Tutorial: The basics¶
This tutorial will walk you through the steps needed to install, configure, and run Cosmic Ray.
First you’ll need to install Cosmic Ray. The simplest (and generally best) way to do this is with
pip install cosmic-ray
You’ll generally want to do this in a virtual environment, but it’s not required.
Installation from source¶
If you need to install Cosmic Ray from source, first change to the directorying containing
setup.py. Then run:
pip install .
Or, if you want to install from source in “editable” mode, you can use the ``-e` flag:
pip install -e .
Source module and tests¶
Mutation testing works by making small mutations to the code under test (CUT) and then running a test suite over the mutated code. For this tutorial, then, we’ll need to create our CUT and a test suite for it.
You should create a new directory which will contain the CUT, the tests, and eventually the Cosmic Ray data.
For the rest of this tutorial we’ll refer to this new directory as
$ROOT if we’re showing shell code).
Now create the file
ROOT/mod.py with these contents:
def func(): return 1234
This file contains your code under test, i.e. the code that Cosmic Ray will mutate. It’s clearly very simple, and it has very few opportunities for mutation, but it’s sufficient for this tutorial. In fact, having simple code like this will make it easier to see what Cosmic Ray is doing without getting bogged down by scale.
Next create the file
ROOT/test_mod.py with these contents:
import unittest import mod class Tests(unittest.TestCase): def test_func(self): self.assertEqual(mod.func(), 1234)
This contains the test suite for
mod.py. Cosmic Ray will not mutate this code. Rather, it will run this test suite
for every mutation that it creates.
Before moving on, let’s make sure that the test suite works correctly:
python -m unittest test_mod.py
This should show that all tests pass:
. ---------------------------------------------------------------------- Ran 1 test in 0.000s OK
If you see one test passing like this, then you’re ready to continue!
Creating a configuration¶
Before you do run any mutation tests, you need to create a configuration. A configuration is a TOML file that specifies the modules you want to mutate, the test scripts to use, and so forth. A configuration is used to create a session, something we’ll look at in the next section.
You can create a configuration by hand if you want. In fact, you’ll generally
need to edit them by hand to get the exact configuration you need. But you can
create an initial configuration using the
new-config command. This will ask
you a series of questions and construct a new configuration based on your
To create your config for this tutorial, do this:
cd $ROOT cosmic-ray new-config tutorial.toml
This will ask you a series of questions. Answer them like this:
[?] Top-level module path: mod.py [?] Test execution timeout (seconds): 10 [?] Test command: python -m unittest test_mod.py -- MENU: Distributor -- (0) http (1) local [?] Enter menu selection: 1
This will create the file
tutorial.toml with these contents:
1 2 3 4 5 6 7 8
[cosmic-ray] module-path = "mod.py" timeout = 10.0 excluded-modules =  test-command = "python -m unittest test_mod.py" [cosmic-ray.distributor] name = "local"
Let’s examine the contents of this file before moving on. On line 1 we define the ‘cosmic-ray’ key in the TOML structure; this key will contain all Cosmic Ray configuration information.
On line 2 we set the ‘module-path’ key to the string “mod.py”:
module-path = "mod.py"
This tells Cosmic Ray that we’re going to be mutating the module in the file
mod.py. Every Cosmic Ray configuration
refers to a single top-level module that will be mutated, and in this case we’re telling Cosmic Ray to mutate the
mod module, contained in the file
The ‘module-path’ is a path to a file or directory, not the name of the module of package. If it’s a file then Cosmic Ray will treat it as a single module, but if it’s a directory then Cosmic Ray will treat it as a package.
When working on a package, Cosmic Ray will apply mutations to all submodules in the package.
Additionally, the ‘module-path’ can be a list of directories or files: module-path = [“file1.py”, “some_directory”]
Line 3 tells Cosmic Ray the maximium amount of time to let a test run before it’s considered a failure:
timeout = 10.0
In this case, we’re telling Cosmic Ray to kill a test if it runs longer than 10 seconds. This timeout is important because some mutations can cause the tests to go into an infinite loop. Without timeout we’d never exit the test! It’s important to set this timeout such that it’s long enough for all legitimate tests.
Next, line 4 tells Cosmic Ray which modules to exclude from mutation:
excluded-modules = 
In this case we’re not excluding any, but there may be times when you need to skip certain modules, e.g. because
you know that you don’t have sufficient tests for them at the moment. This parameter expects glob-patterns, so to exclude
files that end with
_test.py recursively for example, you would add
Line 5 is one of the most critical lines in the configuration. This tells Cosmic Ray how to run your test suite:
test-command = "python -m unittest test_mod.py"
In this case, our test suite uses the standard unittest testing framework, and the tests are in the file
The last two lines tell Cosmic Ray which “distributor” to use:
[cosmic-ray.distributor] name = "local"
A distributor controls how mutation jobs are assigned to one or more workers so that they can (potentially) run in parallel. In this case we’re using the default ‘local’ distributor which only runs one mutation at a time. There are other, more sophisticated distributors which we discuss elsewhere.
Create a session and baseline¶
Cosmic Ray uses a notion of sessions to encompass a full mutation testing suite. Since mutation testing runs can take a long time, and since you might need to stop and start them, sessions store data about the progress of a run.
Most Cosmic Ray commands allow you to increase their “verbosity” via the command line. This will make them print out more information about what they’re doing.
Try adding “–verbosity INFO” to the command you run if you more details about what’s going on!
Initializing a session¶
The first step in a full testing run, then, is to initialize a session:
cosmic-ray init tutorial.toml tutorial.sqlite
This command prepares all the mutations that will later be applied to code. As such, its execution time is proportional to the amount of code and the code complexitly. You can expect about 15-30s per 1kloc.
This will create a database file called
tutorial.sqlite. There is a record in the database for each mutation that
Cosmic Ray will perform, and Cosmic Ray will associate testing results with these records as it executes.
When does init need to be run?¶
init completely rewrites the session file you tell it to use, so you should not re-run init on a session that contains any results that you want to keep. At the same time, if you change your configuration in a way that alters which tests are run and which mutations are made, then you should re-initialize your session.
Generally speaking, if you change the ‘module-path’, ‘timeout’, ‘excluded-modules’, or ‘test-command’ parts of your configuration, or if you change any of the filters you use, then you need to re-initialize your session and start over. Any of these changes can affect the operations that the subsequent exec command will run.
Similarly, you need to create a new session with init whenever your code-under-test or your tests themselves change. This is necessary because changes to the CUT will affect which mutations are made and changes to the tests affect which tests are run.
Before running the full mutation suite, it’s important to make sure that the test suite passes in the absence of any mutations. If the test suite does not pass in the absence of mutations, then the results of the mutation testing are essentially useless.
You can use the
baseline command to check that the test suite passes on unmutated code:
cosmic-ray --verbosity=INFO baseline tutorial.toml
This should report that the tests pass, something like this:
[07/23/21 10:00:20] INFO INFO:root:Reading config from 'tutorial.toml' config.py:103 INFO INFO:cosmic_ray.commands.execute:Beginning execution execute.py:45 INFO INFO:cosmic_ray.testing:Running test (timeout=10.0): python -m unittest test_mod.py testing.py:36 INFO INFO:cosmic_ray.commands.execute:Job baseline complete execute.py:43 INFO INFO:cosmic_ray.commands.execute:Execution finished execute.py:53 INFO INFO:root:Baseline passed. Execution with no mutation works fine.
If this command succeeds, then you’re ready to start mutating code and testing it!
Examining the session with cr-report¶
Our session file,
tutorial.sqlite, is essentially a list of mutations that Cosmic Ray will perform on the
code under test. We haven’t actually tested any mutants, so none of our mutations have testing results yet. With
that in mind, let’s examine the contents of our session with the
cr-report tutorial.sqlite --show-pending
This will produce output like this (though note that the test IDs will be different):
[job-id] f168ef23dff24b75846a730858fe0111 mod.py core/NumberReplacer 0 [job-id] 929a563b613242b48dae0f2de74ad2af mod.py core/NumberReplacer 1 total jobs: 2 no jobs completed
This is telling us that Cosmic Ray detected two mutations that it can make to our code, both using the
mutation operator “core/NumberReplacer”. Without going into details, this means that Cosmic Ray has found
one or more numeric literals in our code, and it plans to make two mutations to those numbers. We can see in our
code that there is only one numeric literal, the value returned from
func() on line 2:
def func(): return 1234
So Cosmic Ray is going to mutate that number in two ways, running the test suite each time.
cr-report tool is useful for examining sessions, and it’s main purpose is to give you summary reports after an
entire session has been executed, which we’ll do in the next step.
Now that you’ve initialized and baselined your session, it’s time to start making mutants and testing them. We do this
exec looks in your session file,
tutorial.sqlite, for any mutations which were
detected in the
init phase that don’t yet have results. For each of these, it performs the specified mutation
and runs the test suite.
As we saw, we only have two mutations to make, and our test suite is very small. As a result the
exec command will
run quite quickly:
cosmic-ray exec tutorial.toml tutorial.sqlite
This should produce no output.
The module and test suite for this tutorial are “toys” by design. As such, they run very quickly. Most real-world
modules and test suites are much more substantial and require much longer to run. For example, if a test suite takes
10 seconds to run and Cosmic Ray finds 1000 mutations, a full
exec will take 10 x 1000 = 10,000 seconds, or
about 2.7 hours.
Committing before exec¶
If you’re using revision control with your code (you are, right?!), you should consider committing your changes before running exec. While it’s not strictly necessary to do this in simple cases, it’s often important to commit if you’re using tools like cr-http-workers that rely on fetching code from a repository.
Also, while Cosmic Ray is designed to be robust in the face of exceptions and crashes, there is always the possibility that Cosmic Ray won’t correctly undo a mutation. Remember, it makes mutations directly on disk, so if a mutation is not correctly undone, and if you haven’t committed your changes prior to testing, you run the risk of introducing a mutation into you code accidentally.
Reporting the results¶
Assuming it ran correctly, we can now use
cr-report to see the updated state of our session:
cr-report tutorial.sqlite --show-pending
This time we see that both mutations were made, tests were run for each, and both were “killed”:
[job-id] f168ef23dff24b75846a730858fe0111 mod.py core/NumberReplacer 0 worker outcome: normal, test outcome: killed [job-id] 929a563b613242b48dae0f2de74ad2af mod.py core/NumberReplacer 1 worker outcome: normal, test outcome: killed total jobs: 2 complete: 2 (100.00%) surviving mutants: 0 (0.00%)
You don’t have to wait for
exec to complete to generate a report. If you have a long-running session and want to
see your progress, you can execute
cosmic-ray exec is running to view the progress the
latter is making.
You can also generate a handy HTML report with cr-html:
cr-html tutorial.sqlite > report.html
You can then open
report.html in your browser to see the details. One nice feature of these HTML reports is
that they show the actual mutation that was used.