Coding Guidelines

This section describes requirements and guidelines that should be followed both by the core package and by coordinated packages, and these are also recommended for affiliated packages.

Interface and Dependencies

  • All code must be compatible with the versions of Python indicated by the python_requires key in the setup.cfg file of the core package.

  • Usage of six, __future__, and 2to3 is no longer acceptable.

  • f-strings should be used when possible, and if not, Python 3 formatting should be used (i.e. "{0:s}".format("spam")) instead of the % operator ("%s" % "spam").

  • The core package should be importable with no dependencies other than components already in the Astropy core, the Python Standard Library, and NumPy >=1.20 or later.

  • Additional dependencies - such as SciPy, Matplotlib, or other third-party packages - are allowed for sub-modules or in function calls, but they must be noted in the package documentation and should only affect the relevant component. In functions and methods, the optional dependency should use a normal import statement, which will raise an ImportError if the dependency is not available. In the astropy core package, such optional dependencies should be recorded in the setup.cfg file in the extras_require entry, under all (or test_all if the dependency is only needed for testing).

    At the module level, one can subclass a class from an optional dependency like so:

        from opdep import Superclass
    except ImportError:
        warn(AstropyWarning('opdep is not present, so <functionality>'
                            'will not work.'))
        class Superclass(object): pass
    class Customclass(Superclass):
  • General utilities necessary for but not specific to the package or sub-package should be placed in a packagename.utils module (e.g. astropy.utils for the core package). If a utility is already present in astropy.utils, packages should always use that utility instead of re-implementing it in packagename.utils module.

Documentation and Testing

  • Docstrings must be present for all public classes/methods/functions, and must follow the form outlined in the Writing Documentation document.

  • 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

  • Packages can include data in a directory named data inside a subpackage source directory as long as it is less than about 100 kB. These data should always be accessed via the get_pkg_data_fileobj() or get_pkg_data_filename() functions. If the data exceeds this size, it should be hosted outside the source code repository, either at a third-party location on the internet or the astropy data server. In either case, it should always be downloaded using the get_pkg_data_fileobj() or get_pkg_data_filename() functions. If a specific version of a data file is needed, the hash mechanism described in should be used.

  • All persistent configuration should use the Configuration System (astropy.config) 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 AstropyUserWarning or inherit from it.

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

The logging system uses the built-in Python logging module. The logger can be imported using:

from astropy import log

Coding Style/Conventions

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

    • astropy itself enforces this style guide using the black code formatter.

    • We recognize that sometimes black will autoformat things in undesirable ways, e.g., matrices. In the cases that black produces undesirable code formatting:

      • one can wrap code the code in # fmt: off and # fmt: on to disable black formatting over multiple lines.

      • or one can add a single # fmt: skip comment to the end of a line to disable black formatting for that line.

      This should be done sparingly, and only when black produces undesirable formatting.

  • Our testing infrastructure currently enforces a subset of the PEP8 style guide. In addition, these checks also enforce isort to sort the module imports.

    • We provide a pre-commit hook which automatically enforces and fixes (whenever possible) the coding style, see Pre-commit for details on how to setup and use this.

    • Alternately, you can manually check and fix your changes by running the following tox command:

      tox -e codestyle
  • Following PEP8’s recommendation, absolute imports are to be used in general. The exception to this is relative imports of the form from . import modname, best when referring to files within the same sub-module. This makes it clearer what code is from the current submodule as opposed to from another.


    There are multiple options for testing PEP8 compliance of code, see Testing Guidelines for more information. See Emacs setup for following coding guidelines for some configuration options for Emacs that helps in ensuring conformance to PEP8.

  • Astropy source code should contain a comment at the beginning of the file (or immediately after the #!/usr/bin env python command, if relevant) pointing to the license for the Astropy source code. This line should say:

    # Licensed under a 3-clause BSD style license - see LICENSE.rst
  • The following naming conventions:

    import numpy as np
    import matplotlib as mpl
    import matplotlib.pyplot as plt

    should be used wherever relevant. On the other hand:

    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 the Acceptable use of from module import * example.

  • 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. The Properties vs. get_/set_ example 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. The super() vs. Direct Calling example below illustrates why this is important.

  • Multiple inheritance should be avoided in general without good reason. Multiple inheritance is complicated to implement well, which is why many object-oriented languages, like Java, do not allow it at all. Python does enable multiple inheritance through use of the C3 Linearization algorithm, which provides a consistent method resolution ordering. Non-trivial multiple-inheritance schemes should not be attempted without good justification, or without understanding how C3 is used to determine method resolution order. However, trivial multiple inheritance using orthogonal base classes, known as the ‘mixin’ pattern, may be used.

  • 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.

  • Command-line scripts should follow the form outlined in the Writing Command-Line Scripts document.

Unicode guidelines

For maximum compatibility, we need to assume that writing non-ASCII characters to the console or to files will not work. However, for those that have a correctly configured Unicode environment, we should allow them to opt-in to take advantage of Unicode output when appropriate. Therefore, there is a global configuration option, astropy.conf.unicode_output to enable Unicode output of values, set to False by default.

The following conventions should be used for classes that define the standard string conversion methods (__str__, __repr__, __bytes__, and __format__). In the bullets below, the phrase “string instance” is used to refer to str, while “bytes instance” is used to refer to bytes.

  • __repr__: Return a “string instance” containing only 7-bit characters.

  • __bytes__: Return a “bytes instance” containing only 7-bit characters.

  • __str__: Return a “string instance”. If astropy.conf.unicode_output is False, it must contain only 7-bit characters. If astropy.conf.unicode_output is True, it may contain non-ASCII characters when applicable.

  • __format__: Return a “string instance”. If astropy.conf.unicode_output is False, it must contain only 7-bit characters. If astropy.conf.unicode_output is True, it may contain non-ASCII characters when applicable.

For classes that are expected to roundtrip through strings (unicode or bytes), the parser must accept the output of __str__. Additionally, __repr__ should roundtrip when that makes sense.

This design generally follows Postel’s Law: “Be liberal in what you accept, and conservative in what you send.”

The following example class shows a way to implement this:

# -*- coding: utf-8 -*-

from astropy import conf

class FloatList(object):
    def __init__(self, init):
        if isinstance(init, str):
            init = init.split('‖')
        elif isinstance(init, bytes):
            init = init.split(b'|')
        self.x = [float(x) for x in init]

    def __repr__(self):
        # Return unicode object containing no non-ASCII characters
        return f'<FloatList [{", ".join(str(x) for x in self.x)}]>'

    def __bytes__(self):
        return b'|'.join(bytes(x) for x in self.x)

    def __str__(self):
        if astropy.conf.unicode_output:
            return '‖'.join(str(x) for x in self.x)
            return self.__bytes__().decode('ascii')

Additionally, there is a test helper, astropy.test.helper.assert_follows_unicode_guidelines to ensure that a class follows the Unicode guidelines outlined above. The following example test will test that our example class above is compliant:

def test_unicode_guidelines():
    from astropy.test.helper import assert_follows_unicode_guidelines
    assert_follows_unicode_guidelines(FloatList(b'5|4|3|2'), roundtrip=True)

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. When C extensions are used, the Python interface 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 not the generated .c files.

  • If a C extension has a dependency on an external C library, the source code for the library should be bundled with the Astropy core, provided the license for the C library is compatible with the Astropy license. Additionally, the package must be compatible with using a system-installed library in place of the library included in Astropy, and a user installing the package should be able to opt-in to using the system version using a ASTROPY_USE_SYSTEM_??? environment variable, where ??? is the name of the library, e.g. ASTROPY_USE_SYSTEM_WCSLIB (see also External C Libraries).

  • 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 via the mechanisms described in C or Cython Extensions.

Requirements Specific to Affiliated Packages

  • Affiliated packages implementing many classes/functions not relevant to the affiliated package itself (for example leftover code from a previous package) will not be accepted - the package should only include the required functionality and relevant extensions.

  • Affiliated packages must be registered on the Python Package Index, with proper metadata for downloading and installing the source package.

  • The astropy root package name should not be used by affiliated packages - it is reserved for use by the core package.


This section shows a few examples (not all of which are correct!) 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 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)

Rather than like this:

>>> s.set_color(0.4)  # Bad form!
>>> print(s.get_color())  # Bad form!

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

This example shows why the use of super leads to a more consistent method resolution order than manually calling methods of the super classes in a multiple inheritance case:

# This is dangerous and bug-prone!

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')

if you then do:

>>> b = B()
>>> b.method()

you will see:

Doing B
Doing A

which is what you expect, and similarly for C. However, if you do:

>>> d = D()
>>> d.method()

you might expect to see the methods called in the order D, B, C, A but instead you see:

Doing D
Doing C
Doing A
Doing B
Doing A

because both B.method() and C.method() call A.method() unaware of the fact that they’re being called as part of a chain in a hierarchy. When C.method() is called it is unaware that it’s being called from a subclass that inherits from both B and C, and that B.method() should be called next. 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 safer

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 foo and AClass, but not numpy.array or numpy.linspace.

Additional Resources

Further tips and hints relating to the coding guidelines are included below.