pytest-cdist

Like pytest-xdist, but for distributed environments - pytest-cdist allows to split a test suite into multiple parts and run them independently.

This is a work in progress

Why?

pytest-xdist can help to parallelize test execution, as long as you can scale horizontally. In many environments, such as GitHub actions with GitHub runners, this is only possible to a fairly limited degree, which can be an issue if your test suite grows large. pytest-cdist can help with this by allowing to execute individual chunks of your test suite in a deterministic order, so you can use multiple concurrent jobs to run each individual chunk.

How?

pytest --cdist-group=1/2  # will run the first half of the test suite
pytest --cdist-group=2/2  # will run the second half of the test suite

In a GitHub workflow

jobs:
  test:
    runs-on: ubuntu-latest
    matrix:
      strategy:
        cdist-groups: [1, 2, 3, 4]

    steps:
      - uses: actions/checkout@v4
      # set up environment here
      - name: Run pytest
        run: pytest --cdist-group=${{ matrix.cdist-group }}/4

Usage

Configuration

Pytest-cdist comes with several CLI and pytest-ini options:

Controlling how items are split up

By default, pytest-cdist will split up the items into groups as evenly as possible. Sometimes this may not be desired, for example if there's some costly fixtures requested by multiple tests, which should ideally only run once.

To solve this, the cdist-justify-items option can be used to configure how items are split up. It can take two possible values:

• file: Ensure all items inside a file end up in the same group
• scope: Ensure all items in the same pytest scope end up in the same group

[pytest]
cdist-justify-items=file

pytest --cdist-group=1/2 --cdist-justify-items=file

Skewing the group sizes

Normally, items are distributed evenly among groups, which is a good default, but there may be cases where this will result in an uneven execution time, if one group contains a number of slower tests than the other ones.

To work around this, the cdist-group-steal option can be used. It allows to specific a certain percentage of items a group will "steal" from other groups. For example --cdist-group-steal=2:30 will cause group 2 to steal 30% of items from all other groups.

[pytest]
cdist-group-steal=2:30

pytest --cdist-group=1/2 --cdist-group-steal=2:30

It is also possible to redistribute items between two specific groups, by specifying both as source and a target group. The following configuration would assign 50% of the items in group 1 to group 2:

pytest --cdist-group=1/3 --cdist-group-steal=1:50:2

With pytest-xdist

When running under pytest-xdist, pytest-cdist will honour tests marked with xdist_group, and group them together in the same cdist group.

With pytest-randomly

To use pytest-cdist with pytest-randomly's test reordering, a randomness seed that's consistent across the different pytest invocations needs to be specified, otherwise, test cases would be dropped, since pytest-cdist has to rely on a consistent collection order to split test cases among its groups.

To achieve this, a random source such as the current timestamp can be used:

jobs:
  setup_randomly_seed:
    runs-on: ubuntu-latest
    outputs:
      randomly_seed: ${{ steps.set-seed.outputs.randomly_seed }}
    steps:
      - name: Set seed
        id: set-seed
        run: echo "randomly_seed=$(date +%s)" >> $GITHUB_OUTPUT
        
  test:
    runs-on: ubuntu-latest
    needs: setup_randomly_seed
    matrix:
      strategy:
        cdist-groups: [1, 2, 3, 4]
    steps:
      - uses: actions/checkout@v4
      - name: Run pytest
        run: > 
          pytest 
          --cdist-group=${{ matrix.cdist-group }}/4 
          --randomly-seed=${{ needs.setup_randomly_seeds.outputs.randomly_seed }}

or a bit simpler (but less random), using the current git hash:

jobs:        
  test:
    runs-on: ubuntu-latest
    needs: setup_randomly_seed
    matrix:
      strategy:
        cdist-groups: [1, 2, 3, 4]
    steps:
      - uses: actions/checkout@v4
      - name: Run pytest
        run: > 
          pytest 
          --cdist-group=${{ matrix.cdist-group }}/4 
          --randomly-seed=$((16#$(git rev-parse HEAD)))