Issue 1075: update snapshots using command-line flag (#1249)

* Adding skippable tests using command-line flag

data/data-pipeline/conftest.py

@@ -0,0 +1,40 @@
+import pytest
+
+
+def pytest_addoption(parser):
+    """Adds --update_snapshots as an optional flag that can be parsed from the
+    command line when calling pytest
+    """
+    parser.addoption(
+        "--update_snapshots",
+        action="store_true",
+        default=False,
+        help="Update snapshot test fixtures",
+    )
+
+
+def pytest_configure(config):
+    """Adds update_snapshots to list of available markers to apply to tests"""
+    config.addinivalue_line(
+        "markers",
+        "update_snapshots: mark test as one that updates snapshot fixtures",
+    )
+
+
+def pytest_collection_modifyitems(config, items):
+    """Applies pytest.mark.skip to all tests marked with update_snapshots
+    unless the --update_snapshots flag is passed to the CLI
+    """
+    # if --update_snapshots passed in cli
+    if config.getoption("--update_snapshots"):
+        # only run the tests marked with update_snapshots
+        skip = pytest.mark.skip(reason="only updating snapshots")
+        for test in items:
+            if "update_snapshots" not in test.keywords:
+                test.add_marker(skip)
+    else:
+        # otherwise skip all tests marked with update_snapshots
+        skip = pytest.mark.skip(reason="must pass --update_snapshots to run")
+        for test in items:
+            if "update_snapshots" in test.keywords:
+                test.add_marker(skip)