dragosmg commented on a change in pull request #12140:
URL: https://github.com/apache/arrow/pull/12140#discussion_r784147236



##########
File path: docs/source/developers/guide/step_by_step/testing.rst
##########
@@ -62,29 +62,103 @@ 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      # unit test files live here
+          └── testthat.R    # runs tests when R CMD check runs (e.g. with 
devtools::check())
+
+      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.
+
+      **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

Review comment:
       My last bullet point covers refactoring, but I've included your entire 
paragraph below the list. Done




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