OverviewTeaching: 20 min
Exercises: 10 minQuestions
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.
Any Python interpreter can be used as a calculator:
3 + 5 * 4
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 track the weight of a patient who weighs 60 kilograms by
assigning the value
60 to a variable
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:
weight0is a valid variable name, whereas
Weightare different variables
Types of data
Python knows various types of data. Three common ones are:
- integer numbers
- floating point numbers, and
In the example above, variable
weight_kg has an integer value of
If we want to more precisely track the weight of our patient,
we can use a floating point value by executing:
weight_kg = 60.3
To create a string, we add single or double quotes around some text. To identify and track a patient throughout our study, we can assign each person a unique identifier by storing it in a string:
patient_id = '001'
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 our patient’s weight in pounds as well as kilograms:
weight_lb = 2.2 * weight_kg
We might decide to add a prefix to our patient identifier:
patient_id = 'inflam_' + patient_id
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
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
We can display multiple things at once using only one
print(patient_id, 'weight in kilograms:', weight_kg)
inflam_001 weight in kilograms: 60.3
We can also call a function inside of another
For example, Python has a built-in function called
type that tells you a value’s data type:
<class 'float'> <class 'str'>
Moreover, we can do arithmetic with variables right inside the
print('weight in pounds:', 2.2 * weight_kg)
weight in pounds: 132.66
The above command, however, did not change the value of
To change the value of the
weight_kg variable, we have to
weight_kg a new value using the equals
weight_kg = 65.0 print('weight in kilograms is now:', weight_kg)
weight in kilograms is now: 65.0
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 subject’s weight in pounds in its own variable:
# There are 2.2 pounds per kilogram weight_lb = 2.2 * weight_kg print('weight in kilograms:', weight_kg, 'and in pounds:', weight_lb)
weight in kilograms: 65.0 and in pounds: 143.0
Similar to above, the expression
2.2 * weight_kgis evaluated to
143.0, and then this value is assigned to the variable
weight_lb(i.e. the sticky note
weight_lbis placed on
143.0). At this point, each variable is “stuck” to completely distinct and unrelated values.
Let’s now change
weight_kg = 100.0 print('weight in kilograms is now:', weight_kg, 'and weight in pounds is still:', weight_lb)
weight in kilograms is now: 100.0 and weight in pounds is still: 143.0
weight_lbdoesn’t “remember” where its value comes from, it is not updated when we change
Check Your Understanding
What values do the variables
agehave 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
`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)
Seeing Data Types
What are the data types of the following variables?
planet = 'Earth' apples = 5 distance = 10.5
type(planet) type(apples) type(distance)
<class 'str'> <class 'int'> <class 'float'>
Basic data types in Python include integers, strings, and floating-point numbers.
variable = valueto 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.
print(something)to display the value of
Built-in functions are always available to use.