• Testing Guidelines

  • Writing Command-Line Scripts

  • C or Cython Extensions

Python Coding Guidelines

This section describes requirements and guidelines.

An Example Python Project

We have created a skeleton Python project which should provide a full introduction to the various recommendations and requirements for the development of Python. The philosophy behind the development of this template was to demonstrate one way to meet the project guidelines and demonstrate a recommended project layout.

The recommended project layout is as follows:

├── docker-requirements.txt
├── docs
│   ├── Makefile
│   └── src
│       ├──
│       ├── index.rst
│       ├── package
│       │   └── guide.rst
│       ├── -> ../../
│       ├── _static
│       │   ├── css
│       │   │   └── custom.css
│       │   ├── img
│       │   │   ├── favicon.ico
│       │   │   ├── logo.jpg
│       │   │   └── logo.svg
│       │   └── js
│       │       └── github.js
│       └── _templates
│           ├── footer.html
│           └── layout.html
├── Makefile
├── Pipfile
├── setup.cfg
├── src
│   └── ska
│       └── skeleton
│           ├──
│           └──
└── tests

Interface and Dependencies

  • All code must be compatible with Python 3.7 and later.

  • The new Python 3 formatting style should be used (i.e. "{0:s}".format("spam") instead of "%s" % "spam"), or use format strings f"{spam}" if variable spam="spam" is in scope.

Documentation and Testing

  • Docstrings must be present for all public classes/methods/functions, and must follow the form outlined by PEP8 Docstring Conventions.

  • Unit tests should be provided for as many public methods and functions as possible, and should adhere to Pytest best practices.

Data and Configuration

  • All persistent configuration should use python-dotenv. Such configuration .env files should be placed at the top of the ska_python_skeleton module and provide a description that is sufficient for users to understand the settings changes.

Standard output, warnings, and errors

The built-in print(...) function should only be used for output that is explicitly requested by the user, for example print_header(...) or list_catalogs(...). Any other standard output, warnings, and errors should follow these rules:

  • For errors/exceptions, one should always use raise with one of the built-in exception classes, or a custom exception class. The nondescript Exception class should be avoided as much as possible, in favor of more specific exceptions (IOError, ValueError, etc.).

  • For warnings, one should always use warnings.warn(message, warning_class). These get redirected to log.warning() by default.

  • For informational and debugging messages, one should always use and log.debug(message).

Logging implementation

There is a standard Python logging module for logging in SKA projects. This module ensures that messages are formatted correctly according to our formatting standards.

For details on how to use the logging module with detailed examples, please refer to:

Coding Style/Conventions

  • The code will follow the standard PEP8 Style Guide for Python Code. In particular, this includes using only 4 spaces for indentation, and never tabs.

  • The import numpy as np, import matplotlib as mpl, and import matplotlib.pyplot as plt naming conventions should be used wherever relevant. from packagename import * should never be used, except as a tool to flatten the namespace of a module. An example of the allowed usage is given in Acceptable use of from module import *.

  • Classes should either use direct variable access, or Python’s property mechanism for setting object instance variables. get_value/set_value style methods should be used only when getting and setting the values requires a computationally-expensive operation. Properties vs. get_/set_ below illustrates this guideline.

  • Classes should use the builtin super() function when making calls to methods in their super-class(es) unless there are specific reasons not to. super() should be used consistently in all subclasses since it does not work otherwise. super() vs. Direct Calling illustrates why this is important.

  • Multiple inheritance should be avoided in general without good reason.

  • files for modules should not contain any significant implementation code. can contain docstrings and code for organizing the module layout, however (e.g. from submodule import * in accord with the guideline above). If a module is small enough that it fits in one file, it should simply be a single file, rather than a directory with an file.

Unicode guidelines

For maximum compatibility, we need to assume that writing non-ASCII characters to the console or to files will not work.

Including C Code

  • When C extensions are used, the Python interface for those extensions must meet the aforementioned Python interface guidelines.

  • The use of Cython is strongly recommended for C extensions. Cython extensions should store .pyx files in the source code repository, but they should be compiled to .c files that are updated in the repository when important changes are made to the .pyx file.

  • In cases where C extensions are needed but Cython cannot be used, the PEP 7 Style Guide for C Code is recommended.


This section shows examples in order to illustrate points from the guidelines.

Properties vs. get_/set_

This example shows a sample class illustrating the guideline regarding the use of properties as opposed to getter/setter methods.

Let’s assuming you’ve defined a ':class:`Star`' class and create an instance like this:

>>> s = Star(B=5.48, V=4.83)

You should always use attribute syntax like this:

>>> s.color = 0.4
>>> print(s.color)

Using Python properties, attribute syntax can still do anything possible with a get/set method. For lengthy or complex calculations, however, use a method:

>>> print(s.compute_color(5800, age=5e9))

super() vs. Direct Calling

By calling super() the entire method resolution order for D is precomputed, enabling each superclass to cooperatively determine which class should be handed control in the next super() call:

# This is safe

class A(object):
    def method(self):
        print('Doing A')

class B(A):
    def method(self):
        print('Doing B')

class C(A):
    def method(self):
        print('Doing C')

class D(C, B):
    def method(self):
        print('Doing D')
>>> d = D()
>>> d.method()
Doing D
Doing C
Doing B
Doing A

As you can see, each superclass’s method is entered only once. For this to work it is very important that each method in a class that calls its superclass’s version of that method use super() instead of calling the method directly. In the most common case of single-inheritance, using super() is functionally equivalent to calling the superclass’s method directly. But as soon as a class is used in a multiple-inheritance hierarchy it must use super() in order to cooperate with other classes in the hierarchy.


For more information on the the benefits of super(), see

Acceptable use of from module import *

from module import * is discouraged in a module that contains implementation code, as it impedes clarity and often imports unused variables. It can, however, be used for a package that is laid out in the following manner:


In this case, packagename/ may be:

A docstring describing the package goes here
from submodule1 import *
from submodule2 import *

This allows functions or classes in the submodules to be used directly as rather than If this is used, it is strongly recommended that the submodules make use of the __all__ variable to specify which modules should be imported. Thus, might read:

from numpy import array, linspace

__all__ = ['foo', 'AClass']

def foo(bar):
    # the function would be defined here

class AClass(object):
    # the class is defined here

This ensures that from submodule import * only imports ':func:`foo' and ':class:`AClass', but not ':class:`numpy.array' or ':func:`numpy.linspace'.

. _python-packaging:

Packaging & Dependencies

SKA python packages use Poetry for packaging & dependency management.


The present document’s coding guidelines are derived from project Astropy’s coding guidelines.