Coding Conventions

The code must be compatible with the oldest supported version of python as given on the Installation page.

Please run flake8 on your code. If flake8 is installed, you can run this unittest:

$ ase test flake8

It will fail if there are too many flakes in total.

The rules are almost identical to those used by the Docutils project:

Contributed code will not be refused merely because it does not strictly adhere to these conditions; as long as it’s internally consistent, clean, and correct, it probably will be accepted. But don’t be surprised if the “offending” code gets fiddled over time to conform to these conventions.

The project follows the generic coding conventions as specified in the Style Guide for Python Code and Docstring Conventions PEPs, clarified and extended as follows:

  • Do not use “*” imports such as from module import *. Instead, list imports explicitly.

  • Use 4 spaces per indentation level. No tabs.

  • Read the Whitespace in Expressions and Statements section of PEP8.

  • Avoid trailing whitespaces.

  • No one-liner compound statements (i.e., no if x: return: use two lines).

  • Maximum line length is 78 characters.

  • Use “StudlyCaps” for class names.

  • Use “lowercase” or “lowercase_with_underscores” for function, method, and variable names. For short names, joined lowercase may be used (e.g. “tagname”). Choose what is most readable.

  • No single-character variable names, except indices in loops that encompass a very small number of lines (for i in range(5): ...).

  • Avoid lambda expressions. Use named functions instead.

  • Avoid functional constructs (filter, map, etc.). Use list comprehensions instead.

  • Use 'single quotes' for string literals, and """triple double quotes""" for docstrings. Double quotes are OK for something like "don't".


Thus spake the Lord: Thou shalt indent with four spaces. No more, no less. Four shall be the number of spaces thou shalt indent, and the number of thy indenting shall be four. Eight shalt thou not indent, nor either indent thou two, excepting that thou then proceed to four. Tabs are right out.

Georg Brandl

General advice

  • Get rid of as many break and continue statements as possible.

  • Write short functions. All functions should fit within a standard screen.

  • Use descriptive variable names.

Writing documentation in the code

ASE follows the NumPy/SciPy convention for docstrings:

Run flake8 on your code

It’s a good idea to run flake8 on your code (or use a text editor that does it automatically):

$ flake8

Run on your code

Another method of enforcing PEP8 is using a tool such as These tools tend to be very effective at cleaning up code, but should be used carefully and code should be retested after cleaning it. Try:

$ --help


There is a common issue with pep8 where spaces are added around the power operator. Code such as “x**2” should not be changed to “x ** 2”. This issue is not fixed in pep8 as of the time of this writing, but a small change to autopep8 has been effective to prevent this change.