Improve reporting of known errors in isort, reachieve 100% test coverage

isort/exceptions.py

@@ -163,9 +163,18 @@ def __init__(self, unsupported_settings: Dict[str, Dict[str, str]]):
 class UnsupportedEncoding(ISortError):
     """Raised when isort encounters an encoding error while trying to read a file"""
 
-    def __init__(
-        self,
-        filename: Union[str, Path],
-    ):
+    def __init__(self, filename: Union[str, Path]):
         super().__init__(f"Unknown or unsupported encoding in {filename}")
         self.filename = filename
+
+
+class MissingSection(ISortError):
+    """Raised when isort encounters an import that matches a section that is not defined"""
+
+    def __init__(self, import_module: str, section: str):
+        super().__init__(
+            f"Found {import_module} import while parsing, but {section} was not included "
+            "in the `sections` setting of your config. Please add it before continuing\n"
+            "See https://pycqa.github.io/isort/#custom-sections-and-ordering "
+            "for more info."
+        )

isort/main.py

@@ -11,11 +11,11 @@
 from warnings import warn
 
 from . import __version__, api, sections
-from .exceptions import FileSkipped, UnsupportedEncoding
+from .exceptions import FileSkipped, ISortError, UnsupportedEncoding
 from .format import create_terminal_printer
 from .logo import ASCII_ART
 from .profiles import profiles
-from .settings import DEFAULT_CONFIG, VALID_PY_TARGETS, Config, WrapModes
+from .settings import VALID_PY_TARGETS, Config, WrapModes
 
 try:
     from .setuptools_commands import ISortCommand  # noqa: F401
@@ -110,17 +110,8 @@ def sort_imports(
         if config.verbose:
             warn(f"Encoding not supported for {file_name}")
         return SortAttempt(incorrectly_sorted, skipped, False)
-    except KeyError as error:
-        if error.args[0] not in DEFAULT_CONFIG.sections:
-            _print_hard_fail(config, offending_file=file_name)
-            raise
-        msg = (
-            f"Found {error} imports while parsing, but {error} was not included "
-            "in the `sections` setting of your config. Please add it before continuing\n"
-            "See https://pycqa.github.io/isort/#custom-sections-and-ordering "
-            "for more info."
-        )
-        _print_hard_fail(config, message=msg)
+    except ISortError as error:
+        _print_hard_fail(config, message=str(error))
         sys.exit(os.EX_CONFIG)
     except Exception:
         _print_hard_fail(config, offending_file=file_name)

isort/parse.py

@@ -8,6 +8,7 @@
 from . import place
 from .comments import parse as parse_comments
 from .deprecated.finders import FindersManager
+from .exceptions import MissingSection
 from .settings import DEFAULT_CONFIG, Config
 
 if TYPE_CHECKING:
@@ -524,6 +525,10 @@ def file_contents(contents: str, config: Config = DEFAULT_CONFIG) -> ParsedConte
                             " Do you need to define a default section?"
                         )
                         imports.setdefault("", {"straight": OrderedDict(), "from": OrderedDict()})
+
+                    if placed_module and placed_module not in imports:
+                        raise MissingSection(import_module=module, section=placed_module)
+
                     straight_import |= imports[placed_module][type_of_import].get(  # type: ignore
                         module, False
                     )