Python fundamentals

Python fundamentals#

At its simplest, Python can be used as a calculator. Throughout this workshop, we’re going to be working with traffic data from an automated sensor that measures how many cars passed in a given hour.

Let’s say we’re interested in looking at morning rush hour traffic, which we take to be the 7am and 8am measurements. On the day in question, there were 500 cars at 7am, and 600 cars at 8am. What’s the total? We can use Python to do this addition.

500 + 600

or any other arithmetic, for example, subtracting to get the difference:

600 - 500

This fine, but we can do all this with a calculator app. To start making use of the power of a full-featured programming language like Python, we can assign our data to variables.

Variables#

We assign a value to a variable using the equals sign =. For example, we can track the number of cars at different times:

cars_at_7am = 500
cars_at_8am = 600

From now on, whenever we use cars_at_7am, Python will substitute the value we assigned to it. You can think of it like this: a variable is a name for a value.

Tip

In Python, variable names:

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

This means that, for example:

cars_7am is a valid variable name, whereas 7am_cars is not
cars and Cars are different variables

Types of data#

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

integer numbers
floating point (a.k.a. decimal) numbers, and
text. In programming textual data is called “strings”.

In the example above, variable cars_at_7am has a integer value of 500. If we want to represent a decimal number, we can use a floating point:

speed_mph = 54.3

To create a string, we add single or double quotes around some text. We might want a label for this group of data, for example:

time_of_day = 'morning'

Using Variables in Python#

Once we have data stored with variable names, we can make use of it in calculations. Now we can do the same calculation of the total rush hour traffic, but using variables:

cars_at_7am + cars_at_8am

This makes it more clear what we’re doing.

We can also “add” strings together to create a new string. For example, we might want to make our label more descriptive:

time_of_day + ' rush hour'

'morning rush hour'

We can also store the results of calculations in variables:

increase_in_cars = cars_at_8am - cars_at_7am

Built-in Python functions#

You might notice that when we assign a value to a variable, the value is not printed to the screen. To display the value of a variable, we can use the print function:

print(increase_in_cars)

When we want to use a function, referred to as calling the function, we type its name, followed by parentheses. The parentheses are important: if you leave them off, the function doesn’t actually run! Often you will include values or variables inside the parentheses for the function to use. These are called arguments, and including them is called passing arguments to a function. In the case of print, we pass the increase_in_cars variable as an argument to the print function, and print displays the value increase_in_cars.

We can display multiple things at once by passing multiple arguments:

print("Cars at 7 AM:", cars_at_7am)

Cars at 7 AM: 500

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(400))
print(type("morning"))

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

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

print('Total rush hour traffic:', cars_at_7am + cars_at_8am)

Total rush hour traffic: 1100

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 total rush hour traffic in its own variable:

total_rush_hour = cars_at_7am + cars_at_8am
print('Cars at 7 AM:', cars_at_7am, 'Total rush hour:', total_rush_hour)

Similar to above, the expression cars_at_7am + cars_at_8am is evaluated to 1100, and then this value is assigned to the variable total_rush_hour (i.e. the sticky note total_rush_hour is placed on 1100). At this point, each variable is “stuck” to completely distinct and unrelated values.

Let’s now change cars_at_7am:

cars_at_7am = 600
print('Cars at 7 AM is now:', cars_at_7am, 'and total rush hour is still:', total_rush_hour)

Since total_rush_hour doesn’t “remember” where its value comes from, it is not updated when we change cars_at_7am.

Challenge: Calculating Traffic Statistics

You have traffic data for three hours: 450 cars at 9am, 550 cars at 10am, and 500 cars at 11am.

Create variables to store these three values.
Calculate and print the total traffic across all three hours.
Calculate and print the average traffic per hour (hint: divide the total by 3).

Challenge: Using Print with Labels

Create variables for the traffic counts at 7am (500) and 8am (600). Then use print() to display a message that shows both values with descriptive labels. The output should look like:

Traffic at 7am: 500
Traffic at 8am: 600

Challenge: Identifying Data Types

What are the data types of the following variables? Use the type() function to check your answers.

hour = 7
traffic_count = 500
location = 'Highway 101'
average_speed = 55.5