Make text editor more configurable

Lookup mechanism:

1. Check whether an editor was given with the -E flag:

     topydo edit -E vim

2. Use the value of $TOPYDO_EDITOR
3. Use the value in the configuration file:

   [edit]
   editor = vim

4. Use the value in $EDITOR
5. Use 'vi' if the above fails.

This makes it easier to invoke an editor as vim with additional
parameters, for instance by adding the full todo.txt file to the
completion options (see the tip in issue #164). This trick was added to
the configuration file as a comment.

topydo.conf

@@ -36,6 +36,15 @@ append_parent_projects      = 0
 ; Add parent contexts when adding sub todo items
 append_parent_contexts      = 0
 
+[edit]
+; Editor to use for the 'edit' subcommand (overrides the EDITOR environment
+; variable, but can be overridden with the TOPYDO_EDITOR environment variable
+; or the -E flag in the edit command)
+; editor = vi
+; Vim tip: enable completion using your complete todo.txt file. Use this as
+; editor command:
+;   vim -c 'autocmd Filetype todo set complete=.,w,b,u,t,i,k~/notes/todo.txt'
+
 [colorscheme]
 ; Configure colorscheme. Accepted values are: black, [light-]red, [light-]green,
 ; [light-]yellow, [light-]blue, [light-]magenta, [light-]cyan, white

topydo/commands/EditCommand.py

@@ -23,10 +23,6 @@ from topydo.lib.Config import config
 from topydo.lib.MultiCommand import MultiCommand
 from topydo.lib.prettyprinters.Numbers import PrettyPrinterNumbers
 from topydo.lib.Todo import Todo
-from topydo.lib.TodoList import TodoList
-
-# the true and only editor
-DEFAULT_EDITOR = 'vi'
 
 def _get_file_mtime(p_file):
     return os.stat(p_file.name).st_mtime
@@ -39,20 +35,32 @@ class EditCommand(MultiCommand):
         super().__init__(p_args, p_todolist, p_output,
                                           p_error, p_input)
 
-        if len(self.args) == 0:
-            self.multi_mode = False
-
+        self.editor = config().editor()
         self.is_expression = False
         self.edit_archive = False
         self.last_argument = False
 
     def get_flags(self):
-        return ("d", [])
+        return ("dE:", [])
 
     def process_flag(self, p_opt, p_value):
         if p_opt == '-d':
             self.edit_archive = True
             self.multi_mode = False
+        elif p_opt == '-E':
+            self.editor = p_value
+
+    def _process_flags(self):
+        """
+        Override to add an additional check after processing the flags: when
+        there are no flags left after argument parsing, then it means we'll be
+        editing the whole todo.txt file as a whole and therefore we're not in
+        multi mode.
+        """
+        super()._process_flags()
+
+        if len(self.args) == 0:
+            self.multi_mode = False
 
     def _todos_to_temp(self):
         f = tempfile.NamedTemporaryFile(delete=False, suffix='.todo.txt')
@@ -74,18 +82,13 @@ class EditCommand(MultiCommand):
 
     def _open_in_editor(self, p_file):
         try:
-            editor = os.environ['EDITOR'] or DEFAULT_EDITOR
-        except(KeyError):
-            editor = DEFAULT_EDITOR
-
-        try:
-            return check_call([editor, p_file])
+            return check_call(config().editor() + [p_file])
         except CalledProcessError:
             self.error('Something went wrong in the editor...')
             return 1
-        except(OSError):
-            self.error('There is no such editor as: ' + editor + '. '
-                       'Check your $EDITOR and/or $PATH')
+        except OSError:
+            self.error('There is no such editor as: ' + self.editor + '. '
+                       'Check your configuration file, $TOPYDO_EDITOR, $EDITOR and/or $PATH')
 
     def _catch_todo_errors(self):
         errors = []
@@ -134,12 +137,13 @@ class EditCommand(MultiCommand):
 
             return self._open_in_editor(todo) == 0
 
+
     def usage(self):
         return """Synopsis:
-  edit
-  edit <NUMBER 1> [<NUMBER 2> ...]
-  edit -e [-x] [EXPRESSION]
-  edit -d"""
+  edit [-E <EDITOR>]
+  edit [-E <EDITOR>] <NUMBER 1> [<NUMBER 2> ...]
+  edit [-E <EDITOR>] -e [-x] [EXPRESSION]
+  edit [-E <EDITOR>] -d"""
 
     def help(self):
         return """\
@@ -150,10 +154,15 @@ edit todo item(s) with the given NUMBER(s) or edit relevant todos matching
 the given EXPRESSION. See `topydo help ls` for more information on relevant
 todo items. It is also possible to open the archive file.
 
-By default it will look to your environment variable $EDITOR, otherwise it will
-fall back to 'vi'.
+The editor is chosen as follows:
+    1. Check whether the -E flag is given and use it;
+    2. Use the value of $TOPYDO_EDITOR in the environment;
+    3. Use the value in the configuration file;
+    4. Use the value of $EDITOR in the environment;
+    5. If all else fails, use 'vi'.
 
 -e : Treat the subsequent arguments as an EXPRESSION.
+-E : Editor to start.
 -x : Edit *all* todos matching the EXPRESSION (i.e. do not filter on
      dependencies or relevance).
 -d : Open the archive file.\

topydo/lib/Config.py

@@ -101,6 +101,9 @@ class _Config:
                 'append_parent_contexts': '0',
             },
 
+            'edit': {
+            },
+
             'colorscheme': {
                 'project_color': 'red',
                 'context_color': 'magenta',
@@ -448,6 +451,23 @@ class _Config:
 
         return (keymap_dict, keystates)
 
+    def editor(self):
+        """
+        Returns the editor to invoke. It returns a list with the command in
+        the first position and its arguments in the remainder.
+        """
+        result = 'vi'
+        if 'TOPYDO_EDITOR' in os.environ and os.environ['TOPYDO_EDITOR']:
+            result = os.environ['TOPYDO_EDITOR']
+        else:
+            try:
+                result = str(self.cp.get('edit', 'editor'))
+            except configparser.NoOptionError:
+                if 'EDITOR' in os.environ and os.environ['EDITOR']:
+                    result = os.environ['EDITOR']
+
+        return shlex.split(result)
+
 
 def config(p_path=None, p_overrides=None):
     """