Fix variable introspection for test helpers in sqlfluff.testing (#863)

* Test introspection

* Add pytest-sugar

* Register helper test functions

* Rewrite assert testing helper functions only once

* Remove fix_str added to check variable introspection

* Add pytest as a dependency as we provide testing tooling in sqlfluff.testing

requirements_dev.txt

@@ -11,6 +11,7 @@ coverage
 hypothesis
 pytest
 pytest-cov
+pytest-sugar
 # MyPy
 mypy
 typing_extensions

setup.py

@@ -96,6 +96,8 @@ def read(*names, **kwargs):
         "dataclasses",
         # better type hints for older python versions
         "typing_extensions",
+        # We provide a testing library for plugins in sqlfluff.testing
+        "pytest",
     ],
     extras_require={
         "dbt": ["dbt>=0.17"],

src/sqlfluff/__init__.py

@@ -1,5 +1,6 @@
 """Sqlfluff is a SQL linter for humans."""
 import sys
+import pytest
 
 # Expose the public API.
 from sqlfluff.api import lint, fix, parse, rules, dialects  # noqa: F401
@@ -25,3 +26,6 @@
 config.read([pkg_resources.resource_filename("sqlfluff", "config.ini")])
 
 __version__ = config.get("sqlfluff", "version")
+
+# Register helper functions to support variable introspection on failure.
+pytest.register_assert_rewrite("sqlfluff.testing")

test/test_testing.py

@@ -44,3 +44,27 @@ def test_rules__test_helper_skipped_when_test_case_skipped():
     with pytest.raises(Skipped) as skipped_test:
         rules__test_helper(rule_test_case)
     skipped_test.match("Skip this one for now")
+
+
+def test_rules__test_helper_has_variable_introspection():
+    """Make sure the helper gives variable introspection information on failure."""
+    rule_test_case = RuleTestCase(
+        rule="L003",
+        fail_str="""
+            select
+                a,
+                    b
+            from table
+        """,
+        # extra comma on purpose
+        fix_str="""
+            select
+                a,
+                b,
+            from table
+        """,
+    )
+    with pytest.raises(AssertionError) as skipped_test:
+        rules__test_helper(rule_test_case)
+    # Enough to check that a query diff is displayed
+    skipped_test.match("select")