Most of these recommendations are copied verbatim from the book Clean Code by Robert C. Martin.
A function name should describes what the function does. Don’t be afraid to make a name long. A long descriptive name is better than a short enigmatic name. A long descriptive name is better than a long descriptive comment.
Don’t be afraid to spend time choosing a name. Indeed, you should try several different names and read the code with each in place.
Be consistent in your names. Use the same phrases, nouns, and verbs in the function or variables names.
The first rule of functions is that they should be small. The second rule of functions is that they should be smaller than that. Between 1 and 10 lines is usually good.
A function must do one thing, and do it well. This is because we want each function to be transparently obvious. For example, passing a boolean into a function is a truly terrible practice. It loudly proclaims that this function does more than one thing: It does one thing if the flag is true and another if the flag is false!
A function should have no side effects. If it does, your function promises to do one thing, but it also does other hidden things. Sometimes it will make unexpected changes to the variables passed as parameters or to system globals. In either case they are devious and damaging mistruths that often result in strange temporal couplings and order dependencies.
Functions should either do something or answer something, but not both. Either your function should change the state of an object, or it should return some information about that object. Doing both often leads to confusion.
Writing software is like any other kind of writing. When you write a paper or an article, you get your thoughts down first, then you massage it until it reads well. The first draft might be clumsy and disorganized, so you wordsmith it and restructure it and refine it until it reads the way you want it to read.
In R. Martin’s words:
“When I write functions, they come out long and complicated. They have lots of indenting and nested loops. They have long argument lists. The names are arbitrary, and there is duplicated code. But I also have a suite of unit tests that cover every one of those clumsy lines of code.
So then I massage and refine that code, splitting out functions, changing names, eliminating duplication.
In the end, I wind up with functions that follow the rules I’ve laid down. I don’t write them that way to start. I don’t think anyone could.”
We want the code to read like a top-down narrative. Each function must tell a story. And each function should led you to the next in a compelling order.
Don’t Repeat Yourself. Duplicating the same code at, say four locations in your program, is a problem because it bloats the code and will require four-fold modification should the algorithm ever have to change. It is also a four-fold opportunity for an error of omission. Structured programming and Object-oriented programming were invented in part as strategies to avoid duplication.
It is an excellent idea to write tests that check your that your modules and function keep working as they should. It can be as simple as having scripts that just run your code on some examples and check that it does not crash after an upgrade, or following a unit testing methodology (see Getting Started With Testing in Python)