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Quick Start

Running the tests in docker

To run the tests in the almost exactly the same environment as CI, use docker. This is recommended to ensure that the tests pass in CI, rather than just on your machine.

  • Make sure you have docker installed (duh)
  • Run ./docker/manual/run.
    • This will use a Vim build with python 3
    • If you want python2, run ./docker/manual/run --py 2
  • You should now be in the container. Your YCM checkout is now mounted in $HOME/YouCompleteMe
  • Run the following setup:
    • cd YouCompleteMe
    • python3 --ts-completer --clangd-completer --java-completer
    • sudo -H pip3 install -r python/test_requirements.txt
  • Run the tests:
    • ./test/run_vim_tests

## Running the tests locally

The CI tests run in the container, so it's probably best to run your tests there too, but not strictly required.

To run locally, you have to be on MacOS or Linux. Sorry, Windows testing is not supported. However, there is a docker image in which you can run the tests.

  • Ensure you have at least the Vim vresion in YCM_VIM_VERSION (in test/docker/ci/image/Dockerfile)
  • Ensure ycmd is compiled with python3 and clangd is enabled python3 --ts-completer --clangd-completer --java-completer
  • Install the test python deps (pip install -r python/test_requirements.txt)
  • Run ./run_vim_tests


The test framework is based on the "new style" Vim tests. These are the tests that are used to test Vim itself. There is good info on this in :help testing.

There's also some useful info in Vim's test readme.

In short, the test framework runs Vim, sources the test script, then executes all of the test functions. This is done as follows :

vim --clean --not-a-term -S lib/run_test.vim <test script>.test.vim

The important thing to know is that run_test.vim is sourced and it in turn sources the test script, which contains the test functions, which are named Test_*.

Test functions should use the vim built-in assert functions to report errors (see :help new-style-testing) and should attempt to reset any changes they make at the end of the function.

You can add set-up and tear-down functions, and can skip tests by throwing a message starting with the word 'Skipped' (e.g. throw "SKipped: <message>").

Test Framework

The test framework has the following components:

  • A vim 'plugin' (in test/lib) containing the framework itself, comprising:
    • run_test.vim, which wraps the test functions and executes them, reporting failures.
    • Some basic support functions in plugin/shared.vim (from Vim)
    • Some screendump support functions in plugin/screendump.vim (from Vim)
    • Some YCM-specific autoloaded functions in autoload/youcompleteme/test/*
  • A script to run the tests, including specific test script and function
  • The actual test scripts in test/*.test.vim
  • CI integration for azure.

Test Scripts

The basic structure of a test is as follows:

function! SetUp()
  " ... set g:ycm_* options here...
  call youcompleteme#test#setup#SetUp()

function! TearDown()
  call youcompleteme#test#setup#CleanUp()

function! Test_MyTest()
  " test goes here, e.g.
  aseert_true( pyxeval( 'ycm_state.ServerIsHealthy()' ) )

" Optional per-test setup/teardown
function! SetUp_Test_MyOtherTest()
  let s:old_ycm_use_clangd = g:ycm_use_clangd
  let g:ycm_use_clangd=1

function! TearDown_Test_MyOtherTest()
  let g:ycm_use_clangd=s:old_ycm_use_clangd

function! Test_MyOtherTest()
  assert_false( 0 )

Test scripts are placed in src/test and are named *.test.vim. Each test script can contain any number of individual tests, each of which is a Vim function named Test_<test name>. Test functions are run in arbitrary order, so must not rely on each other.

Each test script is a fixture, but setup and teardown is done for each and every test. Global (one-time) setup can be done at script level, but this is not recommended.

Set up and tear down functions are run before and after tests. You can define one for the whole script, which is run before every test, and a per-test setup/tear down function which is run before both the global setup function and the test function.

To explain, for each function in the script named Test_<test name>, run_test.vim does the following:

  • If there is a function named SetUp_Test_<test name>, call it
  • If there is a function named SetUp, call it
  • Call Test_<test name>
  • If there is a function named TearDown, call it
  • If there is a function named TearDown_Test_<test name>, call it

If the v:errors list is non-empty at the end of the tests, the test test name is marked as failed.

If any of these functions raises an exception, it is added to v:errors, unless the test is called Test_nocatch_<test name>, in which case exceptions are not caught be the test and should be handled by the test function itself.

If a test fails, run_test.vim attempts to print out all of the log files that YCM's ycm_state object knows about.

The test plugin

The "plguin" provides a handful of things, some of which were simply ported from Vim's test framework, and some were writted specifically for YCM.

Ported from Vim

These are general purpose functions which are commonly used:

  • WaitForAssert: This one is the most useful. It takes a callable (usually a lambda) and waits for it to return 0, but allows the Vim event loop to run in betwen calls. This is key to ensuring that the YCM code can execute while the script is actively trying to test it. NOTE: It waits for the function to return 0, *NOT for the assert to be true/v:errors to be empty!


The autoload functions perform some useful common YCM-sepcific stuff such as setup and teardown, and will likely be built out over time as the suite increases in size and complexity.

Tips and tricks

Things that you need to know to write tests effectively:

  • Don't forget to :%bwipeout! at the end of each test function.

  • Understand the arguments to feedkeys. Importantly, if you want it to behave the way you think it should, use feedkey( "...", 'xt' ). This makes it wait for the input to be actually read before returning, which is important for tests. See :help feedkeys for the other options.

  • Remember that test scripts a vim functions. I know that sounds obvious, but things like "insert mode completions" are hard to test with functions which are typically not invoked in insert mode. In order to actually do anything in insert mode, you need to do the following:

    • If you want TextChangedI to fire, call test_override( 'char_avail', 1)

    • Normally, feedkeys would exit insert mode if you enter it. Tell it not to by passing the ! flag.

    • Now that you've left Vim in insert mode, your test will hang forever unless you exit insert mode, so define a function and call it via a timer or other async callback which performs the actual asserts, and ends by calling feedkeys( "\<ESC>" ) to exit insert mode.

    • Check completion.test.vim for an example.

  • Remember that the assert* functions don't throw exceptions. They return 0 on success, and return nonzero on failure, populating v:errors.

  • Throwing exceptions in tests does fail the test, but this is not recommended because it skips the (local) teardown code.

  • If you're writing a test function, it needs to conform as follows:

    • Only adding to v:errors (e.g. call add(v:errors, 'test')) will cause the test to fail.
    • Don't throw exceptions. This will cause fiddly issues.
    • Return 0 on success and 1 on failure. This allows it to be used with WaitForAssert

Restricting what is run

run_vim_tests takes arguments of the form <test script>:<test function filter>.

For eample to just run the "MyOtherTest" test in the mytests.test.vim:

$ run_vim_tests mytests.test.vim:MyOtherTest

The filter is a Vim regexp. The same script file can be listed multiple times, as in:

$ run_vim_tests 'mytests.test.vim:MyTest' 'mytests.test.vim:MyOtherTest'


The test suite supports covimerage coverage testing. Just set the COVERAGE environment variable when running run_vim_tests.

This generates coverage for both the python code and the vimscript code.

For python, there is some initialisation code in run_test.vim which starts up coverage and saves the result to .coverage.python.

For vim script, we use covimerage which takes the vim :profile data (also initialised in run_test.vim) and converts it to coverage-style data in .coverage.vim (this is done by run_vim_tests after running all the tests).

Finally, we upload that data to codecov. This combines .coverage.python and .coverage.vim into .coverage and uploads it.

To get a local summary:

  • pip install --user -r python/test_requirements.txt
  • COVERAGE=true ./test/run_vim_tests
  • coverage combine -a
  • coverage report or coverage html


We generate and push 4 containers:

  • youcompleteme/ycm-vim-py3:test and youcompleteme/ycm-vim-py3:manual
  • youcompleteme/ycm-vim-py2:test and youcompleteme/ycm-vim-py2:manual

The :test tags are the containers that are used by Azure pipelines to run the tests and contains essentially Ubuntu LTS + the YCM dependencies and a build of Vim at a specific version built with either python3 (-py3) or python 2 (-py2) support..

The :manual tags extend the :test tags with a user account that largely matches the one created by Azure to run our tests. It also installs a basic vimrc so that you can do manual testing too.


To rebuild and push all of the containers:

  • cd test/docker
  • ./rebuild_all

This script essentially runs ./rebuild and ./push in each of the ci and manual directories (corresponding to the :test and :manual tags respectively).

Those scripts are just simple wrappers for docker build and docker push because it's easy to forget the exact syntax.

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