List backup points and revert to specific ones

`topydo revert ls` will now show all saved backup points with position
number (starting with 1), date and time of relevant action, and its
label.

`topydo revert 2` will revert to a point **before** the backup point
marked as 2 in `topydo revert ls` (i.e. it will go 2 changes backwards
or phrasing more naturally: revert last 2 changes).

Warning: changes between between current state and desired backup point
are also deleted to avoid false assumption that our backups can be
treated as some sort of atomic "patches".

topydo/commands/RevertCommand.py

 # Topydo - A todo.txt client written in Python.
-# Copyright (C) 2014 - 2015 Bram Schoenmakers <bram@topydo.org>
+# Copyright (C) 2014 - 2017 Bram Schoenmakers <bram@topydo.org>
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -14,8 +14,10 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from topydo.lib.Command import Command
+import arrow
+
 from topydo.lib.ChangeSet import ChangeSet
+from topydo.lib.Command import Command, InvalidCommandArgument
 from topydo.lib import TodoFile
 from topydo.lib import TodoList
 from topydo.lib.Config import config
@@ -28,36 +30,104 @@ class RevertCommand(Command):
                  p_prompt=lambda a: None):
         super().__init__(p_args, p_todolist, p_out, p_err, p_prompt)
 
-        self._archive = None
+        self._backup = None
         self._archive_file = None
+        self._archive = None
 
     def execute(self):
         if not super().execute():
             return False
 
+        self._backup = ChangeSet()
         archive_path = config().archive()
         if archive_path:
-            self._archive_file = TodoFile.TodoFile(archive_path)
+            self._archive_file = TodoFile.TodoFile(config().archive())
             self._archive = TodoList.TodoList(self._archive_file.read())
 
-        last_change = ChangeSet()
+        if len(self.args) > 1:
+            self.error(self.usage())
+        else:
+            try:
+                arg = self.argument(0)
+                self._handle_args(arg)
+            except InvalidCommandArgument:
+                try:
+                    self._revert_last()
+                except (ValueError, KeyError):
+                    self.error('No backup was found for the current state of '
+                               + config().todotxt())
+
+        self._backup.close()
+
+    def _revert(self, p_timestamp=None):
+        self._backup.read_backup(self.todolist, p_timestamp)
+        self._backup.apply(self.todolist, self._archive)
+
+        if self._archive:
+            self._archive_file.write(self._archive.print_todos())
+
+        self.out("Reverted to state before: " + self._backup.label)
 
+    def _revert_last(self):
+        self._revert()
+        self._backup.delete()
+
+    def _revert_to_specific(self, p_position):
+        timestamps = [timestamp for timestamp, _ in self._backup]
+        position = int(p_position) - 1  # numbering in UI starts with 1
+        try:
+            timestamp = timestamps[position]
+            self._revert(timestamp)
+            for timestamp in timestamps[:position + 1]:
+                self._backup.read_backup(p_timestamp=timestamp)
+                self._backup.delete()
+        except IndexError:
+            self.error('Specified index is out range')
+
+    def _handle_args(self, p_arg):
         try:
-            last_change.get_backup(self.todolist)
-            last_change.apply(self.todolist, self._archive)
-            if self._archive:
-                self._archive_file.write(self._archive.print_todos())
-            last_change.delete()
+            if p_arg == 'ls':
+                self._handle_ls()
+            elif p_arg.isdigit():
+                self._revert_to_specific(p_arg)
+            else:
+                raise InvalidCommandArgument
+        except InvalidCommandArgument:
+            self.error(self.usage())
 
-            self.out("Successfully reverted: " + last_change.label)
-        except (ValueError, KeyError):
-            self.error('No backup was found for the current state of '
-                       + config().todotxt())
+    def _handle_ls(self):
+        num = 1
+        for timestamp, change in self._backup:
+            label = change[2]
+            time = arrow.get(timestamp).format('YYYY-MM-DD HH:mm:ss')
 
-        last_change.close()
+            self.out('{0: >3}| {1} | {2}'.format(str(num), time, label))
+            num += 1
 
     def usage(self):
-        return """Synopsis: revert"""
+        return """Synopsis:
+  revert [ls]
+  revert [NUMBER]"""
 
     def help(self):
-        return """Reverts the last command."""
+        return """\
+Reverts last commands.
+
+* ls       : Lists all backups ordered and numbered chronologically (starting
+             with 1 for latest backup).
+* [NUMBER] : revert to specific point in history specified by NUMBER.
+
+Output example for `revert ls`:
+1 | 1970-01-01 00:00:02 | add Baz
+2 | 1970-01-01 00:00:01 | add Bar
+3 | 1970-01-01 00:00:00 | add Foo
+
+In such example executing `revert 2` will revert todo and archive files to the
+state before execution of `add Bar`.
+
+* `revert` without any further arguments will revert to the latest backup
+  available, provided that this backup matches current state of the todo file.
+  Topydo will refuse to revert, if any changes to todo file were made by
+  external application after the latest backup. To force a `revert` action use
+  it with a NUMBER.\
+"""

topydo/lib/ChangeSet.py

 # Topydo - A todo.txt client written in Python.
-# Copyright (C) 2014 - 2015 Bram Schoenmakers <bram@topydo.org>
+# Copyright (C) 2014 - 2017 Bram Schoenmakers <bram@topydo.org>
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -46,7 +46,7 @@ class ChangeSet(object):
     def __init__(self, p_todolist=None, p_archive=None, p_label=[]):
         self.todolist = deepcopy(p_todolist)
         self.archive = deepcopy(p_archive)
-        self.timestamp = str(int(time.time()))
+        self.timestamp = str(time.time())
         self.label = ' '.join(p_label)
 
         try:
@@ -56,6 +56,11 @@ class ChangeSet(object):
 
         self._read()
 
+    def __iter__(self):
+        items = {key: self.backup_dict[key]
+                 for key in self.backup_dict if key != 'index'}.items()
+        return iter(sorted(items, reverse=True))
+
     def _read(self):
         """
         Reads backup file from json_file property and sets backup_dict property
@@ -158,15 +163,18 @@ class ChangeSet(object):
         for changeset in index[backup_limit:]:
             self.delete(changeset[0], p_write=False)
 
-    def get_backup(self, p_todolist):
+    def read_backup(self, p_todolist=None, p_timestamp=None):
         """
-        Retrieves a backup for p_todolist from backup file and sets todolist,
-        archive and label attributes to appropriate data from it.
+        Retrieves a backup for p_timestamp or p_todolist (if p_timestamp is not
+        specified) from backup file and sets timestamp, todolist, archive and
+        label attributes to appropriate data from it.
         """
-        change_hash = hash_todolist(p_todolist)
-
-        index = self._get_index()
-        self.timestamp = index[[change[1] for change in index].index(change_hash)][0]
+        if not p_timestamp:
+            change_hash = hash_todolist(p_todolist)
+            index = self._get_index()
+            self.timestamp = index[[change[1] for change in index].index(change_hash)][0]
+        else:
+            self.timestamp = p_timestamp
 
         d = self.backup_dict[self.timestamp]