Testing Guidelines

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

Testing Framework

The testing framework used by astropy (and packages using the Astropy package template) is the pytest framework.

Testing Dependencies

The dependencies used by the Astropy test runner are provided by a separate package called pytest-astropy. This package provides the pytest dependency itself, in addition to several pytest plugins that are used by Astropy, and will also be of general use to other packages.

Since the testing dependencies are not actually required to install or use Astropy, they are not included in install_requires in setup.cfg. Instead, they are listed in an extras_require section called test in setup.cfg. Developers who want to run the test suite will need to either install pytest-astropy directly:

pip install pytest-astropy

or install the core package in ‘editable’ mode specifying the [test] option:

pip install -e .[test]

A detailed description of the plugins can be found in the Pytest Plugins section.

Running Tests

There are currently three different ways to invoke Astropy tests. Each method invokes pytest to run the tests but offers different options when calling. To run the tests, you will need to make sure you have the pytest package installed.

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.


The most robust way to run the tests (which can also be the slowest) is to make use of Tox, which is a general purpose tool for automating Python testing. One of the benefits of tox is that it first creates a source distribution of the package being tested, and installs it into a new virtual environment, along with any dependencies that are declared in the package, before running the tests. This can therefore catch issues related to undeclared package data, or missing dependencies. Since we use tox to run many of the tests on continuous integration services, it can also be used in many cases to reproduce issues seen on those services.

To run the tests with tox, first make sure that tox is installed, e.g.:

pip install tox

then run the basic test suite with:

tox -e test

or run the test suite with all optional dependencies with:

tox -e test-alldeps

You can see a list of available test environments with:

tox -l -v

which will also explain what each of them does.

You can also run checks or commands not directly related to tests - for instance:

tox -e codestyle

will run checks using the flake8 tool.

Is is possible to pass options to pytest when running tox - to do this, add a -- after the regular tox command, and anything after this will be passed to pytest, e.g.:

tox -e test -- -v --pdb

This can be used in conjunction with the -P option provided by the pytest-filter-subpackage plugin to run just part of the test suite.


The test suite can also be run directly from the native pytest command, which is generally faster than using tox for iterative development. In this case, it is important for developers to be aware that they must manually rebuild any extensions by running:

pip install -e .[test]

before running the test with pytest with:


Instead of calling pip install -e .[test], you can also build the extensions with:

python setup.py build_ext --inplace

which avoids also installing the developer version of astropy into your current environment - however note that the pip command is required if you need to test parts of the package that rely on certain entry points being installed.

It is possible to run only the tests for a particular subpackage or set of subpackages. For example, to run only the wcs tests from the commandline:

pytest -P wcs

Or, to run only the wcs and utils tests:

pytest -P wcs,utils

You can also specify a single directory or file to test from the commandline, e.g.:

pytest astropy/modeling


pytest astropy/wcs/tests/test_wcs.py

and this works for .rst files too:

pytest astropy/wcs/index.rst


Tests can be run from an installed version of Astropy with:

import astropy

This will run all the default tests for Astropy (but will not run the documentation tests in the .rst documentation since those files are not installed).

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(package='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= and pastebin=.

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

Astropy Test Function


Run the tests for the package.

This method builds arguments for and then calls pytest.main.

packagestr, optional

The name of a specific package to test, e.g. ‘io.fits’ or ‘utils’. Accepts comma separated string to specify multiple packages. If nothing is specified all default tests are run.

argsstr, optional

Additional arguments to be passed to pytest.main in the args keyword argument.

docs_pathstr, optional

The path to the documentation .rst files.

open_filesbool, optional

Fail when any tests leave files open. Off by default, because this adds extra run time to the test suite. Requires the psutil package.

parallelint or ‘auto’, optional

When provided, run the tests in parallel on the specified number of CPUs. If parallel is 'auto', it will use the all the cores on the machine. Requires the pytest-xdist plugin.

pastebin(‘failed’, ‘all’, None), optional

Convenience option for turning on pytest pastebin output. Set to ‘failed’ to upload info for failed tests, or ‘all’ to upload info for all tests.

pdbbool, optional

Turn on PDB post-mortem analysis for failing tests. Same as specifying --pdb in args.

pep8bool, optional

Turn on PEP8 checking via the pytest-pep8 plugin and disable normal tests. Same as specifying --pep8 -k pep8 in args.

pluginslist, optional

Plugins to be passed to pytest.main in the plugins keyword argument.

remote_data{‘none’, ‘astropy’, ‘any’}, optional

Controls whether to run tests marked with @pytest.mark.remote_data. This can be set to run no tests with remote data (none), only ones that use data from http://data.astropy.org (astropy), or all tests that use remote data (any). The default is none.

repeatint, optional

If set, specifies how many times each test should be run. This is useful for diagnosing sporadic failures.

skip_docsbool, optional

When True, skips running the doctests in the .rst files.

test_pathstr, optional

Specify location to test by path. May be a single file or directory. Must be specified absolutely or relative to the calling directory.

verbosebool, optional

Convenience option to turn on verbose output from pytest. Passing True is the same as specifying -v in args.

Test-running options

Testing for open files

Using the pytest-openfiles plugin (which is installed automatically when installing pytest-astropy), we can test whether any of the unit tests inadvertently leave any files open. Since this greatly slows down the time it takes to run the tests, it is turned off by default.

To use it from the commandline, do:

pytest --open-files

To use it from Python, do:

>>> import astropy
>>> astropy.test(open_files=True)

For more information on the pytest-openfiles plugin see pytest-openfiles

Test coverage reports

Coverage reports can be generated using the pytest-cov plugin (which is installed automatically when installing pytest-astropy) by using e.g.:

pytest --cov astropy --cov-report html

There is some configuration inside the setup.cfg file that defines files to omit as well as lines to exclude.

Running tests in parallel

It is possible to speed up astropy’s tests using the pytest-xdist plugin.

Once installed, tests can be run in parallel using the '-n' commandline option. For example, to use 4 processes:

pytest -n 4

Pass -n auto to create the same number of processes as cores on your machine.

Similarly, this feature can be invoked from astropy.test:

>>> import astropy
>>> astropy.test(parallel=4)

Writing tests

pytest 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 pytest.

Simple example

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

def func(x):
    """Add one to the argument."""
    return x + 1

def test_answer():
    """Check the return value of func() for an example argument."""
    assert func(3) == 5

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

pytest test.py

The result is:

============================= test session starts ==============================
python: platform darwin -- Python 3.x.x -- pytest-x.x.x
test object 1: /Users/username/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 ===========================

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, 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:


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.

Working with data files

Tests that need to make use of a data file should use the get_pkg_data_fileobj or get_pkg_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. See the next section for note specific to dealing with the cache in tests.

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 @pytest.mark.remote_data decorator, or, if a doctest, flagged with the REMOTE_DATA flag. Tests marked in this way 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='any' flag. Turn on the remote data tests at the command line with pytest --remote-data=any.

It is possible to mark tests using @pytest.mark.remote_data(source='astropy'), which can be used to indicate that the only required data is from the http://data.astropy.org server. To enable just these tests, you can run the tests with pytest --remote-data=astropy.

For more information on the pytest-remotedata plugin, see pytest-remotedata.


from ...config import get_data_filename

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

def test_2():
    """Test version using a remote file."""
    #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

def doctest_example():
    >>> datafile = get_data_filename('hash/94935')  # doctest: +REMOTE_DATA

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 use the file cache

By default, the Astropy test runner sets up a clean file cache in a temporary directory that is used only for that test run and then destroyed. This is to ensure consistency between test runs, as well as to not clutter users’ caches (i.e. the cache directory returned by get_cache_dir) with test files.

However, some test authors (especially for affiliated packages) may find it desirable to cache files downloaded during a test run in a more permanent location (e.g. for large data sets). To this end the set_temp_cache helper may be used. It can be used either as a context manager within a test to temporarily set the cache to a custom location, or as a decorator that takes effect for an entire test function (not including setup or teardown, which would have to be decorated separately).

Furthermore, it is possible to change the location of the cache directory for the duration of the test run by setting the XDG_CACHE_HOME environment variable.

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 pytest ‘tmpdir’ fixture 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):
    """Initialize the value of NUM."""
    module.NUM = 11

def add_num(x):
    """Add pre-defined NUM to the argument."""
    return x + NUM

def test_42():
    """Ensure that add_num() adds the correct NUM to its argument."""
    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):
    """Store a copy of the remote test file."""
    module.DATAFILE = get_remote_test_data('94935ac31d585f68041c08f87d1a19d4')

def test():
    """Perform test using cached remote input file."""
    f = open(DATAFILE, 'rb')
    # do the test

def teardown_module(module):
    """Clean up remote test file copy."""

Class-level setup/teardown

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

def add_nums(x, y):
    """Add two numbers."""
    return x + y

class TestAdd42(object):
    """Test for add_nums with y=42."""

    def setup_class(self):
        self.NUM = 42

    def test_1(self):
        """Test behavior for a specific input value."""
        added = add_nums(11, self.NUM)
        assert added == 53

    def test_2(self):
        """Test behavior for another input value."""
        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.

Method-level setup/teardown

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):
    """Add two numbers."""
    return x + y

class TestAdd42(object):
    """Test for add_nums with y=42."""

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

    def test_1(self):
    """Test behavior for a specific input value."""
        added = add_nums(11, self.NUM)
        assert added == 53

    def test_2(self):
    """Test behavior for another input value."""
        added = add_nums(13, self.NUM)
        assert added == 55

    def teardown_method(self, method):

Function-level setup/teardown

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):
   """First test."""
    # do test

def test_2(self):
    """Second test."""
    # do test

def teardown_function(function):

Property-based tests

Property-based testing lets you focus on the parts of your test that matter, by making more general claims - “works for any two numbers” instead of “works for 1 + 2”. Imagine if random testing gave you minimal, non-flaky failing examples, and a clean way to describe even the most complicated data - that’s property-based testing!

pytest-astropy includes a dependency on Hypothesis, so installation is easy - you can just read the docs or work through the tutorial and start writing tests like:

from astropy.coordinates import SkyCoord
from hypothesis import given, strategies as st

    st.builds(SkyCoord, ra=st.floats(0, 360), dec=st.floats(-90, 90))
def test_coordinate_transform(coord):
    """Test that sky coord can be translated from ICRS to Galactic and back."""
    assert coord == coord.galactic.icrs  # floating-point precision alert!

Other properties that you could test include:

  • Round-tripping from image to sky coordinates and back should be lossless for distortion-free mappings, and otherwise always below 10^-5 px.

  • Take a moment in time, round-trip it through various frames, and check it hasn’t changed or lost precision. (or at least not by more than a nanosecond)

  • IO routines losslessly round-trip data that they are expected to handle

  • Optimised routines calculate the same result as unoptimised, within tolerances

This is a great way to start contributing to Astropy, and has already found bugs in time handling. See issue #9017 and pull request #9532 for details!

(and if you find Hypothesis useful in your research, please cite it!)

Parametrizing tests

If you want to run a test several times for slightly different values, you can use pytest to avoid writing separate 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 @pytest.mark.parametrize decorator to concisely create a test function for each input:

@pytest.mark.parametrize(('letter'), ['a', 'b', 'c'])
def test(letter):
    """Check that the input is a string."""
    assert type(letter) == str

As a guideline, use parametrize if you can enumerate all possible test cases and each failure would be a distinct issue, and Hypothesis when there are many possible inputs or you only want a single simple failure to be reported.

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.

Using pytest helper functions

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

import pytest

Testing warnings

In order to test that warnings are triggered as expected in certain situations, pytest provides its own context manager pytest.warns that, completely analogously to pytest.raises (see below) allows to probe explicitly for specific warning classes and, through the optional match argument, messages. Note that when no warning of the specified type is triggered, this will make the test fail. When checking for optional, but not mandatory warnings, pytest.warns(None) can be used to catch and inspect them.


With pytest there is also the option of using the recwarn function argument to test that warnings are triggered within the entire embedding function. This method has been found to be problematic in at least one case (pull request 1174).

Testing exceptions

Just like the handling of warnings described above, tests that are designed to trigger certain errors should verify that an exception of the expected type is raised in the expected place. This is efficiently done by running the tested code inside the pytest.raises context manager. Its optional match argument allows to check the error message for any patterns using regex syntax. For example the matches pytest.raises(OSError, match=r'^No such file') and pytest.raises(OSError, match=r'or directory$') would be equivalent to assert str(err).startswith(No such file) and assert str(err).endswith(or directory), respectively, on the raised error message err. For matching multi-line messages you need to pass the (?s) flag to the underlying re.search, as in the example below:

with pytest.raises(fits.VerifyError, match=r'(?s)not upper.+ Illegal key') as excinfo:
assert str(excinfo.value).count('Card') == 2

This invocation also illustrates how to get an ExceptionInfo object returned to perform additional diagnostics on the info.

Testing configuration parameters

In order to ensure reproducibility of tests, all configuration items are reset to their default values when the test runner starts up.

Sometimes you’ll want to test the behavior of code when a certain configuration item is set to a particular value. In that case, you can use the astropy.config.ConfigItem.set_temp context manager to temporarily set a configuration item to that value, test within that context, and have it automatically return to its original value.

For example:

def test_pprint():
    from ... import conf
    with conf.set_temp('max_lines', 6):
        # ...

Marking blocks of code to exclude from coverage

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

if this_rarely_happens:  # pragma: no cover

Image tests with pytest-mpl

Running image tests

We make use of the pytest-mpl plugin to write tests where we can compare the output of plotting commands with reference files on a pixel-by-pixel basis (this is used for instance in astropy.visualization.wcsaxes).

To run the Astropy tests with the image comparison, use:

pytest --mpl --remote-data

However, note that the output can be very sensitive to the version of Matplotlib as well as all its dependencies (e.g. freetype), so we recommend running the image tests inside a Docker container which has a frozen set of package versions (Docker containers can be thought of as mini virtual machines). We have made a set of Docker container images that can be used for this. Once you have installed Docker, to run the Astropy tests with the image comparison inside a Docker container, make sure you are inside the Astropy repository (or the repository of the package you are testing) then do:

docker run -it -v ${PWD}:/repo astropy/image-tests-py35-mpl300:1.3 /bin/bash

This will start up a bash prompt in the Docker container, and you should see something like:


You can now go to the /repo directory, which is the same folder as your local version of the repository you are testing:

cd /repo

You can then run the tests as above:

pip install -e .[test]
pytest --mpl --remote-data

Type exit to exit the container.

You can find the names of the available Docker images on the Docker Hub.

Writing image tests

The README.rst for the plugin contains information on writing tests with this plugin. The only key addition compared to those instructions is that you should set baseline_dir:

from astropy.tests.image_tests import IMAGE_REFERENCE_DIR


This is because since the reference image files would contribute significantly to the repository size, we instead store them on the http://data.astropy.org site. The downside is that it is a little more complicated to create or re-generate reference files, but we describe the process here.

Generating reference images

Once you have a test for which you want to (re-)generate reference images, start up one of the Docker containers using e.g.:

docker run -it -v ${PWD}:/repo astropy/image-tests-py35-mpl300:1.3 /bin/bash

then run the tests inside /repo with the --mpl-generate-path argument, e.g:

cd repo
pip install -e .[test]
pytest --mpl --mpl-generate-path=reference_tmp --remote-data

This will create a reference_tmp folder and put the generated reference images inside it - the folder will be available in the repository outside of the Docker container. Type exit to exit the container.

Make sure you generate images for the different supported Matplotlib versions using the available containers.

Uploading the reference images

Next, we need to add these images to the http://data.astropy.org server. To do this, open a pull request to this repository. The reference images for Astropy tests should go inside the testing/astropy directory. In that directory are folders named as timestamps. If you are simply adding new tests, add the reference files to the most recent directory.

If you are re-generating baseline images due to changes in Astropy, make a new timestamp directory by copying one the most recent one, then replace any baseline images that have changed. Note that due to changes between Matplotlib versions, we need to add the whole set of reference images for each major Matplotlib version. Therefore, in each timestamp folder, there are folders named e.g. 1.4.x and 1.5.x.

Once the reference images are merged in and available on http://data.astropy.org, update the timestamp in the IMAGE_REFERENCE_DIR variable in the astropy.tests.image_tests sub-module. Because the timestamp is hard-coded, adding a new timestamp directory will not mess with testing for released versions of Astropy, so you can easily add and tweak a new timestamp directory while still working on a pull request to Astropy.

Writing doctests

A doctest in Python is a special kind of test that is embedded in a function, class, or module’s docstring, or in the narrative Sphinx documentation, and is formatted to look like a Python interactive session–that is, they show lines of Python code entered at a >>> prompt followed by the output that would be expected (if any) when running that code in an interactive session.

The idea is to write usage examples in docstrings that users can enter verbatim and check their output against the expected output to confirm that they are using the interface properly.

Furthermore, Python includes a doctest module that can detect these doctests and execute them as part of a project’s automated test suite. This way we can automatically ensure that all doctest-like examples in our docstrings are correct.

The Astropy test suite automatically detects and runs any doctests in the astropy source code or documentation, or in packages using the Astropy test running framework. For example doctests and detailed documentation on how to write them, see the full doctest documentation.


Since the narrative Sphinx documentation is not installed alongside the astropy source code, it can only be tested by running pytest directly (or via tox), not by import astropy; astropy.test().

For more information on the pytest-doctestplus plugin used by Astropy, see pytest-doctestplus.

Skipping doctests

Sometimes it is necessary to write examples that look like doctests but that are not actually executable verbatim. An example may depend on some external conditions being fulfilled, for example. In these cases there are a few ways to skip a doctest:

  1. Next to the example add a comment like: # doctest: +SKIP. For example:

    >>> import os
    >>> os.listdir('.')  # doctest: +SKIP

    In the above example we want to direct the user to run os.listdir('.') but we don’t want that line to be executed as part of the doctest.

    To skip tests that require fetching remote data, use the REMOTE_DATA flag instead. This way they can be turned on using the --remote-data flag when running the tests:

    >>> datafile = get_data_filename('hash/94935')  # doctest: +REMOTE_DATA
  2. Astropy’s test framework adds support for a special __doctest_skip__ variable that can be placed at the module level of any module to list functions, classes, and methods in that module whose doctests should not be run. That is, if it doesn’t make sense to run a function’s example usage as a doctest, the entire function can be skipped in the doctest collection phase.

    The value of __doctest_skip__ should be a list of wildcard patterns for all functions/classes whose doctests should be skipped. For example:

    __doctest_skip__ = ['myfunction', 'MyClass', 'MyClass.*']

    skips the doctests in a function called myfunction, the doctest for a class called MyClass, and all methods of MyClass.

    Module docstrings may contain doctests as well. To skip the module-level doctests include the string '.' in __doctest_skip__.

    To skip all doctests in a module:

    __doctest_skip__ = ['*']
  3. In the Sphinx documentation, a doctest section can be skipped by making it part of a doctest-skip directive:

    .. doctest-skip::
        >>> # This is a doctest that will appear in the documentation,
        >>> # but will not be executed by the testing framework.
        >>> 1 / 0  # Divide by zero, ouch!

    It is also possible to skip all doctests below a certain line using a doctest-skip-all comment. Note the lack of :: at the end of the line here:

    .. doctest-skip-all
    All doctests below here are skipped...
  4. __doctest_requires__ is a way to list dependencies for specific doctests. It should be a dictionary mapping wildcard patterns (in the same format as __doctest_skip__) to a list of one or more modules that should be importable in order for the tests to run. For example, if some tests require the scipy module to work they will be skipped unless import scipy is possible. It is also possible to use a tuple of wildcard patterns as a key in this dict:

    __doctest_requires__ = {('func1', 'func2'): ['scipy']}

    Having this module-level variable will require scipy to be importable in order to run the doctests for functions func1 and func2 in that module.

    In the Sphinx documentation, a doctest requirement can be notated with the doctest-requires directive:

    .. doctest-requires:: scipy
        >>> import scipy
        >>> scipy.hamming(...)

Skipping output

One of the important aspects of writing doctests is that the example output can be accurately compared to the actual output produced when running the test.

The doctest system compares the actual output to the example output verbatim by default, but this not always feasible. For example the example output may contain the __repr__ of an object which displays its id (which will change on each run), or a test that expects an exception may output a traceback.

The simplest way to generalize the example output is to use the ellipses .... For example:

>>> 1 / 0
Traceback (most recent call last):
ZeroDivisionError: integer division or modulo by zero

This doctest expects an exception with a traceback, but the text of the traceback is skipped in the example output–only the first and last lines of the output are checked. See the doctest documentation for more examples of skipping output.

Ignoring all output

Another possibility for ignoring output is to use the # doctest: +IGNORE_OUTPUT flag. This allows a doctest to execute (and check that the code executes without errors), but allows the entire output to be ignored in cases where we don’t care what the output is. This differs from using ellipses in that we can still provide complete example output, just without the test checking that it is exactly right. For example:

>>> print('Hello world')  
We don't really care what the output is as long as there were no errors...

Handling float output

Some doctests may produce output that contains string representations of floating point values. Floating point representations are often not exact and contain roundoffs in their least significant digits. Depending on the platform the tests are being run on (different Python versions, different OS, etc.) the exact number of digits shown can differ. Because doctests work by comparing strings this can cause such tests to fail.

To address this issue, the pytest-doctestplus plugin provides support for a FLOAT_CMP flag that can be used with doctests. For example:

>>> 1.0 / 3.0  # doctest: +FLOAT_CMP

When this flag is used, the expected and actual outputs are both parsed to find any floating point values in the strings. Those are then converted to actual Python float objects and compared numerically. This means that small differences in representation of roundoff digits will be ignored by the doctest. The values are otherwise compared exactly, so more significant (albeit possibly small) differences will still be caught by these tests.

Continuous integration


Astropy uses the following continuous integration (CI) services:

  • GitHub Actions for 64-bit Linux, OS X, and Windows setups (Note: GitHub Actions does not have “allowed failures” yet, so you might see a fail job reported for your PR with “(Allowed Failure)” in its name. Still, some failures might be real and related to your changes, so check it anyway!)

  • CircleCI for 32-bit Linux, documentation build, and visualization tests

These continuously test the package for each commit and pull request that is pushed to GitHub to notice when something breaks.

Astropy and many affiliated packages use an external package called ci-helpers to provide support for the generic parts of the CI systems.

Dependencies can be customized for different packages using the appropriate environment variables in the relevant YAML files. For more details on how to set up this machinery, see the package-template and ci-helpers.

The 32-bit tests on CircleCI use the quay.io/pypa/manylinux1_i686 docker image which includes a 32-bit Python environment for each major Python version. See the CircleCI configuration file for the core package for how to access the different Python versions.

In some cases, you may see failures on continuous integration services that you do not see locally, for example because the operating system is different, or because the failure happens with only 32-bit Python. The following sections explain how you can reproduce specific builds locally.

Reproducing failing 32-bit builds

If you want to run your tests in the same 32-bit Python environment that CircleCI uses, start off by installing Docker if you don’t already have it installed. Docker can be installed on a variety of different operating systems.

Then, make sure you have a version of the git repository (either the main Astropy repository or your fork) for which you want to run the tests. Go to that directory, then run Docker with:

$ docker run -i -v ${PWD}:/astropy_src -t quay.io/pypa/manylinux1_i686 bash

This will put you in the bash shell inside the Docker container. Once inside, you can go to the /astropy_src directory, and you should see the files that are in your local git repository:

root@5e2b89d7b07c:/# cd /astropy_src
root@5e2b89d7b07c:/astropy_src# ls
astropy                     conftest.py      licenses              setup.py
cextern                     CONTRIBUTING.md  MANIFEST.in       static
CHANGES.rst         docs             pip-requirements  tox.ini
CITATION            examples             pyproject.toml
codecov.yml         GOVERNANCE.md    README.rst
CODE_OF_CONDUCT.md  LICENSE.rst      setup.cfg

You can then run the tests with e.g.:

root@5e2b89d7b07c:/astropy_src# /opt/python/cp36-cp36m/bin/pip install -e .[test]
root@5e2b89d7b07c:/astropy_src# pytest

Pytest Plugins

The following pytest plugins are maintained and used by Astropy. They are included as dependencies to the pytest-astropy package, which is now required for testing Astropy. More information on all of the plugins provided by the pytest-astropy package (including dependencies not maintained by Astropy) can be found here.


The pytest-remotedata plugin allows developers to control whether to run tests that access data from the internet. The plugin provides two decorators that can be used to mark individual test functions or entire test classes:

  • @pytest.mark.remote_data for tests that require data from the internet

  • @pytest.mark.internet_off for tests that should run only when there is no internet access. This is useful for testing local data caches or fallbacks for when no network access is available.

The plugin also adds the --remote-data option to the pytest command (which is also made available through the Astropy test runner).

If the --remote-data option is not provided when running the test suite, or if --remote-data=none is provided, all tests that are marked with remote_data will be skipped. All tests that are marked with internet_off will be executed. Any test that attempts to access the internet but is not marked with remote_data will result in a failure.

Providing either the --remote-data option, or --remote-data=any, will cause all tests marked with remote_data to be executed. Any tests that are marked with internet_off will be skipped.

Running the tests with --remote-data=astropy will cause only tests that receive remote data from Astropy data sources to be run. Tests with any other data sources will be skipped. This is indicated in the test code by marking test functions with @pytest.mark.remote_data(source='astropy'). Tests marked with internet_off will also be skipped in this case.

Also see Working with data files.


The pytest-doctestplus plugin provides advanced doctest features, including:

  • handling doctests that use remote data in conjunction with the pytest-remotedata plugin above (see Working with data files)

  • approximate floating point comparison for doctests that produce floating point results (see Handling float output)

  • skipping particular classes, methods, and functions when running doctests (see Skipping doctests)

  • optional inclusion of *.rst files for doctests

This plugin provides two command line options: --doctest-plus for enabling the advanced features mentioned above, and --doctest-rst for including *.rst files in doctest collection.

The Astropy test runner enables both of these options by default. When running the test suite directly from pytest (instead of through the Astropy test runner), it is necessary to explicitly provide these options when they are needed.


The pytest-openfiles plugin allows for the detection of open I/O resources at the end of unit tests. This plugin adds the --open-files option to the pytest command (which is also exposed through the Astropy test runner).

When running tests with --open-files, if a file is opened during the course of a unit test but that file not closed before the test finishes, the test will fail. This is particularly useful for testing code that manipulates file handles or other I/O resources. It allows developers to ensure that this kind of code properly cleans up I/O resources when they are no longer needed.

Also see Testing for open files.