Python Basics


Below is a bunch of stuff explaining the basics of Python...

Cheat Sheets

Help can be found at:


Data

Variables

Variables are the most important things in programming.
They let your program remember information.
Variables can store someone's name, their age, or even their favourite food.
Variables simply point to a space in memory, where the data is stored.

A variable always has three things:

# this declares three variables: name, age, male
# and initialises each one with a starting value
name = "Alan Shearer"
age = 45
male = True

Variables generally go through a three stage process:

At various points along this route, the variable will be used.

Constants

Sometimes, you need to store data that doesn't change - these are known as constants.
They are created in the same way, but can't be changed.
In Python, it is common to write the name of constants in all uppercase.

# declare a constant, that we don't want to change
WINNING_SCORE = 100

Data Types

Data types are very important, as they allow the program to have rules: if you add two numbers together, the program should do some maths, but if you add two words together (concatenation) the program will just make a new sentence.

There are five main data types:

Python tries to work out the data type automatically, but it occasionally needs some help, if you want to tell Python what the data type is, try one of the following:

# create 3 new variables by changing the type of var1/var2/var3
name = str(var1)
age = int(var2)
price = float(var3)

Arithmetic Operators

Python allows you to easily perform maths on your data

+ Addition a = b + c
- Subtraction a = b - c
* Multiplication a = b * c
/ Division a = b / c
// Modulus division a = b // c
% Remainder division a = b % c
** Exponent a**b
round() Round Rounds the number to the nearest place
floor() Round-down Rounds the number down
ceil() Round-Up Rounds the number up
sum() Sum Adds together all the items in the list

String Operations

Python also lets us perform various actions on Strings.

varName.upper() Uppercase Makes the entire string uppercase
varName.lower() Lowercase Makes the entire string lowercase
varName.split(" ") Split into a list Converts the String into a list, using spaces
varName.split(",") Split into a list Converts the String into a list, using commas
varName.count("a") Counts occurrences of the text Counts how many times a appears in the String
varName.index("a") Finds the first position of the text Finds the first position of a
varName.startswith("Mr.") Checks what the String starts with Returns True if the String starts with Mr.
varName.endswith("Jarvis") Checks what the String ends with Returns True if the String ends withJarvis

String Splicing

Remember, Strings are just lists of characters.
It's also possible to access parts of a String using array square brackets.

varName[2] Gets a character Returns the third character
varName[2:5] Gets a substring Returns characters from 2 to 4
varName[2:] Gets a substring Returns characters from 2 to the end
varName[:5] Gets a substring Returns characters from the beginning to 4
varName[2:10:2] Gets a substring Returns characters from 2 to 9, skipping every other character
varName[::-1] Reverse a String Returns the String in reverse order

Selection & Operators

Selection is the ability to make a decision. It allows us to do different things, depending on what information is in a variable.
This is done using IF statements:

# if they are 18 or over, print "You can vote"
if age >= 18:
    print("You can vote")
else:
    print("You can't vote")

Making decisions is all about comparing things; is one variable bigger than another, are two other variables the same...

# if they are 18 or over, print "You can vote"
if favArtist == "Picasso":
    print("You like cubist paintings.")
elif favArtist == "Rembrandt":
    print("You like realistic paintings.")
elif favArtist == "Monet":
    print("You like impressionist paintings.")
elif favArtist == "Rembrandt":
    print("You like realistic paintings.")
elif favArtist == "Cezanne":
    print("You like post-impresionistic paintings.")
else:
    print("Unrecognised artist")

To compare things we use standard operators, just like in maths.

Equality Operators

These let us test if things are equal or not:

== Equal to Check if two items are the same
!= Not equal to Check if two items are not the same

Comparison Operators

These let us compare things:

> Greater than Check if the first item is bigger than the second item
< Less than Check if the first item is smaller than the second item
>= Greater than or equal to Check if the first item is bigger than or the same as the second item
<= Less than or equal to Check if the first item is smaller than or the same as the second item

Logical Operators

These let us use logic to combine lots of operators:

and Both comparisons have to be True
or Either or both comparisons have to be True
not This will change the result from True to False, or vice-versa

Iteration

Being able to do selection (make decisions) on its own is nearly enough.
However, if we want our programs to be really useful, we want them to be able to repeat stuff.

We call this iteration.

While Loops

Let us repeat something until we can make a decision (this may never occur)

Infinite While Loops

while True:
    print ( "Looping!" )

Conditional While Loops

num = -1 # keep looping WHILE they don't guess 21
while num != 21:
    num = int ( input ( "Guess a number: " ) )
    print ( "You guessed:", num )
print ( "You finally got it!!!" )

For Loops

Let us repeat something a set number of times 

For Loop Through an Array

# make a list of fruits
fruits = ['banana', 'apple',  'mango']
for item in fruits:
    print ( 'Current fruit :', item)

For Loop x Times

for index in range ( 10 ):
    print ( "Position:", index )

For Loop from x to y

for index in range ( 0, 6 ):
    print ( "Position:", index )

For Loop in Steps

for index in range ( 10, 21, 2 ):
    print ( "Position:", index )

Loop Control

Pyhton provides two really useful commands to help in loop control:

while True:
    guess = int(input("Guess a number: "))
    if guess = answer:
        print("You got it!")
        break
    else:
        print("Wrong!")

Lists

This does exactly what it says on the tin - we can store lots of information all with the same name. If you have 100 books, you don't want to make 100 variables to remember all the names.
Instead, you make one variable called books and then give each book a number...

# make a list
pupils = ["Edd", "Clive", "Kim", "Kat", "Tim", "Tam"]

# print the first pupil in the list - item 0
print(pupils[0])

# add a pupil to the list
pupils.append("Jordan")

# get the number of pupils in the list
num_of_pupils = len(pupils)

# remove a pupil from the list
list.remove("Edd")

We can also iterate through a list, make decisions based on a list, and change items in a list:

# make a list
pupils = ["Edd", "Clive", "Kim", "Kat", "Tim", "Tam"]

# print each pupil in the list
for item in pupils:
   print ( item )

# change an item in the list
pupils [ 2 ] = "Kimberly"

# check if an item is in a list
if "Kat" in pupils:
   print ( "Found her" )

We can also slice up or join lists:

# make a list
pupils = ["Edd", "Clive", "Kim", "Kat", "Tim", "Tam"]

pupils[2:4]     # items from 2 until before 4 = Kim & Kat
pupils[2:]      # items from 2 until the end = Kim, Kat, Tim & Tam
pupils[:3]      # items from the beginning until before 3 = Edd, Clive & Kim
pupils[:]       # everything

# you can also count backwards
pupils[-1]      #last item in the array = Tam
pupils[-2:]     # last two items in the array = Tim & Tam
pupils[:-2]     # items from the beginning until 2 before the end = Edd, CLive, Kim & Kat
pupils[1:-1]    # remove first & last = Clive, Kim, Kat & Ti

# join an array into a String of items
sentence = ", ".join(pupils)    # makes the String "Edd, Clive, Kim, Kat, Tim, Tam"

Modular Development

A modular program is made up of lots of separate parts (modules). Each of these should work independently of the others. In Python, we achieve this by writing functions.

Python has lots of built-in functions that you might have met already: print(), input(), int(), str(), len(), round(), etc... These work by themselves, without needing to be changed. You simply give them some information to work with, and they (sometimes) give you some information back.

Writing a Function

So, how do we write a function? Well, it's pretty simple - you give it a name, and then everything that is indented after the name, is in that function:

def myFunction():
    print("This is in the function")
    print("This is also in the function")

print("This is not in the function.")

This function is called myFunction, but it could be called anything (except for the built-in function names or keywords). Whenever you tell your program to do myFunction it should print the two messages inside.

A more detailed example might be for when you want to keep repeating the same things again and again. It is not a good idea (or any fun) to keep typing the same stuff again and again. Sometimes a loop can solve this, but other times it might not be enough.

# define a function to draw a square
# the size parameter sets the length of the sides
def square ( size ):
   for loop in range ( 4 ) :
        t.forward ( size )
        t.right ( 90 )

# call the function - very important
# setting size to 77
square ( 77 )

Libraries

Libraries are just other people's code that we want to use. To access them, we simply import them. We have learnt about three libraries so far:

Here is an example of importing and using a library:

import random

colours = [ "red", "yellow", "green", "blue" ]

# generate a random number between 1 & 100
num = random.randint ( 1, 100 )

# generate a random colour (using the list above)
colour = random.choice ( colours )

File Access

Accessing files in Python is easy. You simply open the file, read from or write to it, then close it:

outFile = open ( "myfile.txt", "a" )
outFile.write ( "hi there\n" )
outFile.close ( )

This can be simplified even further, using the with command. The with command will ensure the file is closed once you've finished with it:

with open ( "myfile.txt", "a" ) as outFile :
    outFile.write ( "hi there\n" )

When you open a file, you pass the name of the file you want, and an access mode. The following access modes are most commonly used:

When reading from a file, it is most common to read it line-by-line. Again, it is more convenient to use with:

with open ( "myFile.txt", "r") as inFile :
    # read the file into data
    data = inFile.read ( ) 
    # make a list of lines
    myList = data.splitlines ( ) 

This will create a list called myList, with one entry for each line.


Databases in Python

Database Recap

In order to be able to follow this guide, you will need to understand the basics of a database:

Example

So, to model pupils in the real world - we would create a table, let's call it pupils
Each field in pupils will store a piece of information about a pupil
Each record in pupils will hold all the information about a single pupil
We can summarise this in a data-dictionary:

Field Name Data Type Information Rules
PupilID Integer Primary Key
First Name String between 1 and 30 characters
Last Name String between 1 and 30 characters
DOB Date matches format DD/MM/YYYY
Gender String one of Male/Female/Unspecified

To talk to a database you use SQL (Structured Query Language). There are two parts to SQL:

Python comes with a built-in database: SQLite. To gain access to it, you simply import the library:

import sqlite3

Having done that, we simply connect to the database, perform some SQL, and disconnect:

db = sqlite3.connect('pupils.db')
# perform SQL statements
db.close()

As with file access, this can be wrapped up using with, to ensure we always disconnect:

with sqlite3.connect("pupils.db") as db:
    # perform SQL statements

Both of these will either open an existing database called pupils.db or create a new one with that name. This means that the information will always be saved to a file, so every time you run your program, all the old data will still be there. If you don't want to create a file, and would instead like to temporarily create a database in RAM, replace the database name with the String :memory:


Defensive Programming

Defensive programming is a way of designing your program, so that it will continue to work, even when unexpected things happen.

Defensive programming should improve:

Defensive programming can sometimes go too far - you shouldn't try to prevent errors that can't happen, as this could slow your program down, and make it harder to maintain.

It's also important not to be too vague, otherwise you might miss important errors, that need to be found.

Styles of Defensive Programming

There are two primary styles of dealing with potential errors:

Generally, EAFP is preferred in Python.

Examples of Defensive Programming

A common place programs can crash is when you expect NUMERIC input, but the user types a STRING:

choice = int(input( "Enter your choice: " ))

EAFP

Try to do what you wanted to do, then if it breaks, deal with it:

try:
    choice = int(input( "Enter your choice: " ))
except ValueError:
    print( "Invalid number" )

In this example we simply (try to) convert the input into a number - if it fails, we deal with it.

We can then wrap this in a loop, to make it keep repeating until the data is valid:

while True:
    try:
        choice = int(input( "Enter your choice: " ))
        break
    except ValueError:
        print( "Invalid number" )
        continue

LBYL

Check you can do something first, then only do it if it's OK.

choice = input( "Enter your choice: " )
if choice.isdigit():
    choice = int(choice)
else:
    print( "Invalid number" )

In this example, we get the input, check it's a number, and then convert it.

We can then wrap this in a loop, to make it keep repeating until the data is valid:

while True:
    choice = input( "Enter your choice: " )
    if choice.isdigit():
        choice = int(choice)
        break
    else:
        print( "Invalid number" )
        continue

As can be seen, EAFP only catches ValueErrors, it will still crash if some other error occurs.
But that complies with what we said earlier, we don't want to miss strange/important errors.
And, in LBYL, it doesn't actually deal with any errors, but simply tries to prevent them - less reliable?


Turtle Basics

To create a turtle, and get it to move, try the following:

# get the library
import turtle
t = turtle.Pen()

# draw a square
for loop in range(4):
    t.forward(100)
    t.right(90)

GUI Basics

To create a GUI, and show a label, try the following:

# import the library
from appJar import gui

# create the GUI
app = gui("Hello")
app.addLabel("l1", "Hello World!")
app.go()

Colour Map

Below is a useful colour map to use when doing Turtle or GUI programming.
Python Colour Map