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A test fixture is something used to consistently test some item, device, or piece of software. Test fixtures can be found when testing electronics, software and physical devices.
In testing electronic equipment such as circuit boards, electronic components, and chips, a test fixture is a device or setup designed to hold the device under test in place and allow it to be tested by being subjected to controlled electronic test signals.
A software test fixture sets up the system for the testing process by providing it with all the necessary code to initialize it, thereby satisfying whatever preconditions there may be. An example could be loading up a database with known parameters from a customer site before running your test. The Ruby on Rails web framework uses YAML to initialize a database before running a test. This allows for tests to be repeatable, which is one of the key features of an effective test framework.
Test fixtures can be set up three different ways: in-line, delegate, and implicit.
The advantage of a test fixture is that it allows for tests to be repeatable since one is always starting with the same setup every time. Test fixtures also ease test code design by allowing the developer to separate methods into different functions and reuse each function for other tests. It also preconfigures tests into a known state at start instead of working with whatever was left from a previous test run. A disadvantage is that it could lead to duplication of test fixtures if using in-line setup.
It is considered bad practice when test fixtures are too general in implicit setup, or when the test method sets up a test fixture and does not use it during the test. A more subtle issue is if the test methods ignore certain fields within the test fixture. Another bad practice would be a test setup that contains more steps than are needed for the test; this is a problem seen in in-line setup.
A test case is considered unsafe when it modifies the fixtures during the setup phase. This is more pronounced in the fixture setups other than the in-line setup. An unsafe test case is a serious problem which could render subsequent tests useless by leaving the fixture in an unexpected state. It also causes the order of tests to be important, so that you know whether a fixture setup needs to be rerun or not. If a safe test is run after an unsafe test then the fixture setup is rerun if the fixture is common for both but the same is not true for vice versa.
Examples of fixtures include loading a database with a specific known set of data, erasing a hard disk and installing a known clean operating system installation, copying a specific known set of files or the preparation of input data and set-up/creation of mock objects.
Software which is used to run reproducible tests systematically on a piece of software under test is known as a test harness; part of its job is to set up suitable test fixtures.
Frequently fixtures are created by handling setUp() and tearDown() events of the unit testing framework. In setUp() one would create the expected state for the test and intearDown() it would clean up what had been set up.
Four phases of a test:
In physical testing, a fixture is a device or apparatus to hold or support the test specimen during the test. The influence of test fixtures on test results is important and is an ongoing subject of research.
Some fixtures employ clamps, wedge grips and pincer grips.
Further types of construction include eccentric roller fixtures, thread grips and button head grips and rope grips.
Mechanical holding apparatuses provide the clamping force via arms, wedges or eccentric wheel to the jaws. Additionally there are pneumatic and hydraulic fixtures for tensile testing that allow very fast clamping procedures and very high clamping forces.