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Python reference
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Contents
Introduction ................................................................................................................................ 2
Output to the screen ................................................................................................................... 3
Taking input from the user ......................................................................................................... 5
Variable ...................................................................................................................................... 7
Data type .................................................................................................................................. 10
MC tutorial ............................................................................................................................... 23
Introduction
This document is going to cover most of the things you will need to know about Python for
the A453 controlled assessment and for your exam. I have broken the information down into
sections so that it is easier to search it and it looks more organised (if you disagree, do say).
I am hoping that you will use this as a reference guide when you are trying to do something
and you have forgotten the syntax or you are getting some kind of error on the screen and you
are not sure why it is doing that. I would like to say that Google is your tool and Stack
Overflow is your friend. Use them wisely and you should be able to solve most of your
problems.
Remember that programming is about patience and this is coming from someone with Cisco
background that hated programming. My passion lies in networking but I am starting to enjoy
programming. If you are trying to solve a problem and it isn’t working, give yourself a break
and then come back to tackle the problem. You are not going to get it first time round, if you
do then well done.
Finally, if you are someone who doesn’t like to read a lot of text, the code for each example
is put inside the box so that you can easily identify it.
What is python?
Python is a high level programming language that was developed by Guido van Rossum. It is
a language that works in many platforms such as Windows, Linux, Mac etc… A
programming language enables us to communicate instructions to a computer. We use a
programming language to create programs. The program can be used to control the behaviour
of a machine and we do this by creating instructions which are known as algorithm.
If you want to use Python then you will need to install an interpreter. The interpreter will
understand our instructions typed using python syntax and it will convert it into a form that
the computer can understand.
To install the interpreter, you can go to the link below:
https://www.python.org/download
There are different versions of Python. The version we are using at the moment is 3.2.x. You
can install any new version and the code should work perfectly fine. When you install Python
it will install IDLE which is a GUI based interface. At the moment, at school, we are using
PyScripter which is basically an editor. It makes things easier and more manageable when we
are coding. You can think of html as a scripting language and Dreamweaver as the editor we
use to design and code our website.
How do I code?
To create a program, you will need to use IDLE as this is the development environment. You
can then code away and save your project with the following: filename.py. You don’t have to
limit yourself to IDLE, you can use other environments but don’t forget you will need to
interpreter. This can be downloaded from the link above.
Output to the screen
Note: If you copy the code, do not copy the line number. The line number is there for
reference purposes only.
We output to the screen if we want to ask the user a question or if we want to display
something. To output, we use a function called print. The print function is responsible for
displaying text onto the screen and it takes an argument; what you put inside of the brackets.
An example of the print function:
1. print(“Hi. What is your name?”)
1. The first thing you need to remember is that Python is case sensitive and this means
that the word print has to be in lower case otherwise it will not work.
2. It is also important to note that any message you want to display has to be in quotes
unless it is a variable otherwise it will be treated as text (string).
3. You can use single quotes or double quotes but you cannot use them together.
4. Finally, when you print, you need to make sure that you have an open bracket (and a
close bracket ).
Displaying variable
We can use the print function to also display a variable or perform some calculations. You
can perform the calculations inside of the brackets. To perform calculations you must not use
quotes. Every time you use quotes the computer thinks that you want to display string (text)
so “5 + 5” is not the same as print(5+5). The first one will print 5+5 and the second one will
print 10.
Examples:
1. name = “Bob”
2. print(“Hi “ + name)
3. print(10 * 50)
1. Line 1 is showing a variable being assigned a value. In this case the variable is called
name and it is assigned the value “Bob”. We will discuss variables later in detail.
2. Line 2 is showing a variable being printed with some text. This process is known as
concatenation. It is where we add text and variables together.
3. If line 2 was printed, it would output the following text: Hi Bob.
4. Don’t forget that we are adding two strings together and this will work however if we
were adding a string and a number we would need to convert them. I have covered
this under the heading: converting variable data type
5. Finally, line 3 will print 500 as the output. This is because it is not inside quotes and it
will perform the calculation.
Adding line breaks
1. For each line of text you want to display, you will need to use the print statement.
2. You can add a line break line so
1. print(“This is the first line of text”)
2. print(“This is the second line of text”)
1. print(“This is the first sentence
2. And this is the second”)
3. The second example code will not work. You will need separate print statements
lines.
4. If you want the second example to work with line break you can use the following
code:
1. print(“This is going to \n print two lines of code”)
5. \n can be used to print a new line.
6. Don’t forget the space after \n will be printed in the new line. If you don’t want this
space, then use it like so: \n….
Images for printing to the screen
Print function in
lowercase.
Argument that we pass to our function. In
this case we are trying to display text
hence why we are using quotes.
How the code will look when you run it
Tip: hold ctrl and press F9 and it will run the code i.e. debug it.
The green line of text is displaying what our program will look like when we run it. If you
receive any errors you need to make sure that you have used the correct syntax.
Taking input from the user
Taking inputs from the user is an important part of programming because every program
takes some sort of input from the user. If we have a program where the user has to log-in to
the system, we need a way of storing that information somewhere. This is so we can check
whether or not they have the credentials to access the system. This has to be stored
somewhere i.e. in a variable.
To take an input, we use the input function.
1. name = input(“What is your name?”)
2. print(“Hi “ + name)
1. In line 1 we are declaring a variable called name. The variable name is going to be
assigned to whatever the user inputs.
2. In line 1 we have used the input function and the text inside of the brackets is what the
user is going to be displayed with.
3. In line 2 we are printing Hi with whatever the user inputs as their name. If you
remember this process is known as concatenation.
4. We can take as many inputs as we want and we can either store them in separate
variables or update the existing variable.
1.
2.
3.
4.
guess = input(“What is your first guess?”)
print(“You first guesses is: “ + guess)
guess = input(“What is your second guess?”)
print(“You guesses guess is : “ + guess)
5. The example above is showing two inputs taken from the user using one variable.
Images for taking input from the user
1. The image above shows how our code will be formatted in PyScripter.
1. This screenshot shows the window that will be displayed asking the user to input
2. This will be different based on the environment you are using.
1. This screenshot is showing the Python interpreter and the output for running our code.
Variable
A variable is a place holder for a value or expression. We declare a variable by giving it a
name and also the value that it is going to be assigned, i.e. the value it is going to hold. The
value that a variable is assigned can change within our program. A simple way of thinking
about a variable is that it is a box where we store things. As our program develops, we will
be storing more and more of these boxes.
Why would we store an expression or value in a variable?
Well, if the expression was big and if it involved complicated calculations then we might not
want to actually do it again. If we save the value or answer from the expression in a variable
it saves us from running the code again. Remember, programmers are lazy and they don’t
want to repeat things.
Why do we give values a name?
When we give a value from an expression a name, it makes it easy for others to identify our
computation i.e. what we are trying to do in our program and how this value is going to be
used later on.
Declaring variables
In this example I am going to show you expressions that are going to be stored in a variable.
1.
2.
3.
4.
5.
shirt_price = 50
vat_rate = 0.20
vat_amount = shirt_price * 0.20
total_price = shirt_price + vat_amount
print(“The price of the shirt, after vat, is “ + str(total_price))
1. In line 1 and 2 we are assigning the value 50 to shirt_price and 0.20 to vat_rate.
2. When we assign a value to a variable, we do not have to remember what value we
have assigned it, we just have to remember the name (identifier) we have given it such
as shirt_price or vat_rate.
3. The other good thing about storing this value in a variable is that we can perform
calculations on it and if suddenly vat_rate changes to 25%, we just do vat_rate = 0.25
and this will update the value in our algorithm (code).
4. If we didn’t use a variable then we would need to go into our code and manually
change 0.20 to 0.25 which can be time consuming if we are doing many calculations.
5. Line 3 is storing an expression that consists of two operands (50) and (0.20) and the
operator * (multiply). The value of this, which is 10, will be stored in the variable
vat_amount.
6. Line 4 is going to store the calculation for shirt price plus how much the VAT will be
to total_price.
7. Line 5 is printing the string with the variable total price which is converted into string
otherwise it will give an error because of the + sign.
Images for variable
Code
Interpreter window
Suggestion:
In our program we have given the variables static values. This means we have assigned a
value to them. We could create a program where we are taking inputs from the user and
storing those inputs to the variables. This will make our program more dynamic and it
Changing a variable
When we declare a variable and give it a value, we can, anywhere within our program update
this value.
1. name = “Bob”
2. name=”John”
3. print(name)
1. Line 1 is declaring a variable called name that will be assigned Bob
2. Line 2 is updating the variable name and it will be assigned john
3. Line 3 is printing the variable name and it will show the updated name (John)
Output
Valid variable names
When we declare variable names in Python, we have to make sure that they are valid. A valid
variable name will have a combination of letters, numbers and underscore.
Rules:
Allowed:
The name has to start with a letter or an underscore.
Examples:
1.
2.
3.
4.
username
test_score
apple
player_score
Not allowed:
1.
2.
3.
4.
A variable that starts with a number.
Spaces in your variables.
A variables that starts with a special character.
Certain special characters in your variable such as @.
Examples:
1.
2.
3.
4.
13378
1337_score
test score
@52
Tips
1. Variable names are case sensitive so the best advice is to use lowercase for your
variables.
2. Use an underscore for multiple words.
3. Make sure that variables are memorable and they are related to the data it is going to
store or expressions. For example if you were going to store your name then you
would use my_name.
Assignment
You assign a variable to a value by using the equal sign. To test equality we would use == as
this is to check if something is equal to a value.
Data type
To understand data types you need to understand that a variable is a location in memory to
store values or expressions. The memory we are referring to here is your RAM (main
memory). A memory that will lose data when there is no power going to it. Every time we
declare a variable, what we are doing is reserving some space in memory.
Each variable will have a data type, when we declare the data type, the computer will reserve
the appropriate storage space for that data type and it will dictate what can be stored in there.
For example, if we declare a data type of integer, which is whole numbers, the computer will
provide the space in memory for this and only integer (whole numbers: 10, 20 …) data type is
allowed.
Python data type
The reason why Python is an easy language to learn is that we do not have to declare the data
type for the variables as they are automatically assigned. When we assign a value to a
variable, the data type will be automatically assigned. The data type is identified based on the
assignment of data to the variable, the equal sign (=), if you have forgotten. The item to the
left of the equal sign is the variable name and the item to the right of the equal sign is the
value.
Integer
Integers are whole numbers. They are numbers without fractional parts. These are numbers
like: 10, 50, 1,000, 100,000, -100,000 etc…
As you can imagine the computer will use less memory to store integers compared to
numbers that will have fractional parts.
1. cost = 10
2. age = 15
3. cars = 10
1. The examples above shows variables being declared and assigned to different values
2. If you can recall, we mentioned that a variable that will store a number cannot have
quotes. Quotes are only used for string data type.
Uses
I would integers for whole numbers. This could be for example the age of a person in years,
the numbers of cars they have purchased, the numbers houses they own, the numbers kids
they have etc..
String
In Python string is identified by the use of quotes. The quotes can have characters, letters,
numbers etc… You can store string in single quotes or double but you cannot use both in one.
For example:
1. name = “John”
2. name = “John’
1. Line 1 will work perfectly fine however line 2 will not as it is mixing double quotes
and single quote
String manipulation
When we store string in a variable we can then then call that variable or we can manipulate
the string and do lots of different things with it.
Accessing string
1. word = “Hello”
2. letter = word[0]
3. print(word)
Output
1. The example above is going to store “Hello” in the variable word.
2. In line 2 the variable letter is going to store the variable word, 0 index. This is
basically the first letter of the word.
3. Line 3 is going to print this off
Tip:
1. Use [] to access characters in string.
Length
We can also find out the length of the word by using a function called length.
1. name = “Hello”
2. print(len(name))
The output will be 5.
Finding words or letters
We can use the find function to find a word within our string variable or a letter.
Index
1. name = “Hello World”
2. find_letter = name.find(“H”)
3. print(find_letter)
1. When you print the example above it will output 0. The 0 is the index number and
computers count from 0.
2. If you think about it, 0 is H, 1 is e, 2 is l, 3 is l and is l and so on…
3. If you look for a letter that doesn’t exist then it will print -1
Counting letters
1. name = “Hello World”
2. counting_letter = name.count(“H”)
3. print(counting_letter)
1. This will look at the string that is stored in the variable. It will then try to count the
number of letter it has based on what you have asked it to find.
2. In the example above, we are trying to find the letter H.
3. We can improve this example by using the .lower() function to make all the text lower
and using the And operator to find two or three letters that occur.
Example:
1.
2.
3.
4.
name = “Hello World”
name = name.lower()
counting_letter = name.count(“h” and “o”)
print(counting_letter)
1. The example above is going to make the text stored in the variable to lowercase.
2. It will then count the number of times the letter h and o occur in our example.
3. We have used an AND statement so h AND o has to be there.
Count the number of spaces
This could be used to force the user to only use a certain number of spaces.
1. test = “This is going to have too many spaces
2. print(test.count(‘’)
.”
Others you can look through
1.
2.
3.
4.
5.
6.
Start with and end with
Replace function will replace a word or letter
Upper and lower function
Concatenation (+)
Isalnum() check if it is all numbers
Isalpha() checks if all characters in the string are alphabetic
Real
tring summary
Escaping characters
Mention about the type function
Project example
My_name =”F”
Print (my_name)
My_age = 24
Print above
My_age = 24 + 1
better:
my_age = my_age + 1 or my_age += 1
Magic pill
My_age = 24
Magic_pill = 30
Print my_age – magic_pill
Magic_pill cost 300k
Temperature converter
Temp_Fahrenheit = 32
Temp_celcius = 5.0 /9.0 * (temp_Fahrenheit – 32)
Print(temp_celcius)
Know the value before you even run it.
1. What is a function?
Anywhere where you have used a pair of brackets after an instruction in your coding,
you have been using a function. The first function we used was print():
print("I'm an argument")
In the above example, we tell Python the name of the function we want to use (print) and
then in the brackets we put our argument. An argument is the information Python needs
to carry out the function.
When we want to use more than one argument in a function, we separate them with a
comma, as in this example:
1.
2.
name = "Mr Chapman"
print("My name is ", name)
In this example, the string of text is the first argument, and the variable is the second
argument.
It is also possible to have a function that takes no arguments. In this instance, you still
put the brackets after the function name, but leave them empty.
2. Creating a function
We have already used a number of different functions that are built into Python, and you
will no doubt discover many more as you continue your life as a programmer. But there
will also be times where you want to create your own function. It is good practice to
create a function for any piece of code that you will need to use more than once within
your program. This means you don’t have to re-type the whole thing every time you need
to use it, instead you just call your function.
To create a function we use the def keyword, as in this example:
1.
2.
3.
4.
5.
def count(number):
n=1
while n <= number:
print(n)
n = n+1
The def keyword tells Python that we’re creating a new function. We then name the
function, and in the brackets name the argument that the function will use. We then add
all the code to be included in the function indented after the colon. In this example, we’re
creating a variable called n and setting it to equal 1, then starting a loop that will continue
while n is less than or equal to the number variable that was set by our argument. On
each cycle of the loop, we print the value of n, and then add 1 to it. The end result is that
we can now do the following:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
>>> count(10)
1
2
3
4
5
6
7
8
9
10
Python will then count up to the number we supply in the argument.
Over the next couple of tutorials we’ll be creating the final program to finish off this
series, so make sure you check back on Thursday. I’ll be starting a new tutorial series
next week – if there’s anything you’d particularly like to see, let me know.
http://www.openbookproject.net/thinkcs/python/english2e/ch02.html
While loop examples
import random
number = random.randint(1,6)
while True:
choice= input("Would you like to stop")
choice = str(choice)
choice = choice.lower()
if choice == 'y' :
break
elif choice == 'n':
print(number)
import random
timeToStop = "N"
sides = int(input("How many sides would you like to have on your die?"))
while timeToStop != "Y":
number = random.randint(1,sides)
print(number)
timeToStop = input("Would you like to stop?")
print("Thank you for playing")
add the input to it.
3. Creating a list
Another thing we can use the random module for is choosing items from a list. To do
this, we need to set up a list in Python. Setting up a list is very similar to creating a
variable and setting its value. We start out by giving our list a name, followed by an
equals sign. If we were setting up a variable, we would now just type in a value for our
variable. With a list, we use square brackets:
pencilCase = ["Pen", "Pencil", "Ruler", "Rubber", "Fork", "Post-It Notes", "Coffee Stirrer"]
As you can see, I’ve now listed the contents of my own pencil case. If I want to check
what’s in it, I can just type:
print(pencilCase)
And Python tells me:
['Pen', 'Pencil', 'Ruler', 'Rubber', 'Fork', 'Post-It Notes', 'Coffee Stirrer']
If I only want to print a specific item from my list, I put the number the item appears in my
list in square brackets as below:
print(pencilCase[2])
And I get:
Ruler
Note: Python starts counting from zero, so when we asked for item number 2, Python
displayed the third item on the list, ruler. If we wanted Pen to display, we could print
pencilCase[0].
4. Randomly selecting from a list
Now we know how to create and store lists in Python, we can look at how to randomly
select items from them.
In the above example, I created a list of items in my pencil case. If I now wanted to
randomly pick one, I would type:
1.
2.
3.
4.
import random
pencilCase = ["Pen", "Pencil", "Ruler", "Rubber", "Fork", "Post-It Notes", "Coffee Stirrer"]
print(random.choice(pencilCase))
As with the die example above, we must first import the random module to be able to
use functions from it. We then create our list, and to print a random item from it we use
another of random’s built in functions, choice. The output from this code is:
Loop: http://mrchapmanit.com/python-basics-while-loop/
If statements: http://mrchapmanit.com/python-basics-if-statements/
https://docs.python.org/3.3/library/fileinput.html
http://anh.cs.luc.edu/python/hands-on/3.1/handsonHtml/io.html
https://docs.python.org/3/whatsnew/3.0.html
Variables
Concatenation
converting variable data type
MC tutorial
http://voyager.egglescliffe.org.uk/mwc/mukoku/course/view.php?id=16
Keywords
Syntax
Variable
Debug
Code
http://www.openbookproject.net/thinkcs/python/english2e/ch01.html
program idea