How to mock httpx using pytest-mock#

I wrote this test to exercise some httpx code today, using pytest-mock.

The key was to use mocker.patch.object(cli, "httpx") which patches the httpx module that was imported by the cli module.

Here the mocker function argument is a fixture that is provided by pytest-mock.

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from conditional_get import cli
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from click.testing import CliRunner
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def test_performs_conditional_get(mocker):
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    m = mocker.patch.object(cli, "httpx")
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    m.get.return_value = mocker.Mock()
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    m.get.return_value.status_code = 200
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    m.get.return_value.content = b"Hello PNG"
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    m.get.return_value.headers = {"etag": "hello-etag"}
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    runner = CliRunner()
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    with runner.isolated_filesystem():
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        result = runner.invoke(
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            cli.cli, ["https://example.com/file.png", "-o", "file.png"]
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        )
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        m.get.assert_called_once_with("https://example.com/file.png", headers={})
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        assert b"Hello PNG" == open("file.png", "rb").read()
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        # Should have also written the ETags file
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        assert {"https://example.com/file.png": "hello-etag"} == json.load(
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            open("etags.json")
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        )
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        # Second call should react differently
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        m.get.reset_mock()
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        m.get.return_value.status_code = 304
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        result = runner.invoke(
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            cli.cli, ["https://example.com/file.png", "-o", "file.png"]
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        )
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        m.get.assert_called_once_with(
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            "https://example.com/file.png", headers={"If-None-Match": "hello-etag"}
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        )

https://github.com/simonw/conditional-get/blob/485fab46f01edd99818b829e99765ed9ce0978b5/tests/test_cli.py

Mocking a JSON response#

Here’s a mock for a GraphQL POST request that returns JSON:

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@pytest.fixture
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def mock_graphql_region(mocker):
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    m = mocker.patch("datasette_publish_fly.httpx")
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    m.post.return_value = mocker.Mock()
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    m.post.return_value.status_code = 200
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    m.post.return_value.json.return_value = {"data": {"nearestRegion": {"code": "sjc"}}}

https://github.com/simonw/datasette-publish-fly/blob/5253220bded001e94561e215d553f352838e7a1c/tests/test_publish_fly.py#L16-L21

Mocking httpx.stream#

I later had to figure out how to mock the following:

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with httpx.stream("GET", url, headers=headers) as response:
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    ...
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    with open(output, "wb") as fp:
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        for b in response.iter_bytes():
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            fp.write(b)

https://stackoverflow.com/a/6112456 helped me figure out the following:

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def test_performs_conditional_get(mocker):
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    m = mocker.patch.object(cli, "httpx")
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    m.stream.return_value.__enter__.return_value = mocker.Mock()
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    m.stream.return_value.__enter__.return_value.status_code = 200
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    m.stream.return_value.__enter__.return_value.iter_bytes.return_value = [
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        b"Hello PNG"
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    ]

https://github.com/simonw/conditional-get/blob/80454f972d39e2b418572d7938146830fab98fa6/tests/test_cli.py

Mocking an HTTP error triggered by response.raise_for_status()#

The response.raise_for_status() raises an exception if an HTTP error (e.g. a 404 or 500) occurred.

Here’s how I mocked that to return an error:

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def test_airtable_to_yaml_error(mocker):
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    m = mocker.patch.object(cli, "httpx")
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    m.get.return_value = mocker.Mock()
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    m.get.return_value.status_code = 401
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    m.get.return_value.raise_for_status.side_effect = httpx.HTTPError(
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        "Unauthorized", request=None
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    )
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    runner = CliRunner()
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    with runner.isolated_filesystem():
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        result = runner.invoke(
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            cli.cli, [".", "appZOGvNJPXCQ205F", "tablename", "-v", "--key", "x"]
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        )
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        assert result.exit_code == 1
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        assert result.stdout == "Error: Unauthorized\n"