Merge branch 'bug/157' into 'master'

Handle errors reported in empty files

*Description of changes*

Some plugins return errors in empty files which previously caused an IndexError.

*Related to:*  #157 

See merge request !71

src/flake8/formatting/base.py

@@ -123,8 +123,9 @@ def show_source(self, error):
         :rtype:
             str
         """
-        if not self.options.show_source:
-            return None
+        if not self.options.show_source or error.physical_line is None:
+            return ''
+
         pointer = (' ' * error.column_number) + '^'
         # Physical lines have a newline at the end, no need to add an extra
         # one

src/flake8/processor.py

@@ -250,7 +250,13 @@ def generate_tokens(self):
 
     def line_for(self, line_number):
         """Retrieve the physical line at the specified line number."""
-        return self.lines[line_number - 1]
+        adjusted_line_number = line_number - 1
+        # NOTE(sigmavirus24): Some plugins choose to report errors for empty
+        # files on Line 1. In those casese, we shouldn't bother trying to
+        # retrieve a physical line (since none exist).
+        if 0 <= adjusted_line_number < len(self.lines):
+            return self.lines[adjusted_line_number]
+        return None
 
     def next_line(self):
         """Get the next line from the list."""

tests/unit/test_base_formatter.py

@@ -51,7 +51,15 @@ def test_show_source_returns_nothing_when_not_showing_source():
     formatter = base.BaseFormatter(options(show_source=False))
     assert formatter.show_source(
         style_guide.Error('A000', 'file.py', 1, 1, 'error text', 'line')
-    ) is None
+    ) is ''
+
+
+def test_show_source_returns_nothing_when_there_is_source():
+    """Ensure we return nothing when there is no line."""
+    formatter = base.BaseFormatter(options(show_source=True))
+    assert formatter.show_source(
+        style_guide.Error('A000', 'file.py', 1, 1, 'error text', None)
+    ) is ''
 
 
 @pytest.mark.parametrize('line, column', [