Variables

Objectives

• Understand the concept of variables and their role in programming.
• Grasp the dynamic typing system in Python.
• Follow Python's naming conventions when naming variables.
• Learn how to display output using the print() function.
• Know how to obtain and process user input.
• How to check the data type of a variable.
• Definition of a constant and how to use with Python
• Gain practical experience by completing programming tasks related to variables and user input.

In the previous section we set up the Python environment and our IDE for writing Python programs. Here we'll run through one of the key features for all programming languages, variables and constants.

What is a program?

A program, in the context of computers and technology, is like a set of instructions that tells a computer what to do. It's a bit like giving directions to a friend, but instead of telling your friend how to get from one place to another, you're telling the computer how to perform specific tasks.

Imagine you have a robot, and you want that robot to make you a sandwich. You can't just tell the robot, "Make me a sandwich." The robot needs step-by-step instructions, like:

1. Go to the kitchen.
2. Open the refrigerator.
3. Take out the bread, cheese, and lettuce.
4. Put two slices of bread on a plate.
5. Put some cheese and lettuce on one slice of bread.
6. Put the other slice of bread on top.
7. Close the sandwich.
8. Bring it to me.

Each of these steps is like a line of code in a program. A program is a collection of these instructions that a computer can understand and follow to complete a specific task. The computer reads and executes these instructions in order, just like our robot friend follows the steps to make a sandwich. It's the language computers understand to perform the tasks we want them to do.

The syntax of the instructions will differ between programming languages but essentially all have the following components:

• Input: Get data from the keyboard, a file, the network, or some other device.
• Output: Display data on the screen, save it in a file, send it over the network, etc.
• Mathematical and logical operators: To perform basic mathematical operations like addition and multiplication and logical operations such as AND, OR and NOT.
• conditional execution: Check for certain conditions and run the appropriate code.
• Repetition: Perform some action repeatedly, usually with some variation.

Every program you've ever run will be made up of these type of instructions and these will be explored in this and subsequent sections.

Example 1:

# Example 1
name = "Alice"
age = 21
PI = 3.14

print("Name:", name)
print("Age:", age)
print("PI:", PI)

user_input = input("Enter something: ")
print("You entered:", user_input)
print("Data type of user input:", type(user_input))

Read the code and predict what will happen when we run this program, in particular:

• line 1: What will be stored in the variable name?
• line 6: What do you think print("Age:", age) will display?
• line 9: What will happen when the program reaches this line?"
• line 11: What is the purpose of type(user_input)?

Copy the code into your text editor, run the code to see if your were right!

Variables

In the example program, line 1 assigns the value "Alice" to a variable named name.

A variable is a foundational concept for all programming languages. It is a container, or a label, used to store data in our programs. Lines 2 and 3 assign further variables named age and pi.

During the course of a program it is likely the value of that data item will change, it varies, hence it is known as a variable.

# Example 1
name = "Alice"
age = 21
PI = 3.14

print("Name:", name)
print("Age:", age)
print("PI:", PI)

user_input = input("Enter something: ")
print("You entered:", user_input)
print("Data type of user input:", type(user_input))

The opening lines of our program assigns data to three different variables, they contain a different type of data:

line	identifier	value	type
1	name	"Alice"	string
2	age	21	integer
3	PI	3.14	float

Python knows about four primitive types of data:

• Integer: whole numbers, positive and negative
• Floating point numbers: a real number with a decimal point and fractional component
• Boolean: either true or false
• String: any set of characters

Comments

A comment is text in your code that will be ignored by the Python interpreter. Comments can be on a single line when preceded by the # symbol:

# This is a comment, and will be ignored by the interpreter

It's important to recognise the difference between these different data-types. In some programming languages you have to declare the type of data that is expected in a variable, this is known as static data typing, and it is fixed before the code is compiled. For example in C# you would declare these variables as:

string name = "Simon";
int age = 34;
bool plays_piano = true;
float hourly_rate = 34.99;

Python takes a different approach. It is a dynamically typed language. This means the value assigned to the variable will determine the data type for that variable as the program is running. So the data type of the variable is determined by the value it holds.

Identifiers

The names, or identifiers, for each helps to describe the data they each contain. We could have named them as follows:

a = "Alice"
b = 21
c = 3.14

But, the name a does not tell the reader anything about the data being stored in the variablae named a. The name given to the variable should be descriptive of the item of data contained there so it makes sense when you, or others, read your code. This makes the code easier to read and understand.

Tip

When giving a name to a variable, think carefully about what to use. Make if descriptive of the data being stored in the variable.

There are guidelines/rules for naming variables but for now remember they must not start with a digit, nor can they use another term used as a keyword in the language. Python allows for underscores to be used in variable names e.g. is_student but not the hyphen e.g. is-student.

Finally, variable names are case-sensitive, so age is different to Age.

Python Keywords

Python keywords

and, as, assert, break, class, continue, def, del, elif, else, except, finally, for, from, global, if, import, in, is, lambda, nonlocal, not, or, pass, raise, return, try, while, with, yield.

None of these can be used as the name for a variable. To see the latest set of keyboards enter print(keyword.kwlist) in the interactive environment.

Naming style for variables

Python has a style for naming of variables. When an identifier is made up of multiple words separate each word with an underscore:

is_student = True
cumulative_sum_of_numbers = 50

Info

This naming style is called "snake-case". There is extensive documentation for how to write Python code, a style guide known as PEP8. You can read about PEP8 here https://pep8.org

Values are assigned to variables using the assignment operator: =, as shown in the examples above. Variables can be re-assigned with new values thus their value can change during the course of the program. This is why they are called variables.

# Example 1
name = "Alice"
age = 21
PI = 3.14
name = "Imran"  # the value of name has been changed

Note the assignment operator: =. It's important not to confuse this with the mathematical symbol for "equals". Try to get into the habit when reading code and encountering e.g. age = 42 to say "age is assigned the value 42". Also make sure you get it the right way round, 42 = age will not work!

Simple Output

# Example 1
name = "Alice"
age = 21
PI = 3.14

print("Name:", name)
print("Age:", age)
print("PI:", PI)

user_input = input("Enter something: ")
print("You entered:", user_input)
print("Data type of user input:", type(user_input))

To get data displayed on the screen we use the print() function. On it's own this command will just print a new empty line, but elements, or more formally arguments, can be placed inside the brackets. These arguments will then be displayed on the screen.

Having declared and initialised the three variables, we make successive calls to print() passing in a different variable each time to display the values of each variable.

Getting input from the user

# Example 1
name = "Alice"
age = 21
PI = 3.14

print("Name:", name)
print("Age:", age)
print("PI:", PI)

user_input = input("Enter something: ")
print("You entered:", user_input)
print("Data type of user input:", type(user_input))

Assigning data to a variable can be hard-coded in your program or obtained externally such as a user entering data at the keyboard.

To get data from our user we call the input() function, line 11

This function displays the string in brackets and waits for user to type something, this is then assigned to a variable, here named user_input.

All data entered from the keyboard is a string

The data type of the input from the keyboard will always be a string so if we need that data to behave as a integer, a float or some other type then it will need to be cast to that type using the appropriate function. For example, the following will generate an error as we are trying to subtract \(1\) from the string entered by the user. You cannot subtract \(1\) from, say, "15"!

age = input('Enter you age: ")
age = age - 1                   # this generates an error!

Instead, we have to cast it to an integer:

age = input("Enter your age: ")
age = int(age)
age = age - 1

This will work, though it can be shortened by wrapping the int() function around the input():

age = int(input("Enter your age: "))
age = age + 1

• To convert the input to a float i.e. a number with a decimal point such as \(3.14\), use the float() function.

• To convert a numeric value to a string using the str() function,

Checking the data type

# Example 1
name = "Alice"
age = 21
PI = 3.14

print("Name:", name)
print("Age:", age)
print("PI:", PI)

user_input = input("Enter something: ")
print("You entered:", user_input)
print("Data type of user input:", type(user_input))

If you want to check the type of data held by a variable we can use the type() function:

In line 12 we print to the screen the data type of user_input. It will, of course, be a string.

Constants

# Example 1
name = "Alice"
age = 21
PI = 3.14

print("Name:", name)
print("Age:", age)
print("PI:", PI)

user_input = input("Enter something: ")
print("You entered:", user_input)
print("Data type of user input:", type(user_input))

The data on line 4, for the value of pi (\(\pi\)) should not be changed. It's value should remain the same. We call it a constant.

Like a variable a constant is a name given to an item of data. The difference is the value of that item should not change throughout the course of the program.

Warning

Unlike other programming languages Python does not have a strict syntax for constants - this is unfortunate. To get round this we simply declare our constants using upper case identifiers. Yes, you might make a mistake and inadvertently reassign that constant value but if you stick to the convention it should be obvious that the data item is to be treated as a constant value rather than a variable.

Formal vs natural languages

When we think of a language we probably think of the language people speak be that English, French or arabic etc.. These are known as natural languages, no-one sat down and designed the language, they have evolved naturally over time (and are continuing to evolve).

A formal language is a language that has been designed by people to be used in specific circumstances e.g. mathematical expressions and notation or chemical symbols to denote the molecular structure of different chemicals. Similarly programming languages in computer science.

These formal languages have a strict syntax relating to the tokens of the language and how they can be organised. For example, in mathematical notation \(4 + 7 = 11\) combines five tokens of the language into a meaningful expression. By contrast, \(4 += 7\) does not! The tokens might be valid but the rules do not permit one operator to follow another in this way. It is a syntax error.

The Python interpreter

Python is an interpreted language. This means it does not produce a separate executable file but works its way through your code file and executing it on the fly.

The Python Interpreter

How Python works through the code you write and produces the output is a complicated process and not required for the GCSE - you might like to return to this section at another time. But, do remember Python is an interpreted language.

First, Python (the interpreter) will analyse your code and check it for any syntax errors and make sure the rules for indentation have been followed correctly. It carries out a lexical analysis and the divides the source code into a number of tokens which are passed to the parser which generates the Abstract Syntax Tree (AST). The AST is converted into byte codes which can then be saved to a file with a .pyc extension.

Next, the Python interpreter will launch its Python Virtual Machine (PVM) which converts the byte codes into the binary data required by the target computer. This way, your Python code will run on Windows, Linux or MacOS.

Some programming languages will, by default, produce a compiled executable which, on Windows, will be a file with the .exe extension. This means you can create your program and distribute the .exe for another user to run. This file will only be compatible for the target platform e.g. Windows, or MacOS. To distribute your Python program you need to provide the code and your user will have to have Python installed on their machine to run your code.

Climate Quest Project

Throughout this topic we'll be working on a large scale project: Climate Quest. In this project a player embarks on a journey to combat the effects of climate change by making decisions that impact the environment. Each choice affects the outcome of the game, emphasizing the importance of individual actions in addressing climate change.

Go to task 1 - Declaring variables

Questions

1. What is a variable in programming?

A container used to store data that can change during program execution.

A function used to control program flow.

A mathematical expression used to perform calculations.

A keyword used to organize and manage data effectively.

In programming, a variable acts as a container that stores data, such as numbers or strings, which can be modified as the program runs. This allows the program to work with and manipulate different data values.

Submit

2. Which of the following best describes the concept of a constant?

A container used to store data that can change during program execution.

A mathematical expression used to perform calculations.

A value that remains fixed throughout the program.

A keyword used to control program flow.

A constant is a type of variable that is assigned a value once and does not change during the execution of the program. Constants are used to represent fixed values that should not be altered.

Submit

3. Python is a dynamically typed language. What does this mean in terms of variable declaration and data types?

The ability to change a variable's value during program execution.

The requirement to declare variable types before using them.

The process of converting one data type to another.

The determination of a variable's data type based on the value it holds.

In Python, you do not need to explicitly declare the data type of a variable. The type is determined automatically based on the value assigned to it. This feature makes Python a dynamically typed language, allowing for more flexibility.

Submit

4. What is the purpose of the print() function in Python?

To perform mathematical calculations.

To obtain user input.

To display output on the screen.

To define constants.

The print() function is used in Python to display output, such as text or the value of variables, on the screen. This is a basic way to show results and debug programs.

Submit

5. Which of the following is not a legal name for a variable?

_my_name

my name

my_name

myName

In Python, variable names cannot contain spaces. The name my name is illegal because it contains a space, which would cause a syntax error. Instead, use underscores (_) or other styles like my_name or myName.

Submit

6. How can you obtain user input in Python?

Using the input() function.

Using the print() function.

Using the cast() function.

Using the get_input() function.

The input() function is used to capture input from the user in Python. It pauses program execution until the user provides input, which can then be stored in a variable for further processing.

Submit

7. How can you convert a string to an integer in Python?

Using the convert_to_int() function.

Using the str_to_int() function.

Using the int() function.

Using the parse_int() function.

The int() function in Python is used to convert a string that represents a number (e.g., 123) into an integer. This is useful when you need to perform arithmetic operations on values received as strings.

Submit

8. How can you check the data type of a variable in Python?

Use the type() function to determine the data type of a variable in Python.

Convert the variable to a string and then check its length to identify its data type.

Use the typeof() function to inspect the data type of a variable.

Create a new variable and assign it the value of the original variable, then check its data type using the type() function.

The type() function returns the data type of a variable, such as int, str, or list. This function is essential for debugging and understanding how different variables are being used in a program.

Submit

9. Which naming style is recommended for variables in Python?

camelCase

PascalCase

snake_case

kebab-case

The snake_case naming style, where words are separated by underscores (e.g., my_variable), is the preferred convention for naming variables in Python. This style improves readability and consistency in your code.

Submit

10. What is PEP8, and why is it important in Python programming?

PEP8 is a programming language, essential for writing Python code efficiently.

PEP8 is a set of guidelines for writing clean, readable Python code, important for maintaining code quality and consistency.

PEP8 is a Python package used for handling web development tasks, crucial for building web applications.

PEP8 is a Python Enhancement Proposal that provides coding conventions for Python code, ensuring consistency and readability across projects.

PEP8 is a widely accepted style guide that outlines best practices for writing Python code. Following PEP8 ensures that your code is clean, readable, and consistent, which is especially important when working on larger projects or collaborating with others.

Submit

Extra Programming Tasks

For each of the following tasks write a program using Python.

Simple Calculator

Create a program that takes two numbers as input from the user and performs basic arithmetic operations like addition, subtraction, multiplication, and division. Save the file as `simple-calculator.py`

Read more ...

Total Marks

A student has taken three tests as part of his final year exam. He scored 65/100, 45/100 and 57/100 respectively. Write a Python program that asks the user to enter the marks, calculate the total and display the total and the average of the marks to the screen. Save the file as `marks.py`.

Read more ...

Currency Converter

Write a Python program that converts an amount in Pound Sterling (GBP) to Euros (EUR). The program should prompt the user for an amount in pounds sterling (£) and convert the entered amount to Euros using a suitable exchange rate e.g. 1GBP = 1.17EUR. The program should print the result to the screen. Save the program as `currency.py`. The following shows example output:

Read more ...