Testing with pytest

Testing in notebook

By default, marimo discovers and executes tests inside your notebook. When the optional pytest dependency is present, marimo runs pytest on cells that consist exclusively of test code - i.e. functions whose names start with test_, classes whose names start with Test, or functions decorated with @pytest.fixture. If a cell mixes in anything else (helper functions, constants, variables, imports, etc.), that cell is skipped by the test runner (we recommend you move helpers to another cell).

For example,

Reactive tests can be disabled

You can disable this behavior with the runtime.reactive_test option in the configuration file.

Testing at the command-line

Since marimo notebooks are Python programs, you can test them using pytest, a popular testing framework for Python.

For example,

pytest test_notebook.py

runs and tests all notebook cells whose names start with test_, or cells that contain only test_ functions and Test classes (just like in notebook tests).

Naming cells

Name a cell by giving its function a name in the notebook file, or using the cell action menu in the notebook editor.

Use marimo notebooks just like normal pytest tests

Include test notebooks (notebooks whose names start with test_) in your standard test suite, and pytest will discover them automatically. In addition, you can write self-contained notebooks that contain their own unit tests, and run pytest on them directly (pytest my_notebook.py).

Example

Running pytest on

# content of test_notebook.py
import marimo

__generated_with = "0.10.6"
app = marimo.App()


@app.cell
def _():
    def inc(x):
        return x + 1
    return (inc,)


@app.cell
def test_fails(inc):
    assert inc(3) == 5, "This test fails"


@app.cell
def test_sanity(inc):
    assert inc(3) == 4, "This test passes"

@app.cell
def collection_of_tests(inc, pytest):
    @pytest.mark.parametrize(("x", "y"), [(3, 4), (4, 5)])
    def test_answer(x, y):
        assert inc(x) == y, "These tests should pass."

@app.cell
def imports():
    import pytest
    return pytest

prints

============================= test session starts ==============================
platform linux -- Python 3.12.9, pytest-8.3.5, pluggy-1.5.0
rootdir: /notebooks
configfile: pyproject.toml
collected 4 items

test_notebook.py::test_fails FAILED                                       [ 25%]
test_notebook.py::test_sanity PASSED                                      [ 50%]
test_notebook.py::MarimoTestBlock_0::test_parameterized[3-4] PASSED       [ 75%]
test_notebook.py::MarimoTestBlock_0::test_parameterized[4-5] PASSED       [100%]

=================================== FAILURES ===================================
__________________________________ test_fails __________________________________

    # content of test_notebook.py
    import marimo

    __generated_with = "0.10.6"
    app = marimo.App()


    @app.cell
    def _():
        def inc(x):
            return x + 1
        return (inc,)


    @app.cell
    def test_fails(inc):
>       assert inc(3) == 5, "This test fails"
E       AssertionError: This test fails
E       assert 4 == 5
E        +  where 4 = <function inc>(3)

test_notebook.py:17: AssertionError
=========================== short test summary info ============================
FAILED test_notebook.py::test_fails - AssertionError: This test fails
========================= 1 failed, 3 passed in 0.82s ==========================

Using Pytest Fixtures

marimo supports pytest fixtures with some limitations due to how pytest collects tests statically.

Supported Patterns

Fixtures defined in app.setup:

# test_notebook.py
import marimo
app = marimo.App()

with app.setup:
    from fixtures import db_connection, sample_data

@app.cell
def _(sample_data):
    def test_data_loaded(sample_data):
        assert len(sample_data) > 0

Fixtures in the same cell as tests:

@app.cell
def _():
    import pytest
    return pytest


@app.cell
def _(pytest):
    @pytest.fixture
    def temp_file():
        import tempfile
        with tempfile.NamedTemporaryFile() as f:
            yield f

    def test_writes_to_file(temp_file):
        temp_file.write(b"hello")
        temp_file.seek(0)
        assert temp_file.read() == b"hello"

Class fixtures:

@app.cell
def _():
    import pytest
    return pytest


@app.cell
def _(pytest):
    class TestDatabase:
        @pytest.fixture(scope="class")
        def connection(self):
            return create_connection()

        def test_query(self, connection):
            result = connection.query("SELECT 1")
            assert result == 1

conftest.py fixtures work as expected - pytest discovers them automatically.

Fixture Limitations

Fixtures defined in one cell cannot be used by tests in a different cell. This is because pytest collects tests statically by parsing the notebook file without executing it. During collection, pytest can see module-level fixtures (from conftest.py or imported modules) and fixtures defined in the same scope as the test, but it cannot see fixtures defined in other cells.

Why? Running the entire notebook just for fixture discovery would be expensive, and static analysis cannot determine which fixtures will be available after cell execution since cell order is determined at runtime by marimo's dependency graph.

Recommended Patterns

1. Define fixtures in a separate file and import them in app.setup:

   with app.setup:
       from my_fixtures import db_fixture, api_client
   

2. Define fixtures in the same cell as the tests that use them

3. Use conftest.py for fixtures shared across multiple notebooks