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Programming with Python

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Types, Objects, Values and Variables

Overview

Teaching: 20 min
Exercises: 5 min
Questions

  • How can I store and access basic information in Python?

Objectives

  • Explain what Types, Objects, Values and Variables are with respect to Python

  • Introduce the basic variable types

  • Show how to initialise, access and change variables

Now we have learnt the technical aspects of using the Python interpreter, we can move on to the basics of programming using it. The first thing we’ll learn about is how Python stores and interprets information.

Types, Objects, Values and Variables

Any Python interpreter can be used as a calculator:

3 + 5 * 4

23

This is great but not very interesting. To do anything useful with data, we need to assign its value to a variable otherwise we’d have to write the whole analysis as one long calculation.

But what is a variable? And what is it’s relation to how Python organises the information you give it? This is all covered by four terms:

This probably seems quite abstract but it can be very useful to know the difference as we progress!

Another way of looking at this is in terms of a physical object like a laptop:

A Generic laptop

In this case, the Object is the physical laptop sitting on the desk, the Type of the object is ‘Laptop’, the Value is the bits and pieces that actually make up the laptop (forming a MacBook in this case) and finally the Variable would be who’s laptop it was, e.g. Fred’s laptop.

Assigning Variables

In Python, we can assign an object or value to a variable, using the equals sign =. For example, to assign value 60 to a variable weight_kg, we would execute:

weight_kg = 60

From now on, whenever we use weight_kg, Python will refer back to the Object it points to (in this case an integer with value ‘60’) and effectively substitute it’s value where the variable is.

What’s in a name?

In Python, variable names:

  • can include letters, digits, and underscores
  • cannot start with a digit
  • are case sensitive.

This means that, for example:

  • weight0 is a valid variable name, whereas 0weight is not
  • weight and Weight are different variables

When you run this code to create a variable, Python performs the following actions:

This all happens each time you create a new variable, so if you ran the following code:

a = 43
b = 76.2
c = "Hello"

What happens in the computer’s memory can be shown like this:

Schematic computer memory

where the dark grey boxes are elements of the computer’s memory, the light grey boxes assigned memory with the values inside, the coloured outline is the type and the flags indicate where each variable points to.

Types of data

Python knows various types of data. The four basic ones (not including the collection types) are:

In the example above, variable weight_kg has an integer value of 60. To create a variable with a floating point value, we can execute:

weight_kg = 60.0

And to create a string (as for c above) we simply have to add single or double quotes around some text, for example:

weight_kg_text = "weight in kilograms:"

Single or Double quotes

To indicate a string, you need to enclose it in matching single or double quotes. Python will accept both but it’s generally a good idea to use double quotes as if you tried to assign a string with an apostrope in it using single quotes, you will get an error. Give this a try:

msg = 'All's well'

As you are much more likely to have apostrope’s in a string than double quotes, it’s best to stick with double quotes for strings.

Using Variables in Python

To display the value of a variable to the screen in Python, we can use the print function as we did before:

print(weight_kg)

60

Not that we can display multiple things at once using only one print command:

print(weight_kg_text, weight_kg)

weight in kilograms: 60

To change the value of the weight_kg variable, we have to assign weight_kg a new value using the equals = sign:

weight_kg = 65.0
print('weight in kilograms is now:', weight_kg)

weight in kilograms is now: 65.0

The Difference between Strings and Variables

Which of the following would you use to assign the value of the test1 variable to the test2 variable? Note the difference between a string (indicated by quotes) and a variable name (without quotes).

  1. test2 = "test1"
  2. test2 = test1
  3. "test2" = test1
  4. "test2" = "test1"

Solution

  1. No. This will assign the value "test1" to the test2 variable, not the value of the test1 variable
  2. Yes
  3. No. This will raise an error as it is trying to change the value of a constant string ("test2")
  4. No. Same as for 3

Choosing a Name

Which is a better variable name, m, min, or minutes? Why? Hint: think about which code you would rather inherit from someone who is leaving the lab:

  1. ts = m * 60 + s
  2. tot_sec = min * 60 + sec
  3. total_seconds = minutes * 60 + seconds

Solution

minutes is better because min might mean something like “minimum” (and actually does in Python, but we haven’t seen that yet).

Give it a try

Have a go at creating some variables of the three basic types we’ve been looking at: Integer, Floating Point, Boolean and String. Print each out using the print function.

Key Points

  • Types are how python should interpret data in a memory location (‘object’) and what operations can be performed by it

  • Objects are defined areas of memory that holds data (‘Values’) associated with a Type

  • Values are the actual data/bits in memory interpreted by the ‘Type’

  • Variables are a flag or name for a particular area of memory (‘Object’)

  • Variable names must only contain letters, numbers and underscores. They are case-sensitive.

  • You can assign values to a variable using the = operator

  • The basic (non collection) data types are: integers, floating point numbers, strings and booleans.

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