Let's look at some terrible Python code...

Hey there,

Check out this Python snippet newsletter member Cliff sent me (screenshot):

Cliff attended a Python workshop the other day and the instructor there used this as a code example.

I almost spit my breakfast oatmeal across the table when I heard this was used in somebody's class to teach people Python...

Here's why this is a TERRIBLE example for teaching the basics of object-oriented programming in Python:

- class Customer() <-- Why the empty parentheses? Either extend from object on Python 2 or leave them out on Py 3.

- The formatting is way off. What's the deal with all this extra whitespace around assignment operators?

- The random "print" call in there...

- Confusing naming: What's a "customerdet"? A typo for "customer debt" maybe?

I guess it stands for "customer details", but the fact that I had to guess shows how cryptic these method names are...

Also, "cashmethod" anyone?

- And lastly, the elephant in the OOP shop:

Separation of Concerns Violations

You see, this class makes heavy use of the "input()" method to read interactive user input from the command-line.

Let me explain why this is a bad idea:

In my mind, this Customer class shouldn't *care* about whether its data comes from terminal input, or a file, or from the network—

All it needs to know are the actual values for the customer's name, the payment method, and so on.

This is a "model" class that serves as a data store and operations on that particular data and it should NOT concern itself with parsing user input.

(Also note the fact that this class does ZERO input validation or error handling.)

If these different concerns get mixed up we end up with a hot mess that's hard to test:

To write automated tests for this Customer class we'll have to override "input()" or replace it with a mock object.

And as a result, our test cases become more complex and difficult to maintain.

That's a pretty hefty downside.

The fact that someone used this code as an example of good Python code in their "teaching materials" boggles my mind...

There's a lot of demand for Python training right now.

And not everyone who's teaching knows what they're doing.

So if you're going through someone's Python tutorial, book, or course and it doesn't seem to make much sense—it may not be your fault at all.

Happy Pythoning!

— Dan Bader