Snapshots

Snapshot testing lets you assert that a complex value matches a previously stored “snapshot”. On the first run, the snapshot is written to disk. On subsequent runs, bunpy compares the current value to the stored snapshot and fails the test if they differ.

Basic usage

from bunpy.test import test
from bunpy.snapshot import toMatchSnapshot

@test("renders the right HTML")
def _():
    html = render_page(title="Hello")
    toMatchSnapshot(html)

First run (no snapshot exists):

✓ renders the right HTML  [snapshot created]

Second run (snapshot exists):

✓ renders the right HTML  [snapshot matched]

If the output changed:

✗ renders the right HTML  [snapshot mismatch]
  - Expected: <h1>Hello</h1>
  + Received: <h1>Hello!</h1>

Updating snapshots

To accept new output as the new baseline:

bunpy test --update-snapshots

Snapshot file location

Snapshots are stored in __snapshots__/ next to the test file:

tests/
├── test_render.py
└── __snapshots__/
    └── test_render.py.snap

The snapshot file is JSON and should be committed to version control.

Named snapshots

toMatchSnapshot(result, name="with-title")

Names are used to distinguish multiple snapshots within the same test.