Improve error message for malformed per-file-ignores

3 files changed, 48 insertions, 3 deletions

diff --git a/src/flake8/utils.py b/src/flake8/utils.py
index ae0d060..70b45e0 100644
--- a/src/flake8/utils.py
+++ b/src/flake8/utils.py
@@ -12,6 +12,8 @@ from typing import Callable, Dict, Generator  # noqa: F401 (until flake8 3.7)
 from typing import List, Pattern, Sequence  # noqa: F401 (until flake8 3,7)
 from typing import Tuple, Union  # noqa: F401 (until flake8 3.7)
 
+from flake8 import exceptions
+
 if False:  # `typing.TYPE_CHECKING` was introduced in 3.5.2
     from flake8.plugins.manager import Plugin  # noqa: F401 (until flake8 3.7)
 
@@ -109,6 +111,21 @@ def parse_files_to_codes_mapping(value):  # noqa: C901
         State.filenames = []
         State.codes = []
 
+    def _unexpected_token():
+        # type: () -> exceptions.ExecutionError
+
+        def _indent(s):
+            # type: (str) -> str
+            return "    " + s.strip().replace("\n", "\n    ")
+
+        return exceptions.ExecutionError(
+            "Expected `per-file-ignores` to be a mapping from file exclude "
+            "patterns to ignore codes.\n\n"
+            "Configured `per-file-ignores` setting:\n\n{}".format(
+                _indent(value)
+            )
+        )
+
     for token in _tokenize_files_to_codes_mapping(value):
         # legal in any state: separator sets the sep bit
         if token.tp in {_COMMA, _WS}:
@@ -122,7 +139,7 @@ def parse_files_to_codes_mapping(value):  # noqa: C901
                 State.filenames.append(token.src)
                 State.seen_sep = False
             else:
-                raise ValueError("Unexpected token: {}".format(token))
+                raise _unexpected_token()
         # looking for codes
         else:
             if token.tp == _EOF:
@@ -135,7 +152,7 @@ def parse_files_to_codes_mapping(value):  # noqa: C901
                 State.filenames.append(token.src)
                 State.seen_sep = False
             else:
-                raise ValueError("Unexpected token: {}".format(token))
+                raise _unexpected_token()
 
     return ret
 
diff --git a/tests/integration/test_main.py b/tests/integration/test_main.py
index 56f9414..ba4dd56 100644
--- a/tests/integration/test_main.py
+++ b/tests/integration/test_main.py
@@ -58,3 +58,30 @@ t.py:2:1: F401 'sys' imported but unused
 2     F401 'os' imported but unused
 '''
     assert err == ''
+
+
+def test_malformed_per_file_ignores_error(tmpdir, capsys):
+    """Test the error message for malformed `per-file-ignores`."""
+    setup_cfg = '''\
+[flake8]
+per-file-ignores =
+    incorrect/*
+    values/*
+'''
+
+    with tmpdir.as_cwd():
+        tmpdir.join('setup.cfg').write(setup_cfg)
+
+        app = application.Application()
+        app.run(['.'])
+
+    out, err = capsys.readouterr()
+    assert out == '''\
+There was a critical error during execution of Flake8:
+Expected `per-file-ignores` to be a mapping from file exclude patterns to ignore codes.
+
+Configured `per-file-ignores` setting:
+
+    incorrect/*
+    values/*
+'''  # noqa: E501
diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
index b0eac9a..4b70918 100644
--- a/tests/unit/test_utils.py
+++ b/tests/unit/test_utils.py
@@ -4,6 +4,7 @@ import os
 import mock
 import pytest
 
+from flake8 import exceptions
 from flake8 import utils
 from flake8.plugins import manager as plugin_manager
 
@@ -111,7 +112,7 @@ def test_parse_files_to_codes_mapping(value, expected):
 )
 def test_invalid_file_list(value):
     """Test parsing of invalid files-to-codes mappings."""
-    with pytest.raises(ValueError):
+    with pytest.raises(exceptions.ExecutionError):
         utils.parse_files_to_codes_mapping(value)