Use raise ... from syntax to link exceptions

Python handles one exception at a time (per thread), raising a new
exception while still handling another one will result in run-time
errors. By linking the second exception to the original one, Python will
handle it correctly.

This was encountered while developing the alternative column UI, which
bailed out on completing a todo item with (invalid) child todo items.

topydo/lib/Command.py

@@ -71,8 +71,8 @@ class Command(object):
         """ Retrieves a value from the argument list at the given position. """
         try:
             return self.args[p_number]
-        except IndexError:
-            raise InvalidCommandArgument
+        except IndexError as ie:
+            raise InvalidCommandArgument from ie
 
     def getopt(self, p_flags, p_long=None):
         p_long = p_long or []

topydo/lib/Config.py

@@ -327,7 +327,7 @@ def config(p_path=None, p_overrides=None):
         try:
             config.instance = _Config(p_path, p_overrides)
         except configparser.ParsingError as perr:
-            raise ConfigError(str(perr))
+            raise ConfigError(str(perr)) from perr
 
     return config.instance
 

topydo/lib/TodoListBase.py

@@ -95,11 +95,11 @@ class TodoListBase(object):
                         # the expression is a string and no leading zeroes,
                         # treat it as an integer
                         raise TypeError
-                except TypeError:
+                except TypeError as te:
                     try:
                         result = self._todos[int(p_identifier) - 1]
                     except (ValueError, IndexError):
-                        raise InvalidTodoException
+                        raise InvalidTodoException from te
 
             return result
 
@@ -249,8 +249,8 @@ class TodoListBase(object):
                 return self._todo_id_map[p_todo]
             else:
                 return self._todos.index(p_todo) + 1
-        except (ValueError, KeyError):
-            raise InvalidTodoException
+        except (ValueError, KeyError) as ex:
+            raise InvalidTodoException from ex
 
     def _update_todo_ids(self):
         # the idea is to have a hash that is independent of the position of the