How to Make a Roundtrip Test

The PyNWB test suite has tools for easily doing round-trip tests of container classes. These tools exist in the integration test suite in tests/integration/ui_write/ for this reason and for the sake of keeping the repository organized, we recommend you write your tests in the tests/integration/ui_write subdirectory of the Git repository.

For executing your new tests, we recommend using the script in the top of the Git repository. Roundtrip tests will get executed as part of the integration test suite, which can be executed with the following command:

$ python -i

The roundtrip test will generate a new NWB file with the name test_<CLASS_NAME>.nwb where CLASS_NAME is the class name of the Container class you are roundtripping. The test will write an NWB file with an instance of the container to disk, read this instance back in, and compare it to the instance that was used for writing to disk. Once the test is complete, the NWB file will be deleted. You can keep the NWB file around after the test completes by setting the environment variable CLEAN_NWB to 0, false, False, or FALSE. Setting CLEAN_NWB to any value not listed here will cause the roundtrip NWB file to be deleted once the test has completed

Before writing tests, we also suggest you familiarize yourself with the software architecture of PyNWB.


To write a roundtrip test, you will need to subclass the NWBH5IOMixin class and override some of its instance methods.

NWBH5IOMixin provides four methods for testing the process of going from in-memory Python object to data stored on disk and back. Three of these methods–setUpContainer, addContainer, and getContainer–are required for carrying out the roundtrip test. The fourth method is required for testing the conversion from the container to the builder–the intermediate data structure that gets used by HDMFIO implementations for writing to disk.

If you do not want to test step of the process, you can just implement setUpContainer, addContainer, and getContainer.


The first thing (and possibly the only thing – see AcquisitionH5IOMixin) you need to do is override is the setUpContainer method. This method should take no arguments, and return an instance of the container class you are testing.

Here is an example using a generic TimeSeries:

from pynwb.testing import NWBH5IOMixin, TestCase

class TimeSeriesRoundTrip(NWBH5IOMixin, TestCase):

    def setUpContainer(self):
        return TimeSeries(
            list(range(100, 200, 10)),


The next thing is to tell the NWBH5IOMixin how to add the container to an NWBFile. This method takes a single argument–the NWBFile instance that will be used to write your container.

This method is required because different container types are allowed in different parts of an NWBFile. This method is also where you can add additional containers that your container of interest depends on. For example, for the ElectricalSeries roundtrip test, addContainer handles adding the ElectrodeGroup, ElectrodeTable, and Device dependencies.

Continuing from our example above, we will add the method for adding a generic TimeSeries instance:

class TimeSeriesRoundTrip(NWBH5IOMixin, TestCase):

    def addContainer(self, nwbfile):


Finally, you need to tell NWBH5IOMixin how to get back the container we added. As with addContainer, this method takes an NWBFile as its single argument. The only difference is that this NWBFile instance is what was read back in.

Again, since not all containers go in the same place, we need to tell the test harness how to get back our container of interest.

To finish off example from above, we will add the method for getting back our generic TimeSeries instance:

class TimeSeriesRoundTrip(NWBH5IOMixin, TestCase):

    def getContainer(self, nwbfile):
        return nwbfile.get_acquisition(


As mentioned above, there is an optional method to override. This method will add two additional tests. First, it will add a test for converting your container into a builder to make sure the intermerdiate data structure gets built appropriately. Second it will add a test for constructing your container from the builder returned by your overridden setUpBuilder method. This method takes no arguments, and should return the builder representation of your container class instance.

This method is not required, but can serve as an additional check to make sure your containers are getting converted to the expected structure as described in your specification.

Continuing from the TimeSeries example, lets add setUpBuilder:

from import GroupBuilder

class TimeSeriesRoundTrip(NWBH5IOMixin, TestCase):

    def setUpBuilder(self):
        return GroupBuilder(
                'source': 'example_source',
                'namespace': base.CORE_NAMESPACE,
                'neurodata_type': 'TimeSeries',
                'description': 'no description',
                'comments': 'no comments',
                'data': DatasetBuilder(
                    'data', list(range(100, 200, 10)),
                        'unit': 'SIunit',
                        'conversion': 1.0,
                        'resolution': 0.1,
                'timestamps': DatasetBuilder(
                    'timestamps', list(range(10)),
                    attributes={'unit': 'Seconds', 'interval': 1},


If you are testing something that can go in acquisition, you can avoid writing addContainer and getContainer by extending AcquisitionH5IOMixin. This class has already overridden these methods to add your container object to acquisition.

Even if your container can go in acquisition, you may still need to override addContainer if your container depends other containers that you need to add to the NWBFile that will be written.