Problems faced when downstream testing Python packages


Michał Górny



Downstream testing refers to the testing of software package done by their redistributors, such as Linux distributions. It could be done by distro-specific CI systems, package maintainers or — as it frequently is the case with Gentoo — even distribution users.

What makes downstream testing really useful is that it serves a different purpose than upstream testing does. To put it shortly, upstream testing aims to ensure that the current code of the package works in one or more reference environments, and meets quality standards set by the package authors. On the other hand, downstream testing aims to ensure that a particular version of the package (possibly an old one) works in the environment that it will be used on, or one that closely resembles it.

To put it another way, downstream testing may differ from upstreaming testing by:

While these may sound inconvenient and sometimes cause false positives, they have proven in the past to detect issues that went unnoticed by upstream and that could have broken production setups. Downstream testing is important.

Unfortunately, many test suites make assumptions that cause problems for downstream testers. Some of them can be worked around easily, others can not. In this article I'd like to discuss a number of these issues.

Assuming a disposable container

Probably the most extreme category of problematic test suites are suites assuming that they will always be run in a disposable environment. The exact details can vary.

In a simple case, the test suite could be writing to the source directory and causing the subsequent test runs to fail because of preexisting artifacts.

In an extreme case, the test suite could be modifying the package files, effectively causing the installed package to be broken after running the test suite. This could be particularly bad if the breakage couldn't trivially be detected while installing it, so testing would result in the exact opposite of what it was supposed to do — instead of detecting a problem prior to installing the package, it would actually cause one. For example, mkdocstrings used to write into template files installed by mkdocstrings-python [1].

Please do not assume that the test environment is disposable. Your test suite will be run in development and production environments. It may be run more than once using the same sources, it may be run against a package that is installed already or that is going to be installed. Do not modify the source directory, do not modify the installed packages. Use temporary directories, copying the package to one if necessary.

Requiring old package versions

In general, upstreams do understand that pinning the dependencies of their packages to specific versions is problematic for users, and can cause conflicts in larger projects where many different components may require the same library. Therefore, there is a general agreement that packages should be ported to new versions of their dependencies. However, this doesn't seem to be always the case when a specific dependency is needed only to run the test suite.

This is not a problem when the test suite is run in a virtual environment. However, for the most precise downstream testing we are running the test suites against system packages. This means that effectively we need to package and install test dependencies as well. If your test suite works correctly only with old versions of these packages, we need to provide and maintain these old versions.

Unfortunately, this is not a case of "add once and forget". We need to keep these versions working, patch support for new Python versions in, patch regressions due to the changes in their dependencies and so on. Believe me, it is a lot of effort that doesn't really benefit users. So please, keep updating your test suite to work with the most recent releases of all its dependencies.

Incompatibilities caused by stray packages

A somewhat related problem (and quite unique to the Python ecosystem at that) is caused by downstream test environments featuring more packages than the virtual environments normally used in CI configurations. Again, the details vary on per-package basis.

In some cases the test failures are directly caused by the additional packages affecting the behavior of dependencies indirectly. This is particularly a case with pytest plugins and other plugin systems — to the point that Gentoo disables a few known-bad plugins by default, and often resorts to disabling plugin autoloading entirely to work around the problems.

In other cases, the tests are run with the assumption that some package will not be installed. For example, starlette depended on httpx, and httpx has optional support for Brotli that starlette upstream did not use in their test environment. However, if Brotli happened to be installed in the Gentoo test environment, starlette tests failed because of Brotli being used [2].

In the most extreme cases, the test suite would actually expect some optional dependency not to be installed. This is particularly the case with packaging-related tooling that would compare the list of packages installed in the system, or some packages testing handling of missing dependencies without actually verifying that they aren't installed.

Please try to make your test suite robust. Ideally run your own tests in multiple variants with different sets of dependencies installed to test that all fallbacks work correctly. Most importantly, please be patient with us when we reporting all these weird test failures.

Use of Internet resources

This is a very wide topic. A number of test suites use Internet in some way — from downloading test data, to actually testing the package against live API and websites. This causes problems on many fronts.

Sometimes unauthorized automated access to data can range from being a Terms of Service violation to being simply unethical. Just imagine you're paying for a small server, and somebody's test suite keeps repeatedly adding traffic, inflating your bills or decreasing availability and performance for your actual users.

For downstreams, Internet uses poses a number of user-facing risks and issues:

Just to bring one late example, pycares and aiodns both started failing due to DNS records changing [3] [4].

Please make sure that at least a reasonable subset of your tests can work fully offline. If you include online tests in your test suite, please make them opt-in to ensure that the users' privacy is respected. There are a number of solutions to turn online tests into offline, depending on your actual use case.

If your tests need additional data files, please make sure that they can be redistributed legally and either include them in your package, or host them yourself and make it possible for redistributors to supply them externally. To list two examples, cryptography uses a separate cryptography-vectors package to supply the test vectors, and pypdf uses a sample-files repository [5] [6].

If your tests need to reference a website or an API, ideally mock them. There are nice packages, such as betamax and vcrpy that make it trivial to record interactions with a remote server and reproduce them locally — i.e. make it possible to run an "online" test suite offline.

If you need to work against a specific kind of server, consider using a dedicated test server. There are packages such as pytest-localftpserver that make it easy to run a local FTP server and test your package against it. However, please do not use Docker for that. Docker generally requires root privileges and downloading large machine images — it makes Internet use even worse.

Requiring containers (e.g. Docker) for testing

A special case of the above are test suites that require access to a working Docker daemon or a similar container system. For example, aiomcache uses Docker to start memcached in a container [7].

While using Docker is often convenient, it causes a few important problems. Firstly, it requires the test suite to have access to a Docker daemon which poses an important security risk (test suites are normally run using a dedicated user in Gentoo). Secondly, the daemon needs to download (potentially large) images from the Internet, effectively implicating all the Internet access problems.

Should you decide to use Docker in your test suite, please make sure to make it entirely optional. If your tests require a certain server, please either include an option to provide the executable locally or to start it externally and let the user provide connection parameters.

Fragile tests and timeouts

Some tests can be really fragile to the system load. These tests tend to pass on CI (but not always!) when the hardware running them is relatively fast and not heavily loaded. However, when they are run on real Gentoo systems that are sometimes heavily loaded with other builds or test suites, or are running low-end hardware like our Alpha qr HPPA boxes, they start failing unpredictably.

The simplest example are tests that are running with short timeouts or narrow timing assumptions. The extreme example of this are tests that verify that a particular routine is "fast enough". For example, Gentoo is skipping speed tests in aesara because they can't reliably pass on busy systems [8].

In some cases, these limitations may be non-obvious at first. For example, priority used to fail due to "unreliable test timings" coming from hypothesis [9].

When designing your test suite, please bear in mind that it may be run on systems that are under heavy load, and possibly much weaker than an average PC. Be generous in timeout values, or at least provide the ability to override (ideally "multiply") them. If you really need to include speed tests, please either make them opt-in or at least make it easy to opt out of them.

Unconditional test dependencies

Some packages feature a number of optional dependencies. Unfortunately, sometimes what is an optional dependency at runtime becomes an obligatory dependency for the test suite. While in general we want to run as many tests as possible, it is not always feasible for us to maintain a large number of extra test dependencies, in order to run a minor part of the test suite.

In extreme cases these additional dependencies may even make it impossible to run the test suite on certain architectures. For example, the greenlet package supports only a handful of platforms [10]. If the test suite requires it, it cannot be run on any other platform.

Ideally, please make non-essential test dependencies optional. pytest provides a convenient pytest.importorskip() function that can be used to automatically skip tests when an import fails.

If a test dependency is required by an important subset of tests, yet it is problematic, please at least scoping the imports so that it is possible to deselect the tests requiring it. For example, pip scopes cryptography imports to make the tests requiring them skippable (if the relevant symbols were imported globally in conftest, the entire test suite would require that package) [11].

Package quality checks

In the opening paragraphs, I have pointed out that one of the goals of upstream testing is ensuring that the package meets set quality standards. However, this is not necessarily a critical goal for downstream testing. After all, we aren't modifying the package code in any way, merely shipping it in its current (or historical) form.

A few examples of what could qualify as quality checks are:

Integrating these checks directly into the test suite could cause surprising problems. In the best case, they could unnecessarily slow downstream testing down and introduce unnecessary dependencies. In the worst case, they could cause the test suite to start failing over time as the behavior of the used tools (and the quality standards they adhere to) change.

For example, in the pydantic ebuild Gentoo skips mypy testing because it has regressed multiple times after upgrading mypy to a newer version [#PYDANTIC]. While using correct types is important and technically the check could find valid bugs in code, more often than not it finds minor issues that do not require patching downstream.

It is only too common for packages in a variety of programming languages to start failing due to -Werror or an equivalent option. In the case of C, it could be due to using a different compiler or platform than was originally tested. In the case of Python, these are often deprecation warnings coming from dependent packages or the Python interpreter itself. Again, while some of them point to valid bugs, most of them do not require immediate patching and only cause unnecessary build and/or test failures.

A problem specific to pytest plugins is that some of them require additional command-line options when used in the test suite. For example, pytest-cov is configured by passing --cov* options [13]. If these options are forced via addopts configuration variable, pytest throws an error if pytest-cov is not installed (because it doesn't recognize the options). As a result, the test suite ends up requiring pytest-cov unconditionally! That said, this isn't a very big deal since we can strip these options easily.

If you'd like to run quality checks as part of your process, please do so by all means. However, please consider integrating them in such a way as to make them entirely optional. They could either be run outside the test suite entirely, or integrated into it in an opt-in or opt-out basis.

Rather than putting all options specific to pytest plugins in addopts unconditionally, consider either passing them externally in your CI configuration (e.g. calling pytest -Werror --cov — i.e. an opt-in solution) or using pytest-enabler to pass the relevant options only if the specified plugin is installed (an opt-out solution).

Assuming -Werror, catching warnings as exceptions

A surprisingly common side effect of running Python test suites using -Werror is catching warnings as exceptions. To avoid the quality check problems described in the previous section, Gentoo runs test suites with -Wdefault instead. As a result, the warning is never turned into an exception, the test does not catch anything and it fails.

When you intend to check whether a warning is issued, use the method appropriate to catch warnings — e.g. unittest.TestCase.assertWarns() or pytest.warns(). The fix is usually trivial, see e.g. the fix in pydantic [14].

Precision problems and other kinds of platform dependency

Floating-point arithmetic is hard. There are numerous articles covering its pitfalls, including Floating Point Arithmetic: Issues and Limitations in Python documentation (and the links therein). As a rule of thumb, you shouldn't assume that:

  1. any operation will give the "obvious" result (e.g. 0.1 + 0.2 ≠ 0.3),

  2. two mathematically equivalent operations will give the same result ([0.1 + 0.2] + 0.3 ≠ 0.1 + [0.2 + 0.3]),

  3. a printed result will yield the same number when typed back.

Unsurprisingly, occasionally we see a test suite that fails on a specific machine because the floating-point arithmetic gave a different result than on the system used to run CI. This doesn't even have to be an exotic architecture — only recently I've found out that test_sum_function in elementpath fails with Python 3.12 on my amd64 system (while it passes upstream) [15].

Floating-point precision problems aren't the only category of portability problems test suites face. Besides architecture, test suites can fail due to insufficient memory, operating system differences, underlying filesystem (e.g. jupyter-server-fileid has some test failures on tmpfs [16]), missing hardware devices and so on.

Sometimes these test failures are inevitable. For example, we are maintaining a large patch to skip failing tests in psutil because many of the tests are making assumptions that don't hold on the variety of Gentoo systems [17]. Another extreme example is our patch increasing tolerances in matplotlib test suite that covers both architecture differences and mismatches due to different dependency versions [18].

In general, please try to make your tests portable. If you work with floating-point numbers, prefer inequality comparisons over equality, or use approximate equality comparisons (e.g. unittest.TestCase.assertAlmostEqual(), pytest.approx()) ­— and be prepared that someone might report that the tolerance is too small for their hardware.

If your test require specific platform features, try to detect whether they're available and either skip the relevant tests or try to give an explanatory error. Prefer mocking system interfaces when feasible.

Be mindful of hardware access. Your users may run your test suite using their regular user. The last thing you want is to scare them with windows popping up in the middle of the test suite xvfbwrapper to the rescue!) or playing loud sounds via their speakers.

Missing test files, using git repository

Many packages do not include the complete set of files needed to run tests in their source distributions (archives uploaded to PyPI). In some cases this is intentional, in other cases it is accidental — often going unnoticed simply because both the CI and developers run tests against a git checkout.

While technically downstreams can often use an autogenerated git archive (when the source hosting used provides such a feature), official distributions are preferable since the former tarballs are not guaranteed to be stable. Furthermore, in case of some packages the official source distributions include additional generated files. For example, jupyter-server uses node.js scripts to build CSS files [#JUPYTER-SERVER]. The PyPI sdist includes these files, and makes it possible for Gentoo to avoid having to make the npm horror work somehow.

A special case are test suites that actually require a git checkout to work. Probably the most extreme case is GitPython — the package uses its own repository as a test fixture, and therefore Gentoo needs to redistribute GitPython and its submodules as git bundles [20].

Ideally, please include all the test files in source distributions. This makes it possible for downstream distributors and users to run tests against the exact same sources they are using to install the package. It can also be a good idea to build a sdist archive, unpack it and run the tests inside the unpacked contents as part of CI.

Home directory use, config leakage

Another case worth mentioning are test suites that (often directly, e.g. via spawned tools) write into the user's home directory. This is technically not a problem for distribution testing, as we unconditionally provide a temporary directory as HOME but it affects starting the tests as a regular user.

The results can vary from package to package. They can range from packages leaving their own configuration files when they weren't actually used by the user to very large caches (pre-commit can clone a dozen large repositories and leave them in your ~/.cache forever!). They can also be as surprising as e.g. the test suite appending commands to your shell history.

A somewhat similar problem are test suites being affected by different aspects of system configuration. These could be configuration files from the home directory of the user, system configuration files, environment variables or even the kernel configuration. To list just a few examples: nox's test suite fails if NO_COLOR is set in the test environment [21]. Some packages started failing due to readline introducing the "bracketed paste" feature, and therefore we are disabling bracketed-paste explicitly in ebuilds [22]. It isn't uncommon for packages to fail due to the system using a different locale or timezone than the CI environment.

Unfortunately, there is no trivial solution to these problems. While it may be tempting to try to isolate the test run as much as possible (e.g. by stripping most of the environment variables), removing too much can also cause issues and surprising behavior. Just to list one example not strictly related to Python, stripping too much could remove ccache or distcc-related control variables.

If your test suite may write into the home directory, it is generally a good idea to override HOME with a temporary directory instead. Many tools have command-line switches and environment variables to disable or override system configuration. Locale and timezone settings can also be effected via environment variables (e.g. setting TZ=UTC). Known-bad environment variables can be stripped from the test environment but please make sure to scope the stripping right. For example, NO_COLOR should still be respected by the test runner itself.

Requiring specific locales

A very special case of test problems are locale problems. Many projects are actually become aware of them, one way or another. Unfortunately, they are often solved via requiring a specific locale, usually en_US.UTF-8 which is not a good solution either. Some projects also test with a variety of locale, e.g. agate uses German and French locales for testing [23].

Gentoo is probably quite special here as unlike many other Linux distributions, we do not default to building all locales or even a "common subset" of them. It is perfectly valid for a Gentoo system to have only a "C" locale, and possibly a single very specific locale (e.g. pl_PL.UTF-8). As a result, tests assuming or explicitly using en_US.UTF-8 could fail.

A curious case worth mentioning is that BSD libc is less lenient on locale strings than glibc is. natsort project used to assume that FreeBSD locale support is broken while they were incorrectly passing en_US.UTF8 as locale instead of en_US.UTF-8 [24].

Ideally, make your test suite locale-independent. If you need to rely on locale-specific behavior, ideally use the C.UTF-8 locale. If you need to support legacy systems that do not feature it, you can either use the "C" locale, try to find a supported UTF-8 locale, or combine both (e.g. "C" for reliable LC_COLLATE, a UTF-8 locale for LC_CTYPE). If you need to test behavior on very specific locales, please assume that they may not exist on a specific system and skip the relevant tests if they are missing.

Fuzzing as a part of the test suite

Fuzz testing means testing the package's behavior against randomized input. Fuzzing can sometimes find non-obvious bugs. However, it is equally likely to be time-consuming and not produce anything new. It also makes the test suite somewhat unpredictable, potentially making it fail on one run and pass on another.

Gentoo doesn't follow a single rule with regards to fuzzer-based tests. In general, if they are part of the normal test run, we leave them be. However, if they are time consuming or otherwise problematic, and the relevant functionality is covered by other tests, we may deselect them.

Please include static tests for at least the few baseline inputs. If you're including fuzzing as a part of the default test run, please bear in mind not to make them too time consuming and make it easy to deselect them. If they require additional dependencies (such as hypothesis), please confine them to a separate file to make it possible to avoid the dependency when they are being skipped.


In this article, I have listed a fair number of problems and potential pitfalls regarding test suites. Nevertheless, most of it could be summarized in a few sentences.

Please consider your test suite not only as a tool for the project developers to test their work but also as a versatile tool for your users to use in order to ensure that the package is working correctly on as many systems as possible. It is simply technically not possible to cover all possible scenarios with a continuous integration system, and users can provide valuable results from their test runs.

Good support for downstream testing primarily falls into the topic of portability. Make as few assumptions as possible, consider the possible limitations and provide means to customization. Most importantly, be friendly and helpful when receiving bug reports. We all share the common goal of delivering software that works as well as possible!