docs(testing): add testing guide and runnable example (#1044)

Hamilton's DAG model is the backbone of clean ETL, and tests are how that
backbone stays honest. This adds a canonical guide and a runnable example
that cover the four scenarios called out in #1044:

  1. Unit-testing plain Hamilton functions
  2. Unit-testing functions that use decorators (@tag, @parameterize,
     @extract_columns) -- both by calling the underlying callable and by
     building a Driver to verify the decorator wiring
  3. Integration-testing the DAG with `Builder().with_modules(...).build()`,
     including `inputs=` and `overrides=` for short-circuiting upstream nodes
  4. Driving an in-memory module via `ad_hoc_utils.create_temporary_module`
     for self-contained tests (e.g. of custom materializers)

The docs page at `docs/how-tos/test-hamilton-code.rst` uses
`.. literalinclude::` to pull every snippet from `examples/testing/`, so the
guide and the example cannot drift out of sync.

Closes #1044

Signed-off-by: Olajumoke Akinremi <106763970+Hore01@users.noreply.github.com>

Author: Hore01

docs/how-tos/index.rst

@@ -16,6 +16,7 @@ directory. If there's an example you want but don't see, reach out or open an is
    ml-training
    llm-workflows
    run-data-quality-checks
+   test-hamilton-code
    use-hamilton-for-lineage
    scale-up
    microservice

docs/how-tos/test-hamilton-code.rst

@@ -0,0 +1,163 @@
+..
+    Licensed to the Apache Software Foundation (ASF) under one
+    or more contributor license agreements.  See the NOTICE file
+    distributed with this work for additional information
+    regarding copyright ownership.  The ASF licenses this file
+    to you under the Apache License, Version 2.0 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+
+==============================
+Testing Apache Hamilton code
+==============================
+
+A common question on `Slack <https://join.slack.com/t/hamilton-opensource/shared_invite/zt-2niepkra8-DGKGf_tTYhXuJWBTXtIs4g>`_
+is "how do I test my Hamilton functions?" -- often with a worry that decorators
+will get in the way. The good news: a Hamilton function is just a Python
+function, so the standard ``pytest`` patterns you already know apply directly.
+
+This guide walks through four cases, in order of increasing scope:
+
+1. Unit-testing a plain function.
+2. Unit-testing a decorated function.
+3. Integration-testing the full DAG with the ``Driver``, including
+   ``inputs=`` and ``overrides=``.
+4. Driving an in-memory module for self-contained tests (e.g. of custom
+   materializers).
+
+The complete runnable code lives in
+`examples/testing <https://github.com/apache/hamilton/tree/main/examples/testing>`_.
+Every code block on this page is a ``literalinclude`` from that folder, so the
+docs and the example can never drift out of sync.
+
+Prerequisites
+-------------
+
+Install the example's dependencies and run it:
+
+.. code-block:: bash
+
+    cd examples/testing
+    pip install -r requirements.txt
+    pytest
+
+You should see all 13 tests pass.
+
+1. Unit-testing plain functions
+-------------------------------
+
+Hamilton encourages you to put your transformation logic in ordinary modules
+that don't import the Driver. That makes them trivial to unit-test:
+
+.. literalinclude:: ../../examples/testing/my_functions.py
+   :language: python
+   :lines: 18-
+   :caption: ``examples/testing/my_functions.py``
+
+Tests are just calls to the function:
+
+.. literalinclude:: ../../examples/testing/test_my_functions.py
+   :language: python
+   :lines: 18-
+   :caption: ``examples/testing/test_my_functions.py``
+
+Notes
+^^^^^
+
+* No Driver is required. You import the module under test and call its
+  functions like any other Python code.
+* ``pytest.mark.parametrize`` is a clean way to cover edge cases without
+  copy-pasting test bodies.
+* Use ``pd.testing.assert_series_equal`` (or ``assert_frame_equal``) for
+  pandas outputs -- it gives readable diffs on failure.
+
+2. Unit-testing decorated functions
+-----------------------------------
+
+Hamilton's function modifiers (``@tag``, ``@parameterize``, ``@extract_columns``,
+...) tell Hamilton how to wire the function into the DAG. They do **not**
+change what the function does when you call it directly. You can therefore
+mix two complementary techniques:
+
+A. Call the underlying function in a unit test (cheap, fast).
+B. Build a Driver and assert on the expanded DAG, to verify the wiring (slower,
+   but the only way to catch decorator misuse).
+
+The decorated module:
+
+.. literalinclude:: ../../examples/testing/decorated_functions.py
+   :language: python
+   :lines: 18-
+   :caption: ``examples/testing/decorated_functions.py``
+
+The tests:
+
+.. literalinclude:: ../../examples/testing/test_decorated_functions.py
+   :language: python
+   :lines: 18-
+   :caption: ``examples/testing/test_decorated_functions.py``
+
+3. Integration-testing the DAG
+------------------------------
+
+For end-to-end tests, build a Driver from the module(s) under test and call
+``execute(...)`` with controlled inputs.
+
+Two arguments are especially useful:
+
+* ``inputs=`` injects test data at the **inputs** of the DAG -- the parameter
+  names that aren't produced by any function.
+* ``overrides=`` short-circuits an **intermediate** node by pinning its value.
+  This is the integration-test sweet spot: instead of fabricating realistic
+  raw inputs and re-deriving every intermediate, hand the DAG a known value
+  for ``spend`` (or any other node) and assert on the *downstream* logic.
+
+.. literalinclude:: ../../examples/testing/test_driver.py
+   :language: python
+   :lines: 18-
+   :caption: ``examples/testing/test_driver.py``
+
+Tip: ``Driver`` exposes a number of inspection methods --
+``what_is_upstream_of``, ``what_is_downstream_of``, ``list_available_variables``
+-- that are handy for asserting on graph shape, not just values.
+
+4. In-memory modules for self-contained tests
+---------------------------------------------
+
+Sometimes you want a test that defines its own tiny Hamilton module inline
+-- to exercise a custom materializer, regression-test a data-quality bug,
+or demonstrate a pattern in a doctest. You don't need to create a new
+``.py`` file; ``hamilton.ad_hoc_utils.create_temporary_module`` packages
+inline-defined functions into a real module that the Driver can consume:
+
+.. literalinclude:: ../../examples/testing/test_ad_hoc_module.py
+   :language: python
+   :lines: 18-
+   :caption: ``examples/testing/test_ad_hoc_module.py``
+
+This is also how Hamilton itself tests several of its built-in materializers,
+so it scales up to fairly involved scenarios. See
+`tests/test_ad_hoc_utils.py <https://github.com/apache/hamilton/blob/main/tests/test_ad_hoc_utils.py>`_
+in the Hamilton source for more usage examples.
+
+Where to go from here
+---------------------
+
+* Read the :doc:`/concepts/best-practices/code-organization` page -- the
+  module structure it recommends is the same one that makes tests easy to
+  write.
+* Browse the
+  `Hamilton test suite <https://github.com/apache/hamilton/tree/main/tests>`_
+  for ideas; the same patterns work for user code.
+* Have a testing pattern that isn't covered here? Share it on
+  `Slack <https://join.slack.com/t/hamilton-opensource/shared_invite/zt-2niepkra8-DGKGf_tTYhXuJWBTXtIs4g>`_
+  -- we'd love to add it.

examples/testing/README.md

@@ -0,0 +1,79 @@
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# Testing Apache Hamilton code
+
+This is the runnable companion to the
+[Testing Hamilton code](https://hamilton.apache.org/how-tos/test-hamilton-code/)
+how-to. It shows that Hamilton functions are normal Python -- so the standard
+`pytest` patterns you already know apply, including when decorators are
+involved.
+
+The example covers the four cases from issue
+[#1044](https://github.com/apache/hamilton/issues/1044):
+
+1. **Unit-testing plain functions** -- `test_my_functions.py`
+2. **Unit-testing decorated functions** -- `test_decorated_functions.py`
+3. **Integration-testing the DAG with `inputs=` and `overrides=`** -- `test_driver.py`
+4. **In-memory modules with `ad_hoc_utils.create_temporary_module`** -- `test_ad_hoc_module.py`
+
+## File organization
+
+| File | Purpose |
+| ---- | ------- |
+| `my_functions.py` | A small marketing dataflow (no decorators). |
+| `decorated_functions.py` | The same style of dataflow, using `@tag`, `@parameterize` and `@extract_columns`. |
+| `test_my_functions.py` | Unit tests that import and call functions directly. |
+| `test_decorated_functions.py` | Unit + driver-level tests for the decorated module. |
+| `test_driver.py` | End-to-end tests using `Builder().with_modules(...).build()` plus `inputs=` and `overrides=`. |
+| `test_ad_hoc_module.py` | Builds a module from inline-defined functions for self-contained tests. |
+| `conftest.py` | Adds this folder to `sys.path` so `import my_functions` works under pytest. |
+
+## Running the tests
+
+```bash
+pip install -r requirements.txt
+pytest
+```
+
+You should see all tests pass. Each test file is independently runnable:
+
+```bash
+pytest test_my_functions.py -v
+pytest test_driver.py -v
+```
+
+## What to take away
+
+* A Hamilton function is just a Python function. Testing it does **not**
+  require building a Driver.
+* Decorators (`@tag`, `@parameterize`, `@extract_columns`, ...) leave the
+  underlying callable intact. Direct function calls still work; the decorator
+  changes how Hamilton wires the function into the DAG, not what the function
+  computes.
+* For integration tests, `Builder().with_modules(...).build()` is the canonical
+  entry point. Use `inputs=` to inject test data at the DAG inputs and
+  `overrides=` to short-circuit intermediate nodes when you want to assert on
+  downstream logic in isolation.
+* Need to test inline -- e.g. for a regression test or a custom materializer
+  -- without a `.py` file on disk? Use
+  `hamilton.ad_hoc_utils.create_temporary_module`.
+
+If you have questions, or need help with this example,
+join us on [Slack](https://join.slack.com/t/hamilton-opensource/shared_invite/zt-2niepkra8-DGKGf_tTYhXuJWBTXtIs4g).

examples/testing/conftest.py

@@ -0,0 +1,28 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+"""Make the example modules importable when running ``pytest`` from this dir.
+
+Hamilton needs to import your dataflow module by name. Adding this folder to
+``sys.path`` lets the example tests do ``import my_functions`` directly,
+mirroring how a real project would lay out its code.
+"""
+
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(__file__))

examples/testing/decorated_functions.py

@@ -0,0 +1,55 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+"""Functions that use Hamilton decorators.
+
+Decorators are a common source of confusion when testing. The point of this
+module is to show that decorators do not get in the way of unit testing -- the
+function below the decorator is still a plain Python callable, so you can call
+it directly from a test. To test what the decorator *expands to*, drive the
+function through a Driver instead (see ``test_decorated_functions.py``).
+"""
+
+import pandas as pd
+
+from hamilton.function_modifiers import extract_columns, parameterize, source, tag, value
+
+
+@tag(owner="growth-team", pii="false")
+def total_signups(signups: pd.Series) -> int:
+    """Sum of signups across the time window."""
+    return int(signups.sum())
+
+
+@parameterize(
+    spend_in_thousands={"raw_value": source("spend"), "divisor": value(1000.0)},
+    signups_in_hundreds={"raw_value": source("signups"), "divisor": value(100.0)},
+)
+def scaled(raw_value: pd.Series, divisor: float) -> pd.Series:
+    """Scale a series by a constant divisor.
+
+    `@parameterize` produces one node per entry above. The function itself is
+    still a normal callable, so a unit test can call ``scaled(some_series, 1000)``
+    directly without a Driver.
+    """
+    return raw_value / divisor
+
+
+@extract_columns("scaled_spend", "scaled_signups")
+def scaled_features(spend_in_thousands: pd.Series, signups_in_hundreds: pd.Series) -> pd.DataFrame:
+    """Bundle the two scaled series into a frame, then expose each column as a node."""
+    return pd.DataFrame({"scaled_spend": spend_in_thousands, "scaled_signups": signups_in_hundreds})