Temporary tests for feature flagging

Why these changes are being introduced:
* We do not need to heavily test the feature flagging logic,
knowing it will be removed after development work.  But a bit
of testing can ensure that it works as expected while termporarily
in a deployed state.

How this addresses that need:
* Adds new test file tests/test_temporary_feature_flagging.py that
is noted to be removed after development work is complete
* Test branching logic in Format Input handler using 'ETL_VERSION' env var
* Test config function for retrieving 'ETL_VERSION' env var

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/TIMX-412

tests/test_temporary_feature_flagging.py

@@ -0,0 +1,142 @@
+# ruff: noqa: PLR2004
+
+from unittest.mock import patch
+
+import pytest
+
+from lambdas import format_input
+from lambdas.config import get_etl_version
+
+# NOTE: FEATURE FLAG: this test file can be removed completely after v2 parquet work.
+
+
+@pytest.fixture
+def mocked_etl_v1_generate_transform_commands_method():
+    with patch(
+        "lambdas.commands._etl_v1_generate_transform_commands_method"
+    ) as mocked_method:
+        yield mocked_method
+
+
+@pytest.fixture
+def mocked_etl_v2_generate_transform_commands_method():
+    with patch(
+        "lambdas.commands._etl_v2_generate_transform_commands_method"
+    ) as mocked_method:
+        yield mocked_method
+
+
+@pytest.fixture
+def mocked_etl_v1_generate_load_commands_method():
+    with patch("lambdas.commands._etl_v1_generate_load_commands_method") as mocked_method:
+        yield mocked_method
+
+
+@pytest.fixture
+def mocked_etl_v2_generate_load_commands_method():
+    with patch("lambdas.commands._etl_v2_generate_load_commands_method") as mocked_method:
+        yield mocked_method
+
+
+def test_etl_version_helper_function_no_env_var_is_v1(monkeypatch):
+    monkeypatch.delenv("ETL_VERSION")
+    assert get_etl_version() == 1
+
+
+def test_etl_version_helper_function_env_var_is_1_is_v1(monkeypatch):
+    monkeypatch.setenv("ETL_VERSION", "1")
+    assert get_etl_version() == 1
+
+
+def test_etl_version_helper_function_env_var_is_2_is_v2(monkeypatch):
+    monkeypatch.setenv("ETL_VERSION", "2")
+    assert get_etl_version() == 2
+
+
+@pytest.mark.parametrize(
+    "env_value",
+    [
+        "pumpkin_pie",  # throws ValueError because not integer
+        "3",  # throws ValueError because not 1 or 2
+    ],
+)
+def test_etl_version_helper_function_env_var_value_is_unsupported(env_value, monkeypatch):
+    monkeypatch.setenv("ETL_VERSION", env_value)
+    with pytest.raises(ValueError):  # noqa: PT011
+        get_etl_version()
+
+
+def test_lambda_handler_etl_version_v1_with_next_step_transform_invokes_v1_code(
+    mocked_etl_v1_generate_transform_commands_method, s3_client
+):
+    s3_client.put_object(
+        Bucket="test-timdex-bucket",
+        Key="testsource/testsource-2022-01-02-daily-extracted-records-to-index.xml",
+        Body="I am a file",
+    )
+    event = {
+        "run-date": "2022-01-02T12:13:14Z",
+        "run-type": "daily",
+        "next-step": "transform",
+        "source": "testsource",
+        "verbose": "true",
+    }
+    format_input.lambda_handler(event, {})
+    mocked_etl_v1_generate_transform_commands_method.assert_called()
+
+
+def test_lambda_handler_etl_version_v2_with_next_step_transform_invokes_v2_code(
+    mocked_etl_v2_generate_transform_commands_method, monkeypatch, s3_client
+):
+    monkeypatch.setenv("ETL_VERSION", "2")
+    s3_client.put_object(
+        Bucket="test-timdex-bucket",
+        Key="testsource/testsource-2022-01-02-daily-extracted-records-to-index.xml",
+        Body="I am a file",
+    )
+    event = {
+        "run-date": "2022-01-02T12:13:14Z",
+        "run-type": "daily",
+        "next-step": "transform",
+        "source": "testsource",
+        "verbose": "true",
+    }
+    format_input.lambda_handler(event, {})
+    mocked_etl_v2_generate_transform_commands_method.assert_called()
+
+
+def test_lambda_handler_etl_version_v1_with_next_step_load_invokes_v1_code(
+    mocked_etl_v1_generate_load_commands_method, s3_client
+):
+    s3_client.put_object(
+        Bucket="test-timdex-bucket",
+        Key="testsource/testsource-2022-01-02-daily-transformed-records-to-index.xml",
+        Body="I am a file",
+    )
+    event = {
+        "run-date": "2022-01-02T12:13:14Z",
+        "run-type": "daily",
+        "next-step": "load",
+        "source": "testsource",
+    }
+    format_input.lambda_handler(event, {})
+    mocked_etl_v1_generate_load_commands_method.assert_called()
+
+
+def test_lambda_handler_etl_version_v2_with_next_step_load_invokes_v2_code(
+    mocked_etl_v2_generate_load_commands_method, monkeypatch, s3_client
+):
+    monkeypatch.setenv("ETL_VERSION", "2")
+    s3_client.put_object(
+        Bucket="test-timdex-bucket",
+        Key="testsource/testsource-2022-01-02-daily-transformed-records-to-index.xml",
+        Body="I am a file",
+    )
+    event = {
+        "run-date": "2022-01-02T12:13:14Z",
+        "run-type": "daily",
+        "next-step": "load",
+        "source": "testsource",
+    }
+    format_input.lambda_handler(event, {})
+    mocked_etl_v2_generate_load_commands_method.assert_called()