Python
theLonelyWolf
Python20Revision.pptxRevision Round Two
Python Programming
Major Topics
Variables
Input and Output
Functions
Conditionals – if/elif/else
Iterations – while and for loops
Lists
Testing
Professional Practices
Python Variables
Every variable in Python is created when it’s assigned (‘=‘) a value
name = “Bob”
Variable name rules
No spaces
No keywords (words that already have special meaning)
Must start with a letter
camelCase or underscores_for_spaces
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Getting values in Python
Built-in function:
variableName = input(prompt)
Displays prompt and then gets a value from the keyboard. This value will be stored in variableName as a string
Typecast to an int or float if needed
num_name = int(input(prompt))
Math
Basic math can be done with numerical types (int or float)
Symbols
Addition is ‘+’
Subtraction is ‘-’
Division is ‘/’
Multiplication is ‘*’
Because ‘X’ already has a meaning… it is the letter ‘X’
Modulo is ‘%’
Display variables
Use ‘concatenation’ to join things together
In planning and Python we use ‘+’ to indicate this:
display “The sum of “ + num1 + “ and “ + num2 “ is “ + num3
We MUST explicitly insert spaces
In flowgorithm use the ampersand (&)
Python allows us to print a collection of things, separated by commas. It will insert spaces automatically:
print(“The sum of”, num1, “and”, num2, “is”, num3)
functions
Keyword ‘def’
Inputs (if any) in brackets after name, separated by commas
Have empty brackets if there are none
Return at end (if needed)
def getValue(prompt)
num = int(input(prompt))
return num
Calling functions
If it returns a value
We assign the returned value to a variable using the ‘=‘ statement:
result = getResult(num1, num2)
Otherwise:
Just call it by name:
displayResults(score)
Decisions
Basic keyword
if
Decision block structure
if <condition> :
<code goes here>
Note header ends with ‘:’ as well as indentation
Similar basic structure to function block
Compound decisions
Can combine decisions with boolean keywords:
and, or, not
E.g. if temp >35 and temp < 40
Each side MUST be a full decision
temp > 35 and < 40 is not possible
| Path | Condition | Result |
| Porridge is too hot | Temperature > 40° | Complain “It’s too hot” |
| Porridge is too cold | Temperature < 35° | Complain “It’s too cold” |
| Porridge is just right | Temperature > 35° and < 40° | Eat someone else’s porridge |
For Example…
if num > 0 and num <= 10:
print(“Valid Number”)
if num <= 0 or num > 10:
print(“Invalid Number”)
More complex decisions
New Keyword needed:
else
For the ‘false’ path
And more complex again…
New keyword needed:
elif
for each branch other than the first and last
Examples
if num > 0:
print(“You Entered ” & num)
else:
print(“Invalid Number”)
or
if num <= :
print(“Invalid Number”)
else:
print(“You Entered ” & num)
Examples
if score < 50:
print(“Fail”)
elif score <65:
print(“Pass”)
elif score <75:
print(“Credit”)
elif score <85:
print(“Distinction”)
else:
print(“High Distinction”)
Types of Loops
Two ‘categories’ of iterations
Definite (we KNOW how many times)
Indefinite (no way of knowing)
‘while’ loops for indefinite
‘for’ loops for definite
Can use while here too, but for makes it easier
Definite also applies if the computer knows at runtime, even though we cannot predict it at writing time…
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While loops
Also know as a ‘pre-test’ loop
Tests the loop condition BEFORE it runs
May never run if the condition is false to start with
Like decisions, an iteration ‘block’ has 2 parts
Header
while <condition> :
Body
Code within the loop, must be indented
While loops
Let’s implement this in Python:
get value from user
while value is less than zero:
print an error
display prompt
get value
display value
For loops
For loop header is very different
Header
for variable in collection:
collection can be either:
A range
range(10)
A list
Names
More in CP1404…
For loops
On the first iteration variable will have the value of the first thing in the collection
On each iteration the value of variable changes to be the next value in the collection
range(10) is [0,1,2,3,4,5,6,7,8,9]
Up to, but not including, the stop value!
for count in range(10)
print(count)
Will display the numbers
0,1,2,3,4,5,6,7,8 and 9
on separate lines
Arrays (Lists in Python)
Like any variable we create a list by giving it a value:
listName = []
Creates an empty list
Can give it values, separated by commas, instead of making it empty
myList = [1,2,3,4,5]
Creates a list of five integers
1 to 5
Iterating over a list
For loops were designed to access lists, so they are our preferred method here
We know the number of iterations (size of the list)
Builtin function ‘len()’ can tell us this at runtime (avoids magic numbers)
‘For’ loop header can use this value
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Examples
myList = [1,2,3,4,5]
for i in range(len(myList))
print(myList[i])
Iterating over a list
But there is an even simpler way
We can use the list itself as the container to loop over:
for num in myList:
print(num)
Will print each value in the list
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Testing Code
Why?
We (should) have already checked our design, so we know it works
So why do we need to make sure the code works?
We need to check that we implemented our solution correctly
Testing Code
How?
Try running it, see what happens
If and when error messages appear READ THEM!
Right now they may seem unhelpful, but we will become familiar with them over time
What are we testing?
That it runs
Syntax errors
That it gives the correct results
Logic errors
Are there any user inputs which break the program
Runtime errors
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Testing functions
Each function should be tested individually
Called ‘unit testing’
Create a simple ‘main’ which tests each function with sample input
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Testing Decisions
Each decision should be tested individually
Put it in a function by itself, and test it with fixed inputs (or user input)
Test ALL ‘boundary conditions’
E.g.
If temp <= 35 Then
We need to test this with temp = 34, 35 and 36 to ensure it behaves as we expect
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Testing Loops
Each loop should be tested individually
Put it in a function by itself, and test it with fixed inputs
Test the number of runs
E.g.
while count <= 10
Does this exit when count is 9, or 10, or 11?
Does it behave as we expect?
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Professional Practices
Commenting standards
Position (above/end of line)
On functions
Style
Length
Position is a style choice, some people prefer to comment on the line above, some comment at the end of the line
Functions should have a comment on the line above, indicating what the function is for
Style should be clear and concise
Length should be as short as possible without being unclear
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Naming Standards
Valid variable names != good variable names
Naming conventions
Capsfirst – used for Class names
We don’t use these in CP1401, but you will see them in the future.
ALLCAPS – reserved for unchanging (constant) values
Constants – Local and Global
In Python we cannot ‘define’ constants
Use ALLCAPS to indicate constancy
IF a value is only used in one function it should be defined inside that function
If it is used in several places it can be defined at the top of our code, outside of ALL functions (including main) as a ‘global ‘constant’
Global constants allow me to avoid passing around data which is used in several places
Why is this allowed when global variables are not?
