resolves #1331: Warns when sections don't match up.

CHANGELOG.md

@@ -5,6 +5,7 @@ NOTE: isort follows the [semver](https://semver.org/) versioning standard.
 
 ### 5.2.0 TBD
   - Implemented #1335: Official API for diff capturing.
+  - Implemented #1331: Warn when sections don't match up.
 
 ### 5.1.4 July 19, 2020
   - Fixed issue #1333: Use of wrap_length raises an exception about it not being lower or equal to line_length.

isort/settings.py

@@ -290,7 +290,14 @@ def __init__(
                 "known_third_party",
                 "known_first_party",
             ):
-                known_other[key[len(KNOWN_PREFIX) :].lower()] = frozenset(value)
+                import_heading = key[len(KNOWN_PREFIX) :].lower()
+                known_other[import_heading] = frozenset(value)
+                if not import_heading.upper() in combined_config.get("sections", ()):
+                    warn(
+                        f"`{key}` setting is defined, but not {import_heading.upper} is not"
+                        " included in `sections` config option:"
+                        f" {combined_config.get('sections', SECTION_DEFAULTS)}."
+                    )
             if key.startswith(IMPORT_HEADING_PREFIX):
                 import_headings[key[len(IMPORT_HEADING_PREFIX) :].lower()] = str(value)
 
@@ -301,6 +308,17 @@ def __init__(
 
             combined_config[key] = type(default_value)(value)
 
+        for section in combined_config.get("sections", ()):
+            if section in SECTION_DEFAULTS:
+                continue
+            elif not section.lower() in known_other:
+                config_keys = ", ".join(known_other.keys())
+                warn(
+                    f"`sections` setting includes {section}, but no known_{section.lower()} "
+                    "is defined. "
+                    f"The following known_SECTION config options are defined: {config_keys}."
+                )
+
         if "directory" not in combined_config:
             combined_config["directory"] = (
                 os.path.dirname(config_settings["source"])

tests/test_ticketed_features.py

@@ -3,7 +3,10 @@
 """
 from io import StringIO
 
+import pytest
+
 import isort
+from isort import Config
 
 
 def test_semicolon_ignored_for_dynamic_lines_after_import_issue_1178():
@@ -161,3 +164,37 @@ def test_isort_provides_official_api_for_diff_output_issue_1335():
     isort.code("import b\nimport a\n", show_diff=diff_output)
     diff_output.seek(0)
     assert "+import a" in diff_output.read()
+
+
+def test_isort_warns_when_known_sections_dont_match_issue_1331():
+    """Test to ensure that isort warns if there is a mismatch between sections and known_sections.
+    See: https://github.com/timothycrosley/isort/issues/1331.
+    """
+    assert (
+        isort.place_module(
+            "bot_core",
+            config=Config(
+                known_robotlocomotion_upstream=["bot_core"],
+                sections=["ROBOTLOCOMOTION_UPSTREAM", "THIRDPARTY"],
+            ),
+        )
+        == "ROBOTLOCOMOTION_UPSTREAM"
+    )
+    with pytest.warns(UserWarning):
+        assert (
+            isort.place_module(
+                "bot_core",
+                config=Config(
+                    known_robotlocomotion_upstream=["bot_core"],
+                    sections=["ROBOTLOOMOTION_UPSTREAM", "THIRDPARTY"],
+                ),
+            )
+            == "THIRDPARTY"
+        )
+    with pytest.warns(UserWarning):
+        assert (
+            isort.place_module(
+                "bot_core", config=Config(known_robotlocomotion_upstream=["bot_core"])
+            )
+            == "THIRDPARTY"
+        )