Fixed indicator position in code with tabs (#8307)

Author: dosisod

mypy/errors.py

@@ -451,12 +451,17 @@ def format_messages(self, error_info: List[ErrorInfo],
                 # Add source code fragment and a location marker.
                 if severity == 'error' and source_lines and line > 0:
                     source_line = source_lines[line - 1]
+                    source_line_expanded = source_line.expandtabs()
                     if column < 0:
                         # Something went wrong, take first non-empty column.
                         column = len(source_line) - len(source_line.lstrip())
+
+                    # Shifts column after tab expansion
+                    column = len(source_line[:column].expandtabs())
+
                     # Note, currently coloring uses the offset to detect source snippets,
                     # so these offsets should not be arbitrary.
-                    a.append(' ' * DEFAULT_SOURCE_OFFSET + source_line)
+                    a.append(' ' * DEFAULT_SOURCE_OFFSET + source_line_expanded)
                     a.append(' ' * (DEFAULT_SOURCE_OFFSET + column) + '^')
         return a
 

test-data/unit/cmdline.test

@@ -1036,6 +1036,16 @@ some_file.py:1: error: invalid syntax  [syntax]
                                                         ^
 == Return code: 2
 
+[case testTabRenderingUponError]
+# cmd: mypy --pretty tabs.py
+[file tabs.py]
+def test_tabs() -> str:
+	return None
+[out]
+tabs.py:2: error: Incompatible return value type (got "None", expected "str")
+            return None
+                   ^
+
 [case testSpecialTypeshedGenericNote]
 # cmd: mypy --disallow-any-generics --python-version=3.6 test.py
 [file test.py]

test-data/unit/fine-grained-blockers.test

@@ -40,7 +40,7 @@ def f() -> None: pass
 ==
 a.py:1: error: invalid syntax  [syntax]
     def f(x: int) ->
-                     ^
+                    ^
 ==
 main:3: error: Too few arguments for "f"  [call-arg]
     a.f()