Python Fundamentals

Overview

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

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Any Python interpreter can be used as a calculator:

PYTHON

3 + 5 * 4

OUTPUT

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, the GDP per capita of the UK is approximately $46510. We could track this by assigning the value 46510 to a variable gdp_per_capita:

PYTHON

gdp_per_capita = 46_510

From now on, whenever we use gdp_per_capita, Python will substitute the value we assigned to it. In layperson’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:

• gdp_per_capita_2021 is a valid variable name, whereas 2021_gdp_per_capita is not.
• gdp_per_capita and GDP_per_capita are different variables.

Types of data

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

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

In the example above, variable gdp_per_capita has an integer value of 46510. If we want to more precisely track the GDP of the UK, we can use a floating point value by executing:

PYTHON

gdp_per_capita = 46_510.28

To create a string, we add single or double quotes around some text. We could track the language code of a country by storing it as a string:

PYTHON

uk_language_code = 'eng'

Using Variables in Python

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Once we have data stored with variable names, we can make use of it in calculations. We may want to store our country’s raw GDP value as well as the GDP per capita:

PYTHON

gdp = 67_330_000.0 * gdp_per_capita

We also might decide to add a prefix to our language identifier:

PYTHON

uk_language_code = 'ISO_' + uk_language_code

Built-in Python functions

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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:

PYTHON

print(gdp_per_capita)
print(uk_language_code)

OUTPUT

46510.28
ISO_eng

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:

PYTHON

print(uk_language_code, 'GDP per capita is USD $', gdp_per_capita)

OUTPUT

ISO_eng GDP per capita is USD $ 46510.28

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:

PYTHON

print(type(gdp_per_capita))
print(type(uk_language_code))

OUTPUT

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

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

PYTHON

print('GDP in USD $', 67_330_000.0 * gdp_per_capita)

OUTPUT

GDP in USD $ 3131537152400.0

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

PYTHON

print(gdp_per_capita)

OUTPUT

46510.28

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

PYTHON

gdp_per_capita = 46_371.45
print('GDP per capita is now:', gdp_per_capita)

OUTPUT

GDP per capita is now: 46371.45

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’s GDP in its own variable:

PYTHON

# There are 67330000 people in the UK
gdp = 67330000 * gdp_per_capita
print('GDP per capita: USD $', gdp_per_capita, 'Raw GDP: USD $', gdp)

OUTPUT

GDP per capita: USD $ 46371.45 Raw GDP: USD $ 3122189728500.0

Everything in a line of code following the ‘#’ symbol is a comment that is ignored by Python. Comments allow programmers to leave explanatory notes for other programmers or their future selves.

Similar to above, the expression 67_330_000 * gdp_per_capita is evaluated to 3122189728500.0, and then this value is assigned to the variable gdp (i.e. the sticky note gdp is placed on 3122189728500.0). At this point, each variable is “stuck” to completely distinct and unrelated values.

Let’s now change gdp_per_capita:

PYTHON

gdp_per_capita = 45_000.00
print('GDP per capita is now: USD $', gdp_per_capita, 'But raw GDP is still: USD $', gdp)

OUTPUT

GDP per capita is now: USD $ 45000.0 But raw GDP is still: USD $ 3122189728500.0

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

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.

PYTHON

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

Show me the solution

OUTPUT

`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?

PYTHON

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

Show me the solution

OUTPUT

Hopper Grace

Seeing Data Types

What are the data types of the following variables?

PYTHON

planet = 'Earth'
apples = 5
distance = 10.5

Show me the solution

PYTHON

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

OUTPUT

<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.
• Use # some kind of explanation to add comments to programs.
• Built-in functions are always available to use.