A Streamlit app can look fine until unexpected data or a code change exposes a bug. pytest is a Python framework for writing and running automated checks, so you can catch failures before users do. section 11.0 section 11.2.3
Start with one test
Install pytest with `pip install pytest`. In `test_example.py`, define `add(x, y)` and a `test_add()` function that checks `assert add(2, 3) == 5`. Run `pytest -v` in that file’s directory. pytest discovers the test and reports whether it passed. section 11.2.3
Build useful tests one check at a time:
- Name test functions with a `test_` prefix. Use `assert` to compare actual results with expected results, including cases such as an empty search result. section 11.3.2
- Put repeated setup in a function marked `@pytest.fixture`. A test receives that setup by naming the fixture as a parameter. section 11.3.2
- Use `unittest.mock.patch` to replace an external request when unit-testing backend behavior. Check both the returned data and how the request was called. section 11.3.2
Listing 11.5 Backend unit tests
import pytest
from unittest.mock import patch, Mock
from backend import Backend, SEARCH_URL
from mock_data import MOCK_MOVIES, MOCK_RESULTS
@pytest.fixture
def backend():
api_key = "test_api_key"
return Backend(api_key)
@patch('requests.get')
def test_search_movies_success(mock_get, backend):
mock_response = Mock()
mock_response.json.return_value = {'results': MOCK_MOVIES}
mock_get.return_value = mock_response
results = backend.search_movies("test query")
mock_get.assert_called_once_with(
SEARCH_URL,
params={"api_key": backend.api_key, "query": "test query"}
)
assert len(results) == len(MOCK_MOVIES)
for _id, movie in MOCK_RESULTS.items():
assert _id in results
assert results[_id]["title"] == movie["title"]
A complete backend test combines a fixture, a mocked HTTP response, and assertions about the search request and its results. section 11.3.2
When a test fails, read the failing assertion and pytest’s expected-versus-actual output. An intentionally wrong expectation of `[]` reveals that an empty movie search returns `{}`. A frontend test catches a quieter bug: the display reads a missing `rating` field instead of `vote_average`, showing `N/A` rather than the movie’s rating. Fix the code and run `pytest -v` again. Passing isolated tests are useful, but they do not establish that frontend and backend work together; integration tests check that connection. section 11.3.3 section 11.4.2
