---
tags: [python, testing]
---

# Testing Python code

## `pytest`

Pytest is the most popular testing library for Python. It is not included with
the Python standard library so it must be installed with
[pip](Python_package_management.md).
While it does not include a declaration library, it is robust enough to handle
most scenarios having a rich and expressive set of constructs and decorators
that let you declare what your tests should do, under what conditions they
should run, and how they should interact with the rest of your system.

### Using `pytest`

- Pytest will automatically detect test files so long as they are named
  appropriately. E.g. for a module called `lorem`, it will detzect the unit test
  files `lorem_test.py` and `test_lorem.py`.
- In order to detect tests it should be run from a directory level above them

### Examples

Here is a basic example of using pytest for a local module callled `palindrome`:

```py
import palindrome

def test_is_palindrome():
    assert palindrome.is_palindrome('soros')
    assert palindrome.is_palindrome('torot')
    assert not palindrome.is_palindrome('chair')
```

## Mocking

`patch()` and `Mock` enable us to mock objects whilst testing (classes,
functions, methods and properties belonging). They are used in combination.

The `@patch` decorator temporarily replaces a specified object in your code with
a mock object and restores the original object after the test is complete

A Mock object simulates the object it replaces so that the object behaves as
expected during testing. For example, if your code calls
`some_function.some_method()`, and `some_method` is mocked, calling
`some_method` will not execute real logic but will interact with the Mock object
instead. Mock objects record details about how they have been used, like what
methods have been called, with what arguments, etc., allowing you to make
assertions about how they have been used.

> `@patch` and `Mock` work together because a patch is used to replace an object
> or attribute with a Mock object. `Mock` handles the simulated functionality,
> and `@patch` designates the real value we are replacing with the mock.

### Example case

I will use the following example from one my projects:

```py
# get_articles.py
def get_articles(article_type: str) -> Optional[Dict[str, Any]]:
    """Retrieve articles from Pocket API"""

    if POCKET_LAMBDA_ENDPOINT is None:
        logging.error("Error: POCKET_LAMBDA_ENDPOINT envinronment variable is not set")
        return None
    else:
        # Interpolate the article_type into the Pocket request URL
        endpoint = POCKET_LAMBDA_ENDPOINT.format(article_type=article_type)

    try:
        response = requests.get(endpoint)
        response.raise_for_status()
        return response.json()

    except RequestException as e:
        print(f"An error occurred: {e}")
        return None
```

This function: sources a URL from an environment variable, interpolates a query
string into the URL (which comes in as a parameter), makes a request to the URL,
and returns the response as JSON.

It has some safeguards in place:

- It checks that the environment variable is set
- It checks that the request was successful

In the example we could use a Mock object to simulate the response from the
Pocket API. This would allow us to test the function without having to make a
real request to the API:

```py
def test_successful_request():
    # Replace the requests.get function with a Mock object (mock_get)
    with patch("requests.get") as mock_get:
        # Specify the return value of the mock_get object)
        mock_get.return_value = Mock(ok=true)
        mock_get.return_value.json.return_value = {"value": "test"}

        # Call the function under test
        result = get_articles("gaby")

    # Assert expected outcomes
    mock_get.assert_called_once_with(endpoint)
    assert result == mock_json_response
```

The example above follows the **Arrange, Act, Assert** pattern:

| Stage   | Action                                                                          |
| ------- | ------------------------------------------------------------------------------- |
| Arrange | Replace the `requests.get` function with `patch` and set properties with `Mock` |
| Act     | Call the function under test                                                    |
| Assert  | Assert that the function under test behaved as expected                         |

### Alternative mock syntax

The `with patch(...) as mock_name` syntax is fine for small-scale mocking but
can become cumbersome when you are mocking several dependencies.

There is another syntax (which I actually find clearer). Say we have a function
with three dependencies: `update_worksheet`, `process_articles`, `get_articles`.
We could mock like so:

```py
@patch("app.update_worksheet")
@patch("app.process_articles")
@patch("app.get_articles")
def test_success(
    mock_get_articles, mock_process_articles, mock_update_worksheet
):
    mock_get_articles.return_value = [1, 2, 3]
```

Here the patching is done by the decorator and the mocks are defined as
parameters to the test function (always in reverse order)

### Mock assertion lexicon

#### `return_value`

State what the mock should return

```py
my_mocked_function.return_value = ['one', 'two', 'three']
```

#### `call_count`

Test how many times a dependent function is called

```py
assert my_mocked_function.call_count = 3
```

#### `assert_any_call()`

Test that a given mock is called at least once during the execution of the
function under test

```py
my_mocked_function.assert_any_call(some_mocked_return_value)
```

When the output of one function is used as a parameter to another, and we don't
particularly care about the details of what is concerned we can just pass the
executed function, e.g:

```py
my_mocked_function.assert_any_call(preceding_function())
```

#### `call_args_list`

Get a list of all the arguments that a mock object was called with during the
test.

`call_args_list` is useful when you want to check the arguments that a mock
object was called with during the test, especially if the mock object was called
multiple times with different arguments. You can use it to inspect the arguments
of each call and make assertions based on them.

```py
second_my_mocked_function_call = my_mocked_function.call_args_list[1]

# check the first argument of the second call:

assert second_my_mocked_function_call[0][0] == "expected arg"
```

#### `side_effect`

Use to trigger a side effect when returning a value from a mock. Most useful for
mocking exceptions.

```py
my_mocked_function.side_effect = Exception("Some exception raised")
```

## Testing exceptions with `raises`

Testing exceptions is quite straightforward. You can use the `raises` helper
provided by pytest, and combine this with `excinfo` ("exception info") to
inspect the exception message.

```py
if POCKET_LAMBDA_ENDPOINT is None:
    raise ValueError(
        "Error: POCKET_LAMBDA_ENDPOINT environment variable is not set"
    )
```

Then to test this, we would use pytest's `excinfo` fixture along with `raises`:

```py
 with pytest.raises(ValueError) as excinfo:  # Watch for the ValueError
    get_articles("some_type")

    assert "Error: POCKET_LAMBDA_ENDPOINT environment variable is not set" in str(
        excinfo.value
    )
```

We could actually simplify the above test by using the `match` parameter with
`raise`. This way we do not need the separate assertion:

```py
with pytest.raises(ValueError, match="Error: POCKET_LAMBDA_ENDPOINT environment variable is not set"):
    get_articles("some_type")
```

Note that `excinfo` is best used for testing the exception text that you the
developer explicitly `raise`. For exceptions tha may occur naturaly in the code
you are testing, you should use `caplog` or `capsys` (see below).

## Before-each and after-each

When testing functions, we achieve this in Python using `setup_function` and
`teardown_function` methods. These methods are called before and after each test
method respectively.

To apply a "before each" to _every test_ just put your setup function and/or
teardown function at the top level of your test module.

For example, below we set and remove an env var before and after each test:

```py
@pytest.fixture(scope="function") # specify that this fixture should be run before each function test
def setup_function():
    print("Setting up test environment...")
    os.environ["POCKET_LAMBDA_ENDPOINT"] = "https://some_endpoint.com/{article_type}"


def teardown_function():
    print("Tearing down test environment...")
    del os.environ["POCKET_LAMBDA_ENDPOINT"]
```

If the setup/teardown should only be applied to a subset of tests, just pass the
name of the fixture as a parameter to the test function:

```py
def some_function(setup_function):
    # setup_function will be run before this test
```

You don't need to use the names `setup_function` and `teardown_function` so long
as you are passing the fixture as a parameter.

You can also use `yield` to combine the setup and teardown into a single
function:

```py
@pytest.fixture(scope="function")
def setup_function():
    os.environ["POCKET_LAMBDA_ENDPOINT"] = "https://some_endpoint.com/{article_type}"
    yield
    del os.environ["POCKET_LAMBDA_ENDPOINT"]
```

### Another example:

The following test suite uses the same three mocked functions in every test. The
following set-up assigns the mocks before each test and resets after each
individual test has run:

```py
@pytest.fixture(scope="function")
def setup_function():
    with patch("app.get_articles") as mock_get_articles, patch(
        "app.process_articles"
    ) as mock_process_articles, patch("app.update_worksheet") as mock_update_worksheet:
        yield mock_get_articles, mock_process_articles, mock_update_worksheet
```

Then to use:

```py
def individual_test(setup_function):
    mock_get_articles, mock_process_articles, mock_update_worksheet = setup_function
    # Now each mock can be referenced using the vars above

```

## Parameterized tests

For a sequence of tests that are repetitive, to avoid repeating the same code
over and over again, we can use parameterized tests. This is where we pass in a
list of parameters to the test function and the test function is run once for
each parameter.

For example, in the function below I am handling numerous possible Exceptions
that could be raised by the `requests.get` method:

```py
    try:
        response = requests.get(endpoint)
        response.raise_for_status()
        return response.json()

    except HTTPError as http_err:
        logging.error(f"HTTP Error occurred: {http_err}")

    except ConnectionError as conn_err:
        logging.error(f"Connection Error occurred: {conn_err}")

    except Timeout as timeout_err:
        logging.error(f"Timeout Error occurred: {timeout_err}")

    except RequestException as e:
        logging.error(f"Request Exception occurred: {e}")

    return None
```

Instead of writing something like the following for each of the four exceptions:

```py

 def test_exception_generic(caplog):
    with patch("requests.get", side_effect=RequestException("Some error")):
        result = get_articles("some_type")

    assert "Request Exception occurred" in caplog.text
    assert result is None
```

I could parameterize like so:

```py
@pytest.mark.parametrize(
    "exception_type, log_message",
    [
        (RequestException, "Request Exception occurred: "),
        (HTTPError, "HTTP Error occurred: "),
        (Timeout, "Timeout Error occurred: "),
        (ConnectionError, "Connection Error occurred: "),
    ],
)
def test_exceptions(caplog, exception_type, log_message):
    with patch("requests.get", side_effect=exception_type("Some error")):
        result = get_articles("some_type")

    assert log_message in caplog.text
    assert result is None
```

## Caplog, syslog, excinfo

`caplog` and `capsys` are built-in pytest fixtures. `caplog` lets you test log
messages. `capsys` lets you test stdout and stderr. As such they are very useful
when testing that error messages are logged correctly.

### `caplog`

In our example, if the endpoing environment is not set, we log an error message.
We can test that this message is logged correctly using `caplog`:

```py
def test_no_endpoint_env_var(caplog):
    os.environ.pop("POCKET_LAMBDA_ENDPOINT", None)  # Remove env variable if it exists

    with caplog.at_level(logging.ERROR):
        result = get_articles("some_type")

    assert (
        "Error: POCKET_LAMBDA_ENDPOINT environment variable is not set" in caplog.text
    )
    assert result is None
```

> Note tha we pass in `caplog` as a parameter to the test function. This is how
> pytest knows to use it as a fixture.

### `capsys`

In our example, if the request is unsuccessful, we log an error message with
`print` rather than `logging`. We can test that this message is printed
correctly using `capsys` to check the stdout:

```py
def test_http_error(capsys):
    with patch("requests.get") as mock_get:
        mock_get.return_value = Mock(ok=False, status_code=404)

        # Raise an HTTP error when raise_for_status is called
        mock_get.return_value.raise_for_status.side_effect = RequestException()

        result = get_articles("some_type")

    captured = capsys.readouterr()
    assert "An error occurred" in captured.out
```

### `excinfo`