Testing

Simple Interactive Testing of Python functions, and pointers to more advanced testing techniques

There are various strategies for testing Python functions. Each of these is appropriate at a certain stage in your development as a Python programmer.

In this article, we cover simple interactive testing both with IDLE, and without IDLE (using just the python3 or python command a at the command line prompt).

Other articles cover:

• pytest, a simple testing framework that require an extra install
• unittest, a testing framework that is slightly harder to understand for beginners, but is built in to Python

Simple Interactive Testing in IDLE

If you are just getting started with Python, you might be using IDLE as the program through which you interact with Python. IDLE is an “Integrated Development Environment (IDE)” for Python—that is, a tool in which you can create Python programs, make changes to them, save them, and run them to see what they do.

In IDLE, to test a function, you can use the “File->New Window” command to open up a new file in which to write a function such as this one:

def area_rect(length,width):
   return length * width

You can then execute the file by selecting “Run -> Run Module”. This will prompt you to save the file; you should give it a name such as area.py and then save it.

Then you’ll see that in a separate window, the so-called Python Shell window, that the file has been loaded. That looks like this:

Python 3.3.2 (v3.3.2:d047928ae3f6, May 13 2013, 13:52:24) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "copyright", "credits" or "license()" for more information.
>>> ================================ RESTART ================================
>>> 
>>> 

At the >>> prompt, you can type function calls to the area_rect function to test it. That looks like this:

>>> area_rect(10,5)
50
>>> area_rect(2.5,2)
5.0
>>> 

You can see that for each function call, the Python Shell (also known as the Python REPL, for “Read Eval Print Loop”) reads your function call, evaluates it, prints the answer, and then loops back to show you another >>> prompt.

Interactive Testing without IDLE

If you aren’t able to use IDLE for some reason, or simply prefer to use a different editor to create and make changes to files of Python code, it is still possible to do interactive testing as the command line.

Use whatever program code editor you like (e.g. Atom, VSCode, Sublime Text) to create your Python code in a file ending in .py. For example, put this code in a file called area.py:

def area_rect(length,width):
   return length * width

Then, to test it, at the Unix prompt on CSIL, type:

python3 -i area.py

If you are using your own computer, you might need to type either python3 or just plain python, like this:

python -i area.py

This loads the file area.py, and the -i flag means “interactive”—that is, you’ll be given a >>> Python Shell prompt to interact with your Python code after it loads. Here’s what that looks like. In this case, cgaucho $ is the command line prompt. It may look different on your system, and isn’t part of what you type:

cgaucho $ python3 -i area.py
>>> 

At the >>> prompt, you can start typing Python function calls to test your code. You can type quit() when you are finished.

>>> area_rect(3,7)
21
>>> area_rect(3.5,4)
14.0
>>> quit()

Better ways of testing

This way of testing is appropriate when you are first starting with Python. But it is tedious. You have to type in the test cases every single time you make a change to a function, and you can only run one test at a time. So as you make progress as a programmer, you’ll likely want to move on to more powerful ways of testing your code.

A few better ways of testing include:

• pytest
• unittest