1. Python 3.x Programming Key Stages 1-5 (OCR/AQA)
Continuing Professional Development in Computer Science
Secondary School Teachers
Python 3.x Programming
Key Stages 1-5 (OCR/AQA)
Session-1 (of 5) October – November 2017

2. Course Introduction Procedural programming concepts
Develop programming skills in Python 3.x
Sequence
I/O, variables, data types, arithmetic and converting data
Selection
If-else, function calls
Iteration
For, While, nesting
Compound Types
tuples, lists, dictionaries
Files
I/O, strings, exceptions

3. Course Introduction Computational thinking Structured development
Pseudocode, Structured English
Analysis and design of programs
Incremental development

4. Environment Python v3.x Use v3.5 or above Incompatible with v2.x
Edit code in IDLE (Integrated Environment)
Possible to edit in Notepad(++) and paste to IDLE
interpretation, execution, debugging
Represent algorithms as structured code

5. Programming Constructs
Sequence
print statements and input statements
Selection (branching)
if, if…else, break, continue statements
Iteration (repetition)
while and for statements
These constructs apply to ‘data types’

6. Data Types Strings (collection of characters) Numeric (Integer, Float)
No ‘char’ type: It’s a string length = 1 in Python
Case sensitive!
Numeric (Integer, Float)
There is no ‘longint’ or ‘double’ type in Python
Compound (Lists, Tuples, Dictionaries)
Equivalent to the ‘array’ type

7. Output of Strings print() function
Must have parentheses
Strings are enclosed in quotes
use a consistent ‘house style’ (PEP0008)
Default output is left justified
exactly what is between the quotes.
See Appendix-03
Style-1
Style-2
print (‘Hello World’)
print (“Hello World”)
Hello World outputs left-justified with any other characters that are between the quotes

8. Output of strings Escape Sequences
Sess1_Example-1.py
print (“Hello World”)
Hello World
print (“\tHello World”)
print (“Hello \nWorld”)
Hello
World
print (“Hello \”World\””)
Hello “World”
print (“Hello \\World”)
Hello \World
Also see Appendix-03 for managing quotes and escape chars
Use a ‘\’ character to escape special chars
\t for a single tab
\n for a newline
\” for a quote
\\ for a backslash

9. Output of strings More control
Sess1_Example-1.py cont’d
print (“Hello”)
print (“World”)
Hello
World
print (“Hello”,end””)
Hello World
print (“A very “ \
“very long line”)
A very very long line
# print a blank line
print()
Suppress the newline char: end=“”
Splitting a long line (\)
Comments
(#)
Output a blank line

10. Input of strings input ( ) function
Must have parentheses
halts execution until user enters data and presses return key
A variable holds the input (see appendix-01)
Data is assigned to the variable with ‘=‘
No reserved words used for variable names (appendix-02)
Sess1_Example-2.py
# ‘=‘ is the assignment operator – it isn’t the equals operator (==)
userName = input(“Enter Your Name: “)
test1
print (“Hello”,userName)
Hello test1
There are no spaces between the quotes: The comma is a ‘separator’. It will put a space in the output for us

11. Sess1_Exercise-1_Menu Load
Sess1_Exercise-1_Menu_Question.py
Finish the code to output the menu as shown right
Prompt user for input
Echo back what they entered
Answer is in
Sess1_Exercise-1_Menu_Solution.py
Main Menu
D: Division
M: Multiplication
A: Addition
S: Subtraction
X: Exit
Enter an Option:

12. I/O of strings String Methods
Further control of output can be done with string methods. The methods are ‘called’ using dot notation
Practice with the following – study the output
name = (“test ONE”)
print (name)
print (name.upper() )
print (name.lower() )
print (name.title() )
print (name.replace(“one”, “FIVE”) ) #”one” is not a typo
#Methods are similar to functions but can’t be called on their own.
They must be called via a function (like print() ) using dot notation 12

13. I/O of strings String Concatenation (+)
Used to ‘join’ variables or variables with literals
Practice the following:
word1 = “Bat”
word2 = “man”
print (word1,word2) #comma puts a space in
print (word1 + word2 #concatenation of 2 vars
print (word1+” and Robin”) #concatenation of a var and literal 13

14. Numbers Operations BODMAS applies to Python Addition (+)
Subtraction (-)
Multiplication (*) e.g. print (3*2)  6
Exponent (**) e.g. print (3**2)  9
Equality (==) Not the same as the assignment operator (=)
print (1 == 2)  False
print (1 == 1)  True
Division (/, //, %) – see next slide

15. Numbers Operations Division (/): or ‘integer’ or ‘true’
x/y gives untruncated float result (field width 18 inc d.pt) when x or y is integer or float
Floor (//): or ‘DIV’
gives a truncated integer (the ‘whole’ part – always rounded down, e.g 3 and 3.66 3 and 3.753)
x//y gives integer result when x and y both integer
x//y gives truncated float result when x or y is float
Remainder: (%): or ‘modulo’ or ‘MOD’
gives remainder
x%y gives integer when x and y both integer
x%y gives truncated float when x or y is integer
x%y gives untruncated float when both are float

16. Doing maths on strings gives an error
I/O on Integers
All input defaults to a string
Doing maths on strings gives an error
#ask user to input their age
age_Now = input ("Enter your age in years: ") 21
#add 10 to the input
ageInTen = (age_Now + 10)
print (ageInTen)
Traceback (most recent call last):
File "H:/My Documents/Teaching/Teaching /COMP101_ /COMP101_Support/age.py", line 3, in <module>
ageInTen = age+10
TypeError: Can't convert 'int' object to str implicitly
The input of “21” by the user is a STRING
Because it’s a string, we can’t do maths on it – so we get an error when this line executes

17. I/ O on Integer - casting
How to change a string to an integer
Doing maths on integers is okay
#ask user to input their age
age_Now = int(input ("Enter your age in years: ")) 21
#add 10 to the input
ageInTen = (age_Now + 10)
print (ageInTen) 31
This makes “21” into the number 21
Now 21 is a number (it’s an integer here) – it works! Output is 31
You don’t have to put brackets round the calculation, but it can help clarity

18. Doing maths on floats is okay
I/O on Float - casting
How to change a string to a float
Doing maths on floats is okay
#ask user to input their age
age_Now = float(input ("Enter your age in years: ")) 21
#add 10 to the input
ageInTen = (age_Now + 10)
print (ageInTen)
31.0
This makes “21” into the number 21.0
Now 21.0 is a number (it’s a float here) – it works! You don’t have to put brackets round the calculation, but it can help clarity

19. Constants Constants Python doesn’t handle constants
treats all declarations as variables
can be changed!
Some languages allow constant declaration
CONST pi = 3.142;
pi stays as for the duration of the program
error is thrown on attempts to change it
A Python ‘workaround’ is to capitalise the variable name – then remember not to change it!

20. Arithmetic Addition (+) and Subtraction (-) Sess1_Example-3.py
#addition: user input not shown for clarity
num1 = int(input(“Enter first num: “))
num2 = float(input(“Enter next num: “))
answer = (num1+num2)
print (“Sum =“,answer)
#subtraction: user input not shown for clarity
num1 = float(input (“Enter first num: “))
answer = (num1-num2)
print (“Result =“,answer)
Whenever a float is used, output will be a float
Whenever a float is used, output will be a float

21. Arithmetic cont. Multiplication (*) and Exponent (**)
Sess1_Example-4.py
#multiplication: user input not shown for clarity
num1 = int(input (“Enter first num: “))
num2 = int(input(“Enter next num: “))
answer = (num1*num2)
print (“Product =“,answer)
#exponent: user input not shown for clarity
num1 = int(input(“Enter first num: “))
result = (num1**num2)
print (“Solution =“,result)
If both integer, output is integer.
If float used anywhere, output will be a float
If both integer, output is integer.
If float used anywhere, output will be a float

22. Arithmetic cont. True division (/), Floor division (//)
Sess1_Example-5.py
#true division – answer always a float
num1 = int(input(“Enter first num: “)) 10
num2 = int(input(“Enter next num: “) 3
result = (num1/num2)
print (“Solution =“,result)
Solution =
#Floor division – answer is integer if both integer, else truncated float
result = (num1//num2)
Solution = 3
Field width = 18 inc d.pt

23. I/O Numbers Operations
Division Modulo (%) – outputs the remainder
Sess1_Example-6.py
#integer modulo
c = 10
d = 3
print(c%d) 1
#float(decimal) modulo
e = 7.5
f = 2.4
print(e%f)
Can be used as test for divisibility: if x%y = 0 then x is divisible by y

24. Exercise-2_floatingBarge
Load: Sess1_Exercise-2_Barge_Question.py
(Answer is in: Sess1_Exercise-2_Barge_Solution.py)
Develop and implement a Python program which, given a barge defined in terms of inputs for length(L), breadth(B) and height(H), outputs the associated draft.
Breadth(B)
weight of iron = 1.06kg per square metre
surface area = (2 * height) * (length + breadth) + (length * breadth)
mass of barge = surface area of barge * weight of iron
draft = mass of barge / (length*breadth)

25. Sess1_Example-7.py Import Modules (Classes)
Importing Modules
Python comes with pre-built “modules” (classes).
We can import these and call the functions that they contain.
Functions are called using a dot operator.
Sess1_Example-7.py Import Modules (Classes)
#import the math module – all functions come with it
import math
num = int(input(“Enter a number: “)) 16
#use the sqrt function from within the math module
answer = math.sqrt(num)
print(answer)
4.0
print (math.pi)
dot operator for module.function

26. Importing Modules cont.
Sess1_Example-8.py
#import the math module – all functions come with it
import math
import time
num = float(input(“Decimal number: “))
16.2
print(math.floor(num)) 16
myTime = time.time()
print (myTime)

27. Simulate throwing a single dice
Sess1_Example-9.py
#Throw a single dice to output a random number within a given range (usually 1 to 6)
#import ‘random’ module
#use ‘randint’ function from within ‘random’ module
import random
#set the maximum number to be generated
max_Number = 6
#randint function generates integers 1 and 6 inclusive
dice_Throw = (random.randint(1,max_Number))
#output
print (dice_Throw)

28. Sess1_Exercise-3_dice I/O and Import
Write a program to allow a single user to throw two dice, one after the other
Add the total of the two dice and output this back to the user
Hints: import random; don’t cheat and set the random range as 1-12!
Answer is in:
Sess1_Exercise-3_dice.py
Pseudocode
Initialise variables
1.1 var max = 6
1.2 total = 0
1.3 die_1, die_2 = 0
Input (from system)
2.1 die1=random(1,max)
2.2 die2=random(1,max)
3. total = die1 + die2
4. print total

29. string input - we did not use ‘int’ to convert it
Standard Functions
Some functions are available as standard and do not need to be imported, e.g., len - returns the length of its string argument, does not work on integer input
Example-10 len()
nums = input("Enter some numbers: ")
74982
print(len(nums)) 5
course_Name = input("Enter course: ")
Computing
print(len(course_Name)) 9
string input - we did not use ‘int’ to convert it

30. Formatted Output - Integers {:value}.format(value)
.format function is used with print() to control output
Sess1_Example-11_Formatting.py
stud_Id = 123
#output number 123 as a signed integer(d)
#in a field width of 5 spaces
print (“Student number{:5d}”.format(stud_Id))
Student number 123
#Two spaces in front of number 123 as it takes up three of the 5 five spaces in the output format. If number was negative, the minus sign would take up another space of the field width

31. Formatted Output cont. Sess1_Example-12_Position.py
#students paying for a school trip
#input student Id and amount paid
sId = int(input(“Enter student id: “))
sPay = float(input(“Enter payment: “))
#pass arguments by position
#sId is first in argument list so is passed to {:5d}
#sPay is second in argument list so is passed to {:5.2f}
print (“Id{:5d},has paid {:5.2f}”.format(sId,sPay))

32. Sess1_Exercise-4 Formatted Output
Amend
Sess1_Exercise-2.py
b) Inform the user what values they entered for L, B, H and tell them what the draft will be.
Have a practice with some formatting for output, or just keep it simple
Some suggested solutions are in:
Sess1_Exercise-4_Solution.py

33. Sequence Summary Sequence operations run once
Users can input ANY data type other than the one prompted for – so we have to trust the user to enter a number if asked and hope they don’t enter a string etc
Sensible prompts can help, and sometimes the output (especially numeric) may need to be formatted to be useful and understood

34. Appendix-01 Variables for input
Prompt for input needs a user-defined variable to hold the input.
Variable names are case-sensitive and can be made up from:
‘A’ - ’Z’; ‘a’ - ’z’; _ ; 0-9
(e.g. longName_01)
can’t start with a number and no spaces allowed
usually maximum 15 characters long (not critical)
Any name you choose but no reserved words

35. Appendix-02 Reserved Words
and, as , assert
break
class, continue
def, del
elif, else, except
False, finally, for, from
global
if, import, in, is
lambda
none, nonlocal, not or
pass
raise, return
True, try
while, with
yield

36. Appendix-03 Advanced String Handling
Within your program, you wish to prompt the user with the following message:
Create a new directory called “C:\\names”
Solutions – see over 36

37. Appendix-03 cont’d Advanced String Handling
Try the following solutions – escape char used
print (‘ Create a new directory called “C:\names” ’)
# \n within the nested quotes is the escape character for newline
Create a new directory called "C:
ames"
print (“Create a new directory called ” C:\\names “)
#\\ means escape the next character – so we output a single ‘\’
Create a new directory called "C:\names" 37

38. Appendix-03 cont’d Advanced String Handling
Try the following solutions – escape char used
print (“Create a new directory called ” C:\\\names “)
# the first ‘\’ escapes the second ‘\’ so we output a single ‘\’
# the third ‘\’ escapes character n so we get a newline
Create a new directory called "C:\
ames"
print (“Create a new directory called ” C:\\\\names “)
# The first ‘\’ escapes the second ‘\’ so we output a single ‘\’. The third ‘\’ escapes the fourth ‘\’ so we output another single ‘\’
Create new a directory called "C:\\names"
Got it!! 38

39. Appendix-03 cont’d Advanced String Handling
Try the following alternatives – no escape char needed
e) print (‘ Create new directory called ” C:\\names “ ‘)
# the use of single quotes around the entire string does not
conflict with the double quotes within the substring
Create a new directory called "C:\\names"
f) print (‘’’ Create new directory called ” C:\\names “ ‘’’)
#the use of triple quotes around the entire string does not
conflict with the double quotes within the substring.
Useful if single quotes are present in the string also 39