Reserved Words and Variable Naming Conventions

Python has its own set of reserved words that, in general, should not be chosen as variable names. As we dive deeper into the course, it will become clearer how to apply these reserved words. For now, just be aware that your variable name choices should avoid the words on this list. Otherwise, though, you can choose any variable name you like. It is important to think about how variable names should be chosen in practical applications. To help others understand your work, you should choose variable names that fit their applications. For example:


might reflect the balance contained in a bank account. You will build the skill of sensibly choosing variable names naturally as you work through more programming examples.


Just like the familiar variables x and y in mathematics, we use variables in programming to easily manipulate values. In this section, we introduce the assignment operator =, namespaces, and naming conventions for variables.


Assign Values to Variables

We assign a value to a variable using the assignment operator =. For example, assign the integer 2 to the variable x

x = 2

The assignment operator does not produce any output and so the cell above does not produce any output. Use the built-in function print to display the value assigned to a variable:


> 2


Compute new values using variables and operators:

1 + x + x**2 + x**3

> 15


Use the built-in function type to verify the datatype of the value assigned to a variable:

pi = 3.14159

> float


Naming Conventions

We can use any set of letters, numbers and underscores to make variable names however a variable name cannot begin with a number. There are many different kinds of naming conventions and we refer to the Style Guide for Python Code (PEP8) for a summary.

In this book we use lower_case_with_underscores variable names and single lowercase letter variable names such x . It is good practice to use descriptive variable names to make your code more readable for other people.

For example, the distance from Vancouver to Halifax along the Trans-Canada Highway is approximately 5799 kilometres. We write the following code to convert this value to miles:

distance_km = 5799
miles_per_km = 0.6214
distance_miles = distance_km * miles_per_km

> 3603.4986


Names to Avoid

It is good practice to use variable names which describe the value assigned to it. However there are words that we should not use as variable names because these words already have special meaning in Python.


Reserved Words

Summarized below are the reserved words in Python 3. Python will raise an error if you try to assign a value to any of these keywords and so you must avoid these as variable names.

False class finally is return
None continue for lambda try
True def from nonlocal while
and del global not with
as elif if or yield
assert else import pass break
except in raise    


Built-in Function Names

There are several functions which are included in the standard Python library. Do not use the names of these functions as variable names otherwise the reference to the built-in function will be lost. For example, do not use sum, min, max, list, or sorted as a variable name.

Source: Patrick Walls,
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Last modified: Tuesday, February 16, 2021, 2:12 PM