Coding Standards

The purpose of the page is to describe the standards that are expected of all the code in SunPy. All potential developers should read and abide by the following standards. Code which does not follow these standards closely will not be accepted.

We try to closely follow the coding style and conventions proposed by Astropy.

Language Standard

  • All code must be compatible with Python 3.7 and later. Usage of six, __future__, and 2to3 is no longer acceptable.

  • The new Python 3 formatting style should be used (i.e. "{0:s}".format("spam") instead of "%s" % "spam").

  • The core package and affiliated packages should be importable with no dependencies other than components already in SunPy, the Python Standard Library, and packages already required by SunPy. Adding dependencies to SunPy will be considered but are highly discouraged. Such optional dependencies should be recorded in the setup.cfg file in the extras_require entry.

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.

  • Follow the existing coding style within a file and avoid making changes that are purely stylistic. Please try to maintain the style when adding or modifying code.

  • Following PEP8’s recommendation, absolute imports are to be used in general. We allow relative imports within a module to avoid circular import chains.

  • 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 (expect in

  • Classes should either use direct variable access, or Python’s property mechanism for setting object instance variables.

  • 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.

  • 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.

  • General utilities necessary for but not specific to the package should be placed in the sunpy.utils module.


We enforce a minimum level of code style and we have tools that will automate this step for you.

First step is to install pre-commit:

$ pip install pre-commit

Now you can do:

$ pre-commit run --all-files

which will run the tools on all the files and make the necessary adjustments. This will show up as new changes that you can review before you commit your work.

The other option is to:

$ pre-commit install

which installs a command to git/hooks/pre-commit which will run these tools at the time you do git commit and means you don’t have to run the first command each time.

Documentation and Testing

  • American English is the default language for all documentation strings and inline commands. Variables names should also be based on English words.

  • Documentation strings must be present for all public classes/methods/functions, and must follow the form outlined in the SunPy Documentation Rules page. Additionally, examples or tutorials in the package documentation are strongly recommended.

  • Write usage examples in the docstrings of all classes and functions whenever possible. These examples should be short and simple to reproduce–users should be able to copy them verbatim and run them. These examples should, whenever possible, be in the doctest format and will be executed as part of the test suite.

  • Unit tests should be provided for as many public methods and functions as possible, and should adhere to the standards set in the Testing Guidelines document.

Data and Configuration

  • We store test data in sunpy/data/test as long as it is less than about 100 kB. These data should always be accessed via the and functions.

  • We store data used for examples in the sample-data repository. This data should not be used for unit tests but can be within our documentation.

  • All persistent configuration should use the Global Settings mechanism. Such configuration items should be placed at the top of the module or package that makes use of them, and supply a description sufficient for users to understand what the setting 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, but one can still use the standard warning-catching mechanism and custom warning classes. The warning class should be either class:SunPyUserWarning or inherit from it.

Including C Code

  • C extensions are only allowed when they provide a significant performance enhancement over pure Python, or a robust C library already exists to provided the needed functionality.

  • The use of Cython is strongly recommended for C extensions.

  • If a C extension has a dependency on an external C library, the source code for the library should be bundled with SunPy, provided the license for the C library is compatible with the SunPy license. Additionally, the package must be compatible with using a system-installed library in place of the library included in SunPy.

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

  • C extensions (Cython or otherwise) should provide the necessary information for building the extension.