01-intro.md

Python Fundamentals

How to use these notebooks

Note that links in these notes may be broken, please see the course notes here for correct links etc.

Stop! When you see the header "solution"

When you read a challenge asking you to do something, don't read beyond the next header that reads "Solution". Insert a new code cell below the question and attempt to solve it yourself before reading on.

Remember to apply PRIMM

When you see a piece of code, remember:

1. Before you do anything else, predict what the output is going to be.
2. Then run it, and compare the output to your prediction.
3. You can begin to investigate, especially if your guess is way off - what information can you find in the notes, or online?
4. Once you've tried to understand how the code works, try to modify it and make some changes.
5. Then, you're ready to start making your own code!

We will mainly be cycling through stages 1 and 2, with some of the later stages coming in as you build more knowledge.

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:

weight_kg = 60

From now on, whenever we use weight_kg, 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:

• weight0 is a valid variable name, whereas 0weight 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. 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 print function:

print(weight_lb)
print(patient_id)

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(patient_id, 'weight in kilograms:', weight_kg)

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(60.3))
print(type(patient_id))

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

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

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

print(weight_kg)

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)

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)

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 2.2 * weight_kg is evaluated to 143.0, and then this value is assigned to the variable weight_lb (i.e. the sticky note weight_lb is placed on 143.0). At this point, each variable is "stuck" to completely distinct and unrelated values.

Let's now change weight_kg:

weight_kg = 100.0
print('weight in kilograms is now:', weight_kg, 'and weight in pounds is still:', weight_lb)

Since weight_lb doesn't "remember" where its value comes from, it is not updated when we change weight_kg.

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

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

Solution

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

• 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.