dragosmg commented on a change in pull request #12140:
URL: https://github.com/apache/arrow/pull/12140#discussion_r784998817
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File path: docs/source/developers/guide/step_by_step/testing.rst
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@@ -62,29 +62,101 @@ In this section we outline steps needed for unit testing
in Arrow.
If the tests start failing, try to recompile
PyArrow or C++.
-
+
.. note::
**Recompiling Cython**
If you only make changes to `.py` files, you do not need to
recompile PyArrow. However, you should recompile it if you make
changes in `.pyx` or `.pxd` files.
-
+
To do that run this command again:
.. code:: console
$ python setup.py build_ext --inplace
.. note::
-
+
**Recompiling C++**
Similarly, you will need to recompile the C++ code if you have
made changes to any C++ files. In this case,
- re-run the cmake commands again.
+ re-run the cmake commands again.
.. tab:: R tests
- .. TODO
+ We use `testthat <https://testthat.r-lib.org/index.html>`_ for unit
testing in R. More specifically, we use the `3rd edition of testthat
<https://testthat.r-lib.org/articles/third-edition.html>`_. On rare occasions
we might want the behaviour of the 2nd edition of testthat, which is indicated
by ``testthat::local_edition(2)``.
+
+ **Structure**
+
+ Expect the usual testthat folder structure:
+
+ .. code-block:: R
+
+ tests
+ ├── testthat # test files live here
+ └── testthat.R # runs tests when R CMD check runs (e.g. with
devtools::check())
+
+ This is the fundamental structure of testing in R with ``testthat``.
Files such as ``testthat.R`` are not expected to change very often. For the
``arrow`` R package ``testthat.R`` also defines how the results of the various
tests are displayed / reported in the console.
+
+ Usually, most files in the ``R/`` sub-folder have a corresponding test
file in ``tests/testthat``.
+
+ **Running tests**
+
+ To run all tests in a package locally call
+
+ .. code-block:: R
+
+ devtools::test()
+
+ in the R console. Alternatively, you can use
+
+ .. code:: console
+
+ $ make test
+
+ in the shell.
+
+ You can run the tests in a single test file you have open with
+
+ .. code-block:: R
+
+ devtools::test_active_file()
+
+ All tests are also run as part of our continuous integration (CI)
pipelines.
+
+ The Arrow R Developer guide also has a section on running tests. You can
check it out `here
<https://arrow.apache.org/docs/r/articles/developing.html#running-tests>`_.
+
+ **Good practice**
+
+ In general any change to source code needs to be accompanied by unit
tests. All tests are expected to pass before a pull request is merged.
+
+ * Add functionality -> add unit tests
+ * Modify functionality -> update unit tests
+ * Solve a bug -> add unit test before solving it, which helps prove the
bug and its fix
+ * Performance improvements should be reflected in benchmarks (which are
also tests)
+ * An exception could be refactoring functionality that is fully covered
by unit tests
+
+ If the new functionality is a user-facing or API change, you will almost
certainly need to change tests — if no tests need to be changed it might mean
the tests aren't right! If the new functionality is a refactor and no APIs are
changing, there might not need to be test changes.
+
+ **Testing helpers**
+
+ To complement the ``testthat`` functionality, the ``arrow`` R package
has defined a series of specific utility functions (called helpers), such as:
+
+ * expectations - these start with ``expect_`` and are used to compare
objects
+ - for example, ``expect_altrep_roundtrip()`` compares the result
of a function ``fn`` run on a vector ``x`` with the result of the same function
run on the altrep version of ``x``. More generally, ``expect_…_roundtrip()``
functions take an input, convert it to some other format (e.g. arrow) and then
convert it back, confirming that the values are the same.
Review comment:
Done. I changed the order of the sentences and went with general
description of `expect_..._roundtrip()` + example
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