Testing Guidelines

This section describes the testing framework and format standards for tests in Astropy core packages (this also serves as recommendations for affiliated packages).

Testing Framework

The testing framework used by Astropy is the py.test framework.

Running Tests

There are currently three different ways to invoke Astropy tests. Each method invokes py.test to run the tests but offers different options when calling.

In addition to running the Astropy tests, these methods can also be called so that they check Python source code for PEP8 compliance. All of the PEP8 testing options require the pytest-pep8 plugin, which must be installed separately.

setup.py test

The safest way to run the astropy test suite is via the setup command test. This is invoked by running python setup.py test while in the astropy source code directory. Run python setup.py test --help to see the options to the test command.

Turn on PEP8 checking by passing --pep8 to the test command. This will turn off regular testing and enable PEP8 testing.


This method of running the tests defaults to the version of py.test that is bundled with Astropy. To use the locally-installed version, you can set the ASTROPY_USE_SYSTEM_PYTEST environment variable, eg.:

> ASTROPY_USE_SYSTEM_PYTEST=1 python setup.py test


An alternative way to run tests from the command line is to switch to the source code directory of astropy and simply type:


py.test will look for files that look like tests in the currect directory and all recursive directories then run all the code that looks like tests within those files.


To test any compiled C/Cython extensions, you must run python setup.py develop prior to running the py.test command-line script. Otherwise, any tests that make use of these extensions will not succeed. Similarly, in python 3, these tests will not run correctly in the source code, because they need the 2to3 tool to be run on them.

You may specify a specific test file or directory at the command line:

py.test test_file.py

To run a specific test within a file use the -k option:

py.test test_file.py -k "test_function"

You may also use the -k option to not run tests py putting a - in front of the matching string:

py.test test_file.py -k "-test_function"

py.test has a number of command line usage options.

Turn on PEP8 testing by adding the --pep8 flag to the py.test call. By default regular tests will also be run but these can be turned off by adding -k pep8:

py.test some_dir --pep8 -k pep8


This method of running the tests uses the locally-installed version of py.test rather than the bundled one, and hence will fail if the local version it is not up-to-date enough (py.test 2.2 as of this writing).


AstroPy includes a standalone version of py.test that allows to tests to be run even if py.test is not installed. Tests can be run from within AstroPy with:

import astropy

This will run all the default tests for AstroPy.

Tests for a specific package can be run by specifying the package in the call to the test() function:


This method works only with package names that can be mapped to Astropy directories. As an alternative you can test a specific directory or file with the test_path option:


The test_path must be specified either relative to the working directory or absolutely.

By default astropy.test() will skip tests which retrieve data from the internet. To turn these tests on use the remote_data flag:

astropy.test('io.fits', remote_data=True)

In addition, the test function supports any of the options that can be passed to pytest.main(), and convenience options verbose=, pastebin= and coverage=.

Enable PEP8 compliance testing with pep8=True in the call to astropy.test. This will enable PEP8 checking and disable regular tests.


This method of running the tests defaults to the version of py.test that is bundled with Astropy. To use the locally-installed version, you should set the ASTROPY_USE_SYSTEM_PYTEST environment variable (see Configuration system (astropy.config)) or the py.test method described above.

Regression tests

Any time a bug is fixed, and wherever possible, one or more regression tests should be added to ensure that the bug is not introduced in future. Regression tests should include the ticket URL where the bug was reported.

Where to put tests

Package-specific tests

Each package should include a suite of unit tests, covering as many of the public methods/functions as possible. These tests should be included inside each sub-package, either in a tests directory, or in a test.py file, e.g:




tests directories should contain an __init__.py file so that the tests can be imported and so that they can use relative imports.

Interoperability tests

Tests involving two or more sub-packages should be included in:


and using:


then runs both these interoperability tests, and all the unit tests in the sub-packages. This functionality is especially important for people who install packages through bundles and package managers, where the original source code for the tests is not immediately available.

Writing tests

py.test has the following test discovery rules:

  • test_*.py or *_test.py files
  • Test prefixed classes (without an __init__ method)
  • test_ prefixed functions and methods

Consult the test discovery rules for detailed information on how to name files and tests so that they are automatically discovered by py.test.

Simple example

The following example shows a simple function and a test to test this function:

def func(x):
    return x + 1

def test_answer():
    assert func(3) == 5

If we place this in a test.py file and then run:

py.test test.py

The result is:

============================= test session starts ==============================
python: platform darwin -- Python 2.7.2 -- pytest-1.1.1
test object 1: /Users/tom/tmp/test.py

test.py F

=================================== FAILURES ===================================
_________________________________ test_answer __________________________________

    def test_answer():
>       assert func(3) == 5
E       assert 4 == 5
E        +  where 4 = func(3)

test.py:5: AssertionError
=========================== 1 failed in 0.07 seconds ===========================

Working with data files

Tests that need to make use of a data file should use the get_data_fileobj or get_data_filename functions. These functions search locally first, and then on the astropy data server or an arbitrary URL, and return a file-like object or a local filename, respectively. They automatically cache the data locally if remote data is obtained, and from then on the local copy will be used transparently.

They also support the use of an MD5 hash to get a specific version of a data file. This hash can be obtained prior to submitting a file to the astropy data server by using the compute_hash function on a local copy of the file.

Tests that may retrieve remote data should be marked with the @remote_data decorator. Tests marked with this decorator will be skipped by default by astropy.test() to prevent test runs from taking too long. These tests can be run by astropy.test() by adding the remote_data=True flag. Turn on the remote data tests at the command line with py.test --remote-data.


from ...config import get_data_filename
from ...tests.helper import remote_data

def test_1():
    #if filename.fits is a local file in the source distribution
    datafile = get_data_filename('filename.fits')
    # do the test

def test_2():
    #this is the hash for a particular version of a file stored on the
    #astropy data server.
    datafile = get_data_filename('hash/94935ac31d585f68041c08f87d1a19d4')
    # do the test

The get_remote_test_data will place the files in a temporary directory indicated by the tempfile module, so that the test files will eventually get removed by the system. In the long term, once test data files become too large, we will need to design a mechanism for removing test data immediately.

Tests that create files

Tests may often be run from directories where users do not have write permissions so tests which create files should always do so in temporary directories. This can be done with the py.test tmpdir function argument or with Python’s built-in tempfile module.

Setting up/Tearing down tests

In some cases, it can be useful to run a series of tests requiring something to be set up first. There are four ways to do this:

Module-level setup/teardown

If the setup_module and teardown_module functions are specified in a file, they are called before and after all the tests in the file respectively. These functions take one argument, which is the module itself, which makes it very easy to set module-wide variables:

def setup_module(module):
    module.NUM = 11

def add_num(x):
    return x + NUM

def test_42():
    added = add_num(42)
    assert added == 53

We can use this for example to download a remote test data file and have all the functions in the file access it:

import os

def setup_module(module):
    module.DATAFILE = get_remote_test_data('94935ac31d585f68041c08f87d1a19d4')

def test():
    f = open(DATAFILE, 'rb')
    # do the test

def teardown_module(module):


Tests can be organized into classes that have their own setup/teardown functions. In the following

def add_nums(x, y):
    return x + y

class TestAdd42(object):

    def setup_class(self):
        self.NUM = 42

    def test_1(self):
        added = add_nums(11, self.NUM)
        assert added == 53

    def test_2(self):
        added = add_nums(13, self.NUM)
        assert added == 55

    def teardown_class(self):

In the above example, the setup_class method is called first, then all the tests in the class, and finally the teardown_class is called.


There are cases where one might want setup and teardown methods to be run before and after each test. For this, use the setup_method and teardown_method methods:

def add_nums(x, y):
    return x + y

class TestAdd42(object):

    def setup_method(self, method):
        self.NUM = 42

    def test_1(self):
        added = add_nums(11, self.NUM)
        assert added == 53

    def test_2(self):
        added = add_nums(13, self.NUM)
        assert added == 55

    def teardown_method(self, method):


Finally, one can use setup_function and teardown_function to define a setup/teardown mechanism to be run before and after each function in a module. These take one argument, which is the function being tested:

def setup_function(function):

def test_1(self):
    # do test

def test_2(self):
    # do test

def teardown_method(function):

Parametrizing tests

If you want to run a test several times for slightly different values, then it can be advantageous to use the py.test option to parametrize tests. For example, instead of writing:

def test1():
    assert type('a') == str

def test2():
    assert type('b') == str

def test3():
    assert type('c') == str

You can use the parametrize decorator to loop over the different inputs:

@pytest.mark.parametrize(('letter'), ['a', 'b', 'c'])
def test(letter):
    assert type(letter) == str

Using py.test helper functions

If your tests need to use py.test helper functions, such as pytest.raises, import pytest into your test module like so:

from ...tests.helper import pytest

You may need to adjust the relative import to work for the depth of your module. tests.helper imports pytest either from the user’s system or extern.pytest if the user does not have py.test installed. This is so that users need not install py.test to run AstroPy’s tests.

Using data in tests

Tests can include very small datafiles, but any files significantly larger than the source code should be placed on a remote server. The base URL for the test files will be:


and files will be accessed by their MD5 hash, for example:


Tests then retrieve data via this URL. This implicitly allows versioning, since different versions of data files will have different hashes. Old data files should not be removed, so that tests can be run in any version of AstroPy.

The details of the server implementation have yet to be decided, but using these static hash-based URLs ensures that even if we change the backend, the URL will remain the same.

Tests requiring optional dependencies

For tests that test functions or methods that require optional dependencies (e.g. Scipy), pytest should be instructed to skip the test if the dependencies are not present. The following example shows how this should be done:

import pytest

    import scipy
    HAS_SCIPY = True
except ImportError:
    HAS_SCIPY = False

@pytest.mark.skipif('not HAS_SCIPY')
def test_that_uses_scipy():

In this way, the test is run if Scipy is present, and skipped if not. No tests should fail simply because an optional dependency is not present.

Test coverage reports

Astropy can use coverage.py to generate test coverage reports. To generate a test coverage report, use:

python setup.py test --coverage

There is a coveragerc file that defines files to omit as well as lines to exclude. It is installed along with astropy so that the astropy.test function can use it. In the source tree, it is at astropy/tests/coveragerc.

Marking blocks of code to exclude from coverage

Blocks of code may be ignored by adding a comment containing the phrase pragma: no cover to the start of the block:

if this_rarely_happens:  # pragma: no cover

Blocks of code that are intended to run only in Python 2.x or 3.x may also be marked so that they will be ignored when appropriate by coverage.py:

if sys.version_info[0] >= 3:  # pragma: py3
else:  # pragma: py2