This module is a plugin for the popular Python testing framework pytest. It manages dependencies of tests: you may mark some tests as dependent from other tests. These tests will then be skipped if any of the dependencies did fail or has been skipped.
In the theory of good test design, tests should be self-contained and independent. Each test should cover one single issue, either verify that one single feature is working or that one single bug is fixed. Tests should be designed to work in any order independent of each other.
So far the theory. The practice is often more complicated then that. Sometimes, the principle of independency of tests is simply unrealistic or impractical. Program features often depend on each other. If some feature B depends on another feature A in such a way that B cannot work without A, then it may simply be pointless to run the test for B unless the test for A has succeeded. Another case may be if the subject of the tests has an internal state that unavoidably is influenced by the tests. In this situation it may happen that test A, as a side effect, sets the system in some state that is the precondition to be able to run test B. Again, in this case it would be pointless to try running test B unless test A has been run successful.
It should be emphasized however that the principle of independency of tests is still valid. Before using pytest-dependency, it is still advisable to reconsider your test design and to avoid dependencies of tests whenever possible, rather then to manage these dependencies.
The pytest-dependency module defines a marker that can be applied to tests. The marker accepts an argument that allows to list the dependencies of the test. Both tests, the dependency and the dependent test should be decorated with the marker. Behind the scenes, the marker arranges for the result of the test to be recorded internally. If a list of dependencies has been given as argument, the marker verifies that a successful outcome of all the dependencies has been registered previously and causes a skip of the test if this was not the case.
The benefit of skipping dependent tests is the same as for skipping tests in general: it avoids cluttering the test report with useless and misleading failure reports from tests that have been known beforehand not to work in this particular case.
If tests depend on each other in such a way that test B cannot work unless test A has been run successfully, a failure of test A will likely result in failure messages from both tests. But the failure message from test B will not be helpful in any way. It will only distract the user from the real issue that is the failure of test A. Skipping test B in this case will help the user to concentrate on those results that really matter.
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