Python Fundamentals

Overview

Teaching: 20 min
Exercises: 5 min

Questions

• What basic data types can I work with in Python?

• How can I create a new variable in Python?

• Can I change the value associated with a variable after I create it?

Objectives

• Assign values to variables.

Preparation

For this lesson we will begin using the Python programming language.
For examples used in class:

1. Log onto the Hopper Desktop using the browser-based Dashboard, launch a terminal window and create a new directory in your home directory called: swc-python

2. Copy the following two files from /home/lortizur/clim680/ to your new directory: python-novice-inflammation-code.zip python-novice-inflammation-data.zip

3. Unzip the two files You should see two new folders called data and code in the swc-python directory

4. Load a Python module so that you will have access to the language and its interpreter. If you already have a Python environment set up, you may load/use that. If you do not or are unsure, you may run the following command:

$ module load python

To start an interactive Python session, type:

$ python

Variables

Any Python interpreter can be used as a calculator:

3 + 5 * 4

23

This is fine, but not very interesting. To do anything useful with data, we need to assign its value to a variable. In Python, we can assign a 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 substitute the value we assigned to it. In layman’s terms, a variable is a name for a value.

In Python, variable names:

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

This means that, for example: - weight1 is a valid variable name, whereas 1weight is not. - weight and Weight are different variables.

Types of data

Python knows various types of data. Three common ones are:

• integer numbers
• floating point numbers, and
• strings.

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, we surround the text with single or double quotes, for example:

weight_kg_text = 'weight in kilograms:'

Using Variables in Python

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

print(weight_kg)

60.0

We can display multiple things at once using only one print command:

print(weight_kg_text, weight_kg)

weight in kilograms: 60.0

Moreover, we can do arithmetic with variables right inside the print function:

print('weight in pounds:', 2.2 * weight_kg)

weight in pounds: 132.0

The above command, however, did not change the value of weight_kg:

print(weight_kg)

60.0

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

Python is an Object-Oriented Programming Language

Everything in Python, variables, functions, libraries,… everything is an object. And all objects are instances of classes and have attributes. These are characteristics of object-oriented programming (OOP).

• A class has a set of common characteristics, e.g. in biology, “humans” can be thought of as a class.
  • Classes may contain other classes, e.g., the class “mammals” contains the class “humans”.
• An instance of a class is an object.
  • A specific person is an instance of “humans”. In OOP, you would be an object within the class “humans”.
• An object has attributes, and different objects in the same class may or may not share attributes.
  • You may have the attribute “tall” or “short”, “young” or “middle-aged” or “old”, etc.
• Additionally, some of the attributes of an object can be procedures the object is capable of - these are called methods.
  • Some of your attributes could be things you can do (i.e., methods), like “juggle” or “ride a bike” or “sing”.

You may be surprised at all the attributes and methods possessed a simple object, like the variable weight_kg you defined above. It might seem like a basic floating point number with value 60.0, and no more. But because of the class it belongs to in Python, it is actually so much more.

We will look into the characteristics of objects later as we learn more about Python.

Check Your Understanding

What values do the variables mass and age have after each statement in the following program? Test your answers by executing the commands.

mass = 47.5
age = 122
mass = mass * 2.0
age = age - 20
print(mass, age)

Solution

95.0 102

Sorting Out References

What does the following program print out?

first, second = 'Hopper', 'Grace'
third, fourth = second, first
print(third, fourth)

Solution

Grace Hopper

Who is Grace Hopper?

Key Points

• Basic data types in Python include integers, strings, and floating-point numbers.

• Use variable = value to assign a value to a variable in order to record it in memory.

• Variables are created on demand whenever a value is assigned to them.

• Use print(something) to display the value of something.