Writing clean code

Code should be readable, simple, and concise. There are a few things to consider when writing code.

When naming stuff like variables and functions, be clear and concise. Furthermore, if applicable, imply its type. For example, when the variable holds an array, you can append _list to its name (e.g., grade_list vs grades). If it's a boolean, you can prepend the name with is_ or has_.
Use whitespace properly and consistently. For example, in Python, spaces are preferred to tabs and a "tab" is made up of 4 spaces following PEP 8 standard which is a style guide for Python.

This could be inapplicable if you're worrying about how to create the code that works so be sure to apply it when refactoring.