

This is what Chris Bosh explained to us.

The function name/parameters gives you all that you need to know. all that you are allowed to know

.

The implementation details are hidden inside.

You should introduce nice abstractions (functions) in your programs.Slide13

Programming concept #3: ModularityModules can be inserted or removed from a project; each module code blocks can be changed to make it run faster, to make it more readable, and so on.

Just make sure that the module still performs the same task

.

a function/method introduces modularitya group of functions/methods in a Python module is a module (e.g., the turtle module, the random module)Slide14

Programming concept #4: Information HidingInformation hiding is any attempt to prevent people from being able to see information.

The code for functions and methods are not visible to the program that invokes them.

The code inside a module (that is imported in Python) is not visible even to the programmer.Slide15

You can add docstrings (in triple quotes)immediately after your function header

This is a triple

quoted string.

You can use one

anywhere (as a

lengthy comment).

When you put it

immediately after

the function header,

it serves as

documentation

for the function.

triple quoted strings

can span multiple

lines.Slide16

Planmaking your programs more readable:

comments

coding style

booleans – if elif else while in Pythonadditional Python instruction:Python console (aka Python interpreter)subscripts and decryption/encryptionSlide17

New data type:

Boolean

(you already know integer and string data types)

Exactly two constants;

written

exactly this way:

True

False

Six comparators – always return True or False:

< less than

> greater than

<= (Note: you must put the < first

) less than or equal to

>= (Note: you must put the > first

) greater than or equal to

== (Note: you MUST use two equal signs

) equal to

!= (can also be written as <>

) not equal to

17Slide18

A program with a while loop

This condition compares two

variables from this program.

If they are not equal (if the condition returns True),

then the block is executed.

guess_my_number1Slide19

Also – count and print the number of guesses

Initialize a new variable to 0 using

an assignment statement.

Replace

num_of_guesses

with

the current value of

num_of_guesses

plus 1.

This program will add 1 to

num_of_guesses

every time the code block in the while loop

is executed.

Here, we print the

num_of_guesses

guess_my_number2Slide20

use a while True loop with break

use an if/else in loop to make sure user input is

valid (1)

U

sing the in predicate here.

will return True if the value of

user_guess

is equal to one of the

values in this list.

guess_my_number3Slide21

use a

while True

loop with

breakuse an if/else in loop to make sure user input is valid (2)

The constant True always evaluates

to True. This

l

oop will run forever.

An if statement has a

condition. If the condition

evaluates to True, the code

block is executed.

The break statement exits the loop.

The statement executed after break is this one.

else block is executed if

user_guess

was not in [1, 2, 3, 4, 5]

guess_my_number3Slide22

use a while True loop with break

use an if/else in loop to make sure user input is

valid (3)

one equal sign (=) in an

assignment

statement

two equal signs (==) in a

condition that is

checking for equality.

guess_my_number3Slide23

the 4 blocks

guess_my_number3Slide24

Practice with while loops and if statementstrinket.io

Learn/Start Learning/If-Else Statements

(3 lessons)

Learn/Tutorials (down near the bottom of the page)click on the Learn button in the upper right cornerscroll down to the tutorialschoose the one on Conditionals (for if statements)choose the one on Loops (for while loops)

you can also practice with the conditional expressions (used in if and while) with the tutorial on Logic ExpressionsSlide25

Plan for Day 4making your programs more readable:

comments

coding style

cybersecurity first principlesbooleans – if elif else while in Pythonadditional Python instruction:Python console (aka Python interpreter)

subscripts and decryption/encryptionSlide26

You can choose to run the Python console (also called the interpreter)

On this screen,

Click on this button and then choose >_ Console.

Go to My Trinkets; choose New Trinket (Python)Slide27

The Python Console (interpreter)will be on the right side of the screenSlide28

If you use turtle graphics, the Console ison the lower right side of the screen

python console here:

turtle canvas here:Slide29

The console runs one Python statementat a time (immediately when you type it)

As soon as I typed

t1.forward

and hit return,

the turtle

immediately

went forward.

Enter another

command, and it

will be executed

immediately.Slide30

The console runs one Python statementat a time (immediately when you type it)

Don’t write entire

programs this way.

Use the left window

in trinket to write

programs

and then save and

run them.

You know you’re in

the interpreter when

you see >>> on the line.Slide31

Encrypting in PythonSimplified versions of code that

Dr. Wu-

chang

Feng used to generate the crypto puzzles.Slide32

Demo of transposition encoding(09_SUBSTITUTION_simple on trinket)Slide33

Python program for a substitution cypher What needs to happen?

# s is the original message

s = "the key for number nine is

xxxxx"

alphabet = "

abcdefghijklmnopqrstuvwxyz

"

scramble = "

zyxwvutsrqponmlkjihgfedcba

"Slide34

Take the first letter in s

s = "the key for number nine is

xxxxx

"alphabet = "abcdefghijklmnopqrstuvwxyz "

scramble = "

zyxwvutsrqponmlkjihgfedcba

"Slide35

find it in the alphabet

s = "the key for number nine is

xxxxx

"alphabet = "abcdefghijklmnopqrstuvwxyz "scramble = "

zyxwvutsrqponmlkjihgfedcba

"Slide36

replace it with the correspondingletter in scramble

s = "the key for number nine is

xxxxx

"alphabet = "abcdefghijklmnopqrstuvwxyz "

scramble = "

zyxwvutsrqponmlkjihgfedcba

"Slide37

put the letter from scrambleinto the new msg we are building

s = "the key for number nine is

xxxxx

"alphabet = "abcdefghijklmnopqrstuvwxyz

"

scramble = "

zyxwvutsrqponmlkjihgfedcba

"

encrypted_s

= "g"Slide38

Take the second letter in s

s = "the key for number nine is

xxxxx

"alphabet = "abcdefghijklmnopqrstuvwxyz "

scramble = "

zyxwvutsrqponmlkjihgfedcba

"

encrypted_s

= "

g"Slide39

find it in the alphabet

s = "the key for number nine is

xxxxx

"alphabet = "abcdefghijklmnopqrstuvwxyz "scramble = "

zyxwvutsrqponmlkjihgfedcba

"

encrypted_s

=

"g"Slide40

replace it with the correspondingletter in scramble

s = "the key for number nine is

xxxxx

"alphabet = "abcdefghijklmnopqrstuvwxyz "

scramble = "

zyxwvutsrqponmlkjihgfedcba

"

encrypted_s

= "

g"Slide41

put the letter from scrambleinto the new msg we are building

s = "the key for number nine is

xxxxx

"alphabet = "abcdefghijklmnopqrstuvwxyz

"

scramble = "

zyxwvutsrqponmlkjihgfedcba

"

encrypted_s

= "

gs

"Slide42

continue through all lettersSlide43

final result

s = "the key for number nine is

xxxxx

"alphabet = "abcdefghijklmnopqrstuvwxyz "scramble = "

zyxwvutsrqponmlkjihgfedcba

"

encrypted_s

= "

gsv

pvb

uli

mfnyvi

mrmv

rh

ccccc

"Slide44

Several runs of the programTalk to your neighbor; does it look correct?

alphabet = "

abcdefghijklmnopqrstuvwxyz

"

scramble = "

zyxwvutsrqponmlkjihgfedcba

"Slide45

Let's write the programSlide46

Setting things up

09_subsitution_simpleSlide47

Find out how long alphabet is

09_subsitution_simpleSlide48

Start out with an empty encrypted message

09_subsitution_simpleSlide49

Process each letter in s using a for loop

09_subsitution_simpleSlide50

Check each letter in alphabet

09_subsitution_simpleSlide51

check to see if current character matches position i in alphabet (alphabet[i])

09_subsitution_simpleSlide52

if yes … concatenate scramble[i] to the encrypted message we are building

09_subsitution_simpleSlide53

after all characters in s are seen, print

09_subsitution_simpleSlide54

Class Activity

Talk to your partner: what happens when the original message has punctuation?

Run the program (with a message that has punctuation) to see if you were right.

How would you change this program if you wanted to encrypt both lower and upper case letters?Slide55

Python constructs used in the substitution programNote: these examples are run using the Python console – where each statement is run immediately when typed in. You can recognize that because the beginning of the line has the >>> symbols.Slide56

for loop running through a string(using the Python console here)Slide57

use any variable name you want(using the Python console here)Slide58

use your variable name within loop(using the Python console here)Slide59

for loop running through a string

09_subsitution_simpleSlide60

Using subscripts to run through a string(using the Python console here)Slide61

Subscripts running through a string

09_subsitution_simpleSlide62

You can use len function in range function

(

using the Python console here)

In the first example,

we used an assignment

statement to hold the

length of the message.

Then, we used the range

function in the for loop.

i

will take on the values

0, 1, 2, 3,

… up to one less

than the value of length.

In the second example, we

used

len

(message)

directly in the range function.

There’s no need for an

assignment statement.Slide63

Practice Activity

Modify the

09_substitution_simple.py

program so that the spaces between words in the original message do NOT show up in the encrypted message.Modify the 09_substitution_simple.py program so that a period, a comma, and a question mark are placed into the encrypted message unchanged.

09_subsitution_simpleSlide64

a few tipsWhen in doubt, print it out

(add print statements anywhere – while debugging/developing)

Test as you go

(write a little code; try it/fix it; THEN write more code)Wondering how something works? Try it in the Console(not sure how for i in range(5): works? play with it)For parameters, type function name plus open parentheses (in Shell or program)Slide65

more tipsIn Shell, you can use ctrl-p to see previous Shell command. (see previous in history)

You can use ctrl-n to see next Shell command (from the one you're looking at). (see next in history).

These two tips save typing – in Shell.

You don't have to use the print function in the Shell to see the answer. Shell will show you the answer anyway. (You must use print in a program.)Slide66

arithmetic in Pythonx + y addition

x – y subtraction

x * y multiplication

x / y division (floating point result)x // y division (integer result)x**y exponentiationx % y modulo (or remainder)This one is new for us.

i

nteger division

gives the integer

portion of the answer.

this one is new for us.

modulo gives us the

integer remainder

after dividing x

by y.Slide67

Arithmetic expressions in Console(console always shows you the returned value)

>>> 5 + 6

11

>>> 5 + 11.316.3

>>> 20/4

5.0

>>> 20/3

6.666666666666667

>>> 20//3

6

>>> 5 * 2

10

>>> 5 * 2.5

12.5

>>> 2 ** 3

8

>>> 12%5

2

>>> # 2 is the remainder after dividing 12 by 5Slide68

python program forString to ASCII representationSlide69

Convert text (ASCII) characters to decimal number equivalent

'Hello' is 72 101 108 108 111Slide70

Useful functions in Python(examples shown in the Console)

ord

function – takes an ASCII character as a parameter and returns the decimal number equivalent Slide71

Useful functions (cont.)str

turns a decimal number into an ASCII string of number symbols

>>>

ord('a')97

>>>

ord

('e')

101

>>>

str

(97)

'97'

>>>

str

(101)

'101'Slide72

Python programs: the original messaget: the encrypted message

01_dec_ascii_simpleSlide73

sample run

of

01_dec_ascii_simple

01_dec_ascii_simpleSlide74

You are invited to write decryption programsthe correspond to these encryption programsfor the programming showcase

01_dec_ascii_simple

02_HEX_ASCII_simple

06_COL_XPOSE_simple07_SCYTALE_simple08_CAESAR_simple

09_subsitution_simple

These are all linked to the

code.cyberpdx.org

site

and linked here. Remix any of these; then you’ll have a copy of the code in your trinkets to change as you like.Slide75

Extra material:More detail about parameters for functions/methodsSlide76

terminology

76

this is called the

formal parameter

(for this function)

this is the

parameter

(for this function

invocation)

add_one_function

Slide77

The function on the bottom works but it isn’t usingits formal parameter!

Don’t do this!

using_variables_from_main2

using_variables_from_main1Slide78

both of these programs use variable t1 from the outer program (it would be better to define another parameter)

using_variables_from_main2

using_variables_from_main1Slide79

Better: use one more formal parameter for the turtle that you want to draw the square.

using_variables_from_main3Slide80

Practice activityAccess these programs:

using_variables_from_main1

using_variables_from_main2

using_variables_from_main3Remix them (to have a local copy of them – in your trinkets), run them; try to break themSlide81

No Show and Tell for Day 4Ask for help – whenever you need it

Get ready for the Programming Showcase on Friday afternoon.

Each team needs to select two

subteams to present their program.one subteam with previously inexperienced programmersone other subteam The program must be either:

turtle art

decryption (of any of the message types from crypto)