Python Fundamentals

Overview

Teaching: 20 min
Exercises: 10 min

Questions

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

• How can I create a new variable in Python?

• How do I use a function?

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

Objectives

• Assign values to variables.

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. In Python, we can assign a value to a variable, using the equals sign =. For example we can capture the gross domestic power per capita for a country by assinging the value ? to a variable gdpPercapUSD:

gdpPercapUSD = 2449

From now on, whenever we use gdpPercapUSD, 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:

• gdpPercap0 is a valid variable name, whereas 0gdpPercap is not
• gdp_percap and GDP_percap 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 gdpPercapUSD has an integer value of 2449. If we want to capture the gdp per capita of the country more precisely we can use a floating point value by executing:

gdpPercapUSD = 2449.008185

To create a string, we add single or double quotes around some text. To keep track of the year we are working with we can store the year in a string:

year = "1952"

Using Variables in Python

Once we have data stored with variable names, we can make use of it in calculations. We may want to store the country’s gdp percapita in US dollars and in British pounds:

gdpPercapGBP = gdpPercapUSD * 0.779063

We could add the year to the label gdpPercap_

columnlabel = 'gdpPercap_' + year

Built-in Python functions

To carry out common tasks with data and variables in Python, the language provides us with several built-in functions. To display information to the screen, we use the print function:

print(gdpPercapUSD)
print(gdpPercapGBP)
print(columnlabel)

2249.008185
1752.119063630655
gdpPercap_1952

When we want to make use of a function, referred to as calling the function, we follow its name by parentheses. The parentheses are important: if you leave them off, the function doesn’t actually run! Sometimes you will include values or variables inside the parentheses for the function to use. In the case of print, we use the parentheses to tell the function what value we want to display. We will learn more about how functions work and how to create our own in later episodes.

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

print(columnlabel , 'for Algeria:' , gdpPercapUSD , '(USD)')

gdpPercap_1952 for Algeria: 2249.008185 (USD)

We can also call a function inside of another function call. For example, Python has a built-in function called type that tells you a value’s data type:

print(type(2249.008185))
print(type(columnlabel))

<class 'float'>
<class 'str'>

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

print('gdpPercap in GBP:', gdpPercapUSD * 0.779063)

gdpPercap in GBP: 1752.119063630655

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

print(gdpPercapUSD)

2249.008185

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

gdpPercapUSD = 3520.610273	
print('The gdp per capita for Angola in USD is:', gdpPercapUSD)

The gdp per capita for Angola in USD is: 3520.610273

Variables as Sticky Notes

A variable in Python is analogous to a sticky note with a name written on it: assigning a value to a variable is like putting that sticky note on a particular value.

Using this analogy, we can investigate how assigning a value to one variable does not change values of other, seemingly related, variables. For example, let’s store the, country Angola’s, gdp per capita GPB in its own variable:

# gdp per capita for Angola
gdpPercapGBP = gdpPercapUSD * 0.779090
print('gdp per capita in GBP', gdpPercapGBP, 'and in USD:', gdpPercapUSD)

gdp per capita in GBP 2742.8722575915695 and in USD: 3520.610273

Similar to above, the expression gdpPercapUSD * 0.779090 is evaluated to 2742.8722575915695, and then this value is assigned to the variable gdpPercapGBP (i.e. the sticky note gdpPercapGBP is placed on 2742.8722575915695). At this point, each variable is “stuck” to completely distinct and unrelated values.

Let’s now change gdpPercapUSD:

gdpPercapUSD = 851.241141
print('gdp percapita in USD is now:', gdpPercapUSD, 'and gdp per capita in GBP is still: ', gdpPercapGBP)

gdp percapita in USD is now: 851.241141 and gdp per capita in GBP is still:  2742.8722575915695

Since gdpPercapGBP doesn’t “remember” where its value comes from, it is not updated when we change gdpPercapUSD.

Check Your Understanding

What values do the variables mass and age have after each of the following statements? Test your answer by executing the lines.

mass = 47.5
age = 122
mass = mass * 2.0
age = age - 20

Solution

`mass` holds a value of 47.5, `age` does not exist
`mass` still holds a value of 47.5, `age` holds a value of 122
`mass` now has a value of 95.0, `age`'s value is still 122
`mass` still has a value of 95.0, `age` now holds 102

Sorting Out References

Python allows you to assign multiple values to multiple variables in one line by separating the variables and values with commas. What does the following program print out?

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

Solution

Hopper Grace

Seeing Data Types

What are the data types of the following variables?

planet = 'Earth'
apples = 5
distance = 10.5

Solution

type(planet)
type(apples)
type(distance)

<class 'str'>
<class 'int'>
<class 'float'>

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.

• Built-in functions are always available to use.