Contents

• Overview
• Variable Name Rules in Python
• Variable Value Assignment
• Variables & Memory
• Python is a Dynamically Typed Language
• Multiple Variable Assignments
• Checking a Variable's Data Type
• Variable Case
• Python Reserved Words
• Practice Problem

Overview

Variables in Python are labels (names) that we assign to represent datum elements. As programmers, we choose what we want to use as variable names, within some constraints based on rules (see the list below). Variable naming is often based on naming standards within a course or company where you're working. Generally speaking, variable naming is based on clear connections between the datum value and the concept it represents. Continuing the example of age, the ideal variable name to store an age is age. We could use any variable name for that value, however using something like x to store age, while valid, is less understandable for those reading or maintaining our code.

Note: The discussion on this page includes a few highlights from the Style Guide for Python Code which contains more extensive coverage of coding conventions for Python code.

• Variable names are case-sensitive. The variables age, Age, AGe, AGE are 4 separate variables.
• Variables must start with an alphabetic character or an underscore.
• Variables cannot start with a number.
• Variables can only contain alphabetic characters, digits 0 thru 9, and the underscore character.

It is important to be aware that Python is dynamically typed. What does this mean? In some programming languages to assign a value to a variable, we must first declare the data type of that variable. That is called static typing, which is also often called safe typing. With this safety in place, if I declare a variable as an int then I cannot assign a value of another data type to that variable.

In Python, on the other hand, we do not declare the data type of our variables. Python dynamically recognizes the data type based on the value we are assigning to the variable. So in our example above, writing age = 25 is interpreted by Python to mean that the variable age is of type integer. This is convenient, but it can be potentially problematic if later in the program we write age = "Young". In a strict-typed language, this would produce an error. This is not an error in Python, age would now contain "Young" as its value and its data type would now be a string. If we did not mean to change the data type of age, we could run into a problem with this reassignment. Here's what this would look like in PyCharm:

Let's take a look at an example of variable assignments for our data entry screen example from the previous page about Data & Data Types. The image below depicts our data entry form filled in by Bob Smith. Also, in my Python code file (Demo.py) I've written out variables to show how that data could be set to variables:

Note the following about the Python code in the Demo.py file shown above:

• The variable names follow the variable naming rules outlined for Python.
• Each datum is assigned to a variable.
• Each variable name chosen represents its datum using understandable and relatable words.
• The three str values are surrounded by quotes, which Python will set the data type of those three variables as strings.
• The variable age is assigned the value 25, so Python will set the data type of the variable to an integer.
• The variable employed is assigned True, which is a boolean value, so that variable will be set to data type boolean. Note that this is not a string, there are no quotes. Python recognizes True and False as boolean values.
• The variable hourly_wage is set to 18.50, which is a decimal value, so Python will set the data type to that variable to float.

Python supports assigning the same value to more than one variable, like this:

x = y = z = 0
print(x)
print(y)
print(z)
person1, person2, person3 = "Bob", "Sally", "Joe"
print(person1)
print(person2)
print(person3)

The output of the above example would look like this:

Based on the example above, answer the following question...

Since Python is dynamically typed, there are times when we may not be sure what data type a particular variable is at any given point in a program. Python has a function called type() that we can use to check the type of a variable. Here is an example of Python's dynamic typing and using the type() function:

x = 10
print(type(x))
x = "Bob"
print(type(x))
x = 3.14
print(type(x))

The output of the above example would look like this:

Note that we dynamically changed the data type of the variable x three times, and each print statement of type(x) demonstrated that x changes each time we assign a value.

Based on the example above, answer the following question...

a = 3000
b = 10.6
a = "Three Thousand"
a = True
a = b
print(type(a))

< class 'float' >

As indicated previously, Python is case-sensitive. When we are creating variables for data values we have some choices on how we decide to apply the case to those variables. For example, we have used age as a variable several times now, which is a simple and clear variable name. What if we wanted to create a variable like aggregatemonthlysales? It is a descriptive variable name but can be hard to read. There are a few ways to help the readability of variables like this multi-word variable. Each of these approaches has names:

The Python language has a set of reserved words which are used strictly by the language. When we create variables, we cannot use any of these reserved words. The following table provides a list of these reserved words (current as of Python 3.9):