Development Guidelines

Coding Guidelines

For the most part we try to follow PEP 8 guidelines which can be viewed here:

Testing Guidelines

Swift has a comprehensive suite of tests and pep8 checks that are run on all submitted code, and it is recommended that developers execute the tests themselves to catch regressions early. Developers are also expected to keep the test suite up-to-date with any submitted code changes.

Swift’s tests and pep8 checks can be executed in an isolated environment with tox:

To execute the tests:

  • Ensure pip and virtualenv are upgraded to satisfy the version requirements listed in the OpenStack global requirements:

    pip install pip -U
    pip install virtualenv -U
  • Install tox:

    pip install tox
  • Generate list of distribution packages to install for testing:

    tox -e bindep

    Now install these packages using your distribution package manager like apt-get, dnf, yum, or zypper.

  • Run tox from the root of the swift repo:


To run a selected subset of unit tests with pytest:

  • Create a virtual environment with tox:

    tox devenv -e py3 .env


Alternatively, here are the steps of manual preparation of the virtual environment:

virtualenv .env
source .env/bin/activate
pip3 install -r requirements.txt -r test-requirements.txt -c py36-constraints.txt
pip3 install -e .
  • Activate the virtual environment:

    source .env/bin/activate
  • Run some unit tests, for example:

    pytest test/unit/common/middleware/crypto
  • Run all unit tests:

    pytest test/unit


If you installed using cd ~/swift; sudo python develop, you may need to do cd ~/swift; sudo chown -R ${USER}:${USER} swift.egg-info prior to running tox.

  • By default tox will run all of the unit test and pep8 checks listed in the tox.ini file envlist option. A subset of the test environments can be specified on the tox command line or by setting the TOXENV environment variable. For example, to run only the pep8 checks and python2.7 unit tests use:

    tox -e pep8,py27


    TOXENV=py27,pep8 tox

    To run unit tests with python3.8:

    tox -e py38


As of tox version 2.0.0, most environment variables are not automatically passed to the test environment. Swift’s tox.ini overrides this default behavior so that variable names matching SWIFT_* and *_proxy will be passed, but you may need to run tox --recreate for this to take effect after upgrading from tox <2.0.0.

Conversely, if you do not want those environment variables to be passed to the test environment then you will need to unset them before calling tox.

Also, if you ever encounter DistributionNotFound, try to use tox --recreate or remove the .tox directory to force tox to recreate the dependency list.

Swift’s tests require having an XFS directory available in /tmp or in the TMPDIR environment variable.

Swift’s functional tests may be executed against a SAIO (Swift All In One) or other running Swift cluster using the command:

tox -e func

The endpoint and authorization credentials to be used by functional tests should be configured in the test.conf file as described in the section Setting up scripts for running Swift.

The environment variable SWIFT_TEST_POLICY may be set to specify a particular storage policy name that will be used for testing. When set, tests that would otherwise not specify a policy or choose a random policy from those available will instead use the policy specified. Tests that use more than one policy will include the specified policy in the set of policies used. The specified policy must be available on the cluster under test.

For example, this command would run the functional tests using policy ‘silver’:

SWIFT_TEST_POLICY=silver tox -e func

To run a single functional test, use the --no-discover option together with a path to a specific test method, for example:

tox -e func -- --no-discover test.functional.tests.TestFile.testCopy

In-process functional testing

If the test.conf file is not found then the functional test framework will instantiate a set of Swift servers in the same process that executes the functional tests. This ‘in-process test’ mode may also be enabled (or disabled) by setting the environment variable SWIFT_TEST_IN_PROCESS to a true (or false) value prior to executing tox -e func.

When using the ‘in-process test’ mode some server configuration options may be set using environment variables:

  • the optional in-memory object server may be selected by setting the environment variable SWIFT_TEST_IN_MEMORY_OBJ to a true value.

  • encryption may be added to the proxy pipeline by setting the environment variable SWIFT_TEST_IN_PROCESS_CONF_LOADER to encryption.

  • a 2+1 EC policy may be installed as the default policy by setting the environment variable SWIFT_TEST_IN_PROCESS_CONF_LOADER to ec.

  • logging to stdout may be enabled by setting SWIFT_TEST_DEBUG_LOGS.

For example, this command would run the in-process mode functional tests with encryption enabled in the proxy-server:

    tox -e func

This particular example may also be run using the func-encryption tox environment:

tox -e func-encryption

The tox.ini file also specifies test environments for running other in-process functional test configurations, e.g.:

tox -e func-ec

To debug the functional tests, use the ‘in-process test’ mode and pass the --pdb flag to tox:

SWIFT_TEST_IN_PROCESS=1 tox -e func -- --pdb \

The ‘in-process test’ mode searches for proxy-server.conf and swift.conf config files from which it copies config options and overrides some options to suit in process testing. The search will first look for config files in a <custom_conf_source_dir> that may optionally be specified using the environment variable:


If SWIFT_TEST_IN_PROCESS_CONF_DIR is not set, or if a config file is not found in <custom_conf_source_dir>, the search will then look in the etc/ directory in the source tree. If the config file is still not found, the corresponding sample config file from etc/ is used (e.g. proxy-server.conf-sample or swift.conf-sample).

When using the ‘in-process test’ mode SWIFT_TEST_POLICY may be set to specify a particular storage policy name that will be used for testing as described above. When set, this policy must exist in the swift.conf file and its corresponding ring file must exist in <custom_conf_source_dir> (if specified) or etc/. The test setup will set the specified policy to be the default and use its ring file properties for constructing the test object ring. This allows in-process testing to be run against various policy types and ring files.

For example, this command would run the in-process mode functional tests using config files found in $HOME/my_tests and policy ‘silver’:

   SWIFT_TEST_POLICY=silver tox -e func

S3 API cross-compatibility tests

The cross-compatibility tests in directory test/s3api are intended to verify that the Swift S3 API behaves in the same way as the AWS S3 API. They should pass when run against either a Swift endpoint (with S3 API enabled) or an AWS S3 endpoint.

To run against an AWS S3 endpoint, the /etc/swift/test.conf file must be edited to provide AWS key IDs and secrets. Alternatively, an AWS CLI style credentials file can be loaded by setting the SWIFT_TEST_AWS_CONFIG_FILE environment variable, e.g.:

SWIFT_TEST_AWS_CONFIG_FILE=~/.aws/credentials pytest ./test/s3api


When using SWIFT_TEST_AWS_CONFIG_FILE, the region defaults to us-east-1 and only the default credentials are loaded.

Coding Style

Swift uses flake8 with the OpenStack hacking module to enforce coding style.

Install flake8 and hacking with pip or by the packages of your Operating System.

It is advised to integrate flake8+hacking with your editor to get it automated and not get caught by Jenkins.

For example for Vim the syntastic plugin can do this for you.

Documentation Guidelines

The documentation in docstrings should follow the PEP 257 conventions (as mentioned in the PEP 8 guidelines).

More specifically:

  1. Triple quotes should be used for all docstrings.

  2. If the docstring is simple and fits on one line, then just use one line.

  3. For docstrings that take multiple lines, there should be a newline after the opening quotes, and before the closing quotes.

  4. Sphinx is used to build documentation, so use the restructured text markup to designate parameters, return values, etc. Documentation on the sphinx specific markup can be found here:

To build documentation run:

pip install -r requirements.txt -r doc/requirements.txt
sphinx-build -W -b html doc/source doc/build/html

and then browse to doc/build/html/index.html. These docs are auto-generated after every commit and available online at


For sanity check of your change in manpage, use this command in the root of your Swift repo: