PyHamcrest is a framework for writing matcher objects, allowing you
to declaratively define "match" rules. There are a number of
situations where matchers are invaluable, such as UI validation, or
data filtering, but it is in the area of writing flexible tests that
matchers are most commonly used. This tutorial shows you how to use
PyHamcrest for unit testing.
When writing tests it is sometimes difficult to get the balance right
between overspecifying the test (and making it brittle to changes),
and not specifying enough (making the test less valuable since it
continues to pass even when the thing being tested is broken). Having
a tool that allows you to pick out precisely the aspect under test
and describe the values it should have, to a controlled level of
precision, helps greatly in writing tests that are "just right." Such
tests fail when the behavior of the aspect under test deviates from
the expected behavior, yet continue to pass when minor, unrelated
changes to the behaviour are made.