Use "raise … from …"

rplugin/python3/semshi/node.py

@@ -53,10 +53,12 @@ def __init__(self, name, lineno, col, env, target=None, hl_group=None):
         else:
             try:
                 self.symbol = self.env[-1].lookup(self.symname)
-            except KeyError:
+            except KeyError as exc:
                 # Set dummy hl group, so all fields in __repr__ are defined.
                 self.hl_group = '?'
-                raise Exception('%s can\'t lookup "%s"' % (self, self.symname))
+                raise Exception(
+                    '%s can\'t lookup "%s"' % (self, self.symname)
+                ) from exc
         if hl_group is not None:
             self.hl_group = hl_group
         else:

rplugin/python3/semshi/parser.py

@@ -47,7 +47,7 @@ def parse(self, *args, **kwargs):
             return self._parse(*args, **kwargs)
         except (SyntaxError, RecursionError) as e:
             logger.debug('parsing error: %s', e)
-            raise UnparsableError(e)
+            raise UnparsableError(e) from e
         finally:
             self.tick += 1
 
@@ -135,22 +135,22 @@ def _fix_syntax_and_make_ast(self, code, lines, change_lineno):
         new_code = lines_to_code(new_lines)
         try:
             ast_root = self._make_ast(new_code)
-        except SyntaxError:
+        except SyntaxError as exc:
             # Restore original line
             new_lines[error_idx] = orig_line
             # Fixing the line of the syntax error failed, so try again with the
             # line of last change.
             if change_lineno is None or change_lineno == error_idx:
                 # Don't try to fix the changed line if it's unknown or the same
                 # as the one we tried to fix before.
-                raise orig_error
+                raise orig_error from exc
             new_lines[change_lineno] = self._fix_line(new_lines[change_lineno])
             new_code = lines_to_code(new_lines)
             try:
                 ast_root = self._make_ast(new_code)
             except SyntaxError:
                 # All fixing attempts failed, so raise original syntax error.
-                raise orig_error
+                raise orig_error from exc
         return ast_root, new_code, new_lines, orig_error
 
     @staticmethod

rplugin/python3/semshi/plugin.py

@@ -285,4 +285,6 @@ def _convert_excluded_hl_groups(items):
             return [hl_groups[g] for g in items]
         except KeyError as e:
             # TODO Use err_write instead?
-            raise Exception('"%s" is an unknown highlight group.' % e.args[0])
+            raise Exception(
+                '"%s" is an unknown highlight group.' % e.args[0]
+            ) from e