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Modifier Tests


The current framework supports tests for:

  • Mesh Modifiers
    • Generate
    • Deform
    • Physics
  • Curve Modifiers
  • Operators

There are some exceptions and limitations as each modifier is a bit different, some require to switch between Object and Edit mode, some are just random and non-reproducible, and some output an Empty Mesh, hence its difficult or not possible to test them.


The current framework support is only for "mesh" and "curve" modifiers, not other objects like grease pencil and volume. Additionally, as more functionality moves to geometry nodes tests might move as well.

Adding of new tests

The current tests for Mesh Modifiers are located in tests/python/ file. Navigate to this file and add another test for "Array" Modifier.

Steps to follow:

  1. Navigate to the supporting blend files which are located in the `lib\tests\ folder.
  2. Open the file modifiers.blend which is located in the modeling folder.
  3. Open the blend file and add 2 Cubes.
  4. Rename them to "LearnTestCubeArray", and the second one "LearnExpCubeArray" respectively.
  5. Now open the python file and add this line in the tests list
 SpecMeshTest("LearnTestingArray", "LearnTestCubeArray", "LearnExpCubeArray",
[ModifierSpec('array', 'ARRAY', {})]),


To have a better understanding for each parameter, navigate to tests/python/modules/ and look at the comments in "ModifierSpec" class, this will give a clearer idea of what's possible.

Different passed arguments

  1. --verbose: First set your environment variable. Type set BLENDER_VERBOSE=1 in the command prompt (cmd). Note, it doesn't work for Windows Powershell.
  2. --run-test: To run a single test for visualization and comparison purpose in Blender.
  3. --run-all-tests: To run all tests.

Advanced Steps for Automated Testing

How do the tests work?

Good question, the trick is you don't have to manually add and apply the modifier in Blender at all, on the "LearnExpCubeArray" that is the expected object, you can but then that is doing it manually. So here are the steps-

  1. After you add the test and expected object, no need to apply the modifier to the expected object.
  2. Set the BLENDER_TEST_UPDATE flag to 1 (or any value look at Stackoverflow on how variables are "set", 1 is just the simplest), and it will update the expected object for you.
  3. There is a good chance that the test will fail for the first time, if you look closely at the console, then get updated and then should pass.
  4. The file will get saved with the updated expected object, so the next time you run the test, it will pass in first go.


Failing of a test doesn't necessarily mean its a bug.

Changing Parameters of the Added Tests

Turn on the Python tool tip in the Preferences in Blender. You can now hover the setting you want to change and look at the attribute name. For example, let's increase the count in the Array modifier. Then the code will look like this -

 "LearnTestingArray", "LearnTestCubeArray", "LearnExpCubeArray",
[ModifierSpec('array', 'ARRAY', {'count': 4})]],


The setting name in Blender and actual setting might be different, that's where the tooltip will help you.

Contributing, Reviewing and Committing

For Contributing: The tests can be contributed by anyone.

For Reviewing: One module member of the affected area should be included, Jacques or Hans (Geometry Nodes), Bastien or Campbell (modifiers/operators), Jeroen Baker (Compositor) and Brecht for performance tests. Apart from the core members, volunteers to add for first pass and initial feedback:

  • Habib Gahbiche (zazizizou): Operators, modifiers, compositors and geometry nodes tests.
  • Himanshi Kalra (calra): Operators, modifiers and geometry nodes tests.

For committing: One requires commit access and id and password of, I use SmartSVN to commit the test files. Like other patches, test files can also be committed on other's behalf.

Points to remember

  • Try to keep the meshes as simple as possible.
  • Make sure the tests pass after the BLENDER_TEST_UPDATE flag is un-set.

Future Scope

Adding tests for new modifiers and extending the framework for new object types.


What to do my test is failing?

Use Ctrl+F and find a similar test, look closely at the comments and see if its a corner case.

Its still failing? Let's dig deeper...

For Windows: Set the BLENDER_TEST_UPDATE flag=1 in the environment variable, so now when you run the test, the "expected object" gets updated automatically, if all's good it should pass.

If it still fails, you can manually investigate by running a single test using the --run-test <test_name>, see the example below

blender c:\blender-git\lib\tests\modeling\modifiers.blend --python c:\blender-git\blender\tests\python\ -- --run-test CurveWeld -- --verbose

Here you will find a new object called "evaluated_object", you can try applying the modifiers to it and see if its the same as the expected object you added.

In case you are interested in testing operators, they follow a similar approach.

What to do when getting errors in console?

Try slowly reading through it, it could be a typo in parameter name, so the exception thrown would be an AttributeError and some common programming mistake.

Where are the Physics modifiers' tests located?

Each Physics modifier has been given a separate python file and blend file, the python file name usually starts with physics_<modifier_name>, so you can test them as well, have a look at already added tests to see how to send the parameters, they use nested dictionary to send nested parameters. The supporting blend files are in physics folder.

Deform modifiers have a common blend file for Deform tests of both Mesh and Curves, it contains mostly the one that requires user input, which has been achieved by animating the object.