Prompt mode: reload the todo.txt file if it was modified.

The todo.txt file can be modified while the prompt is running. Make sure
to reload the file when the mtime has changed since the previous load.

This addresses the issue that changes after the 'edit' subcommand were
not visible when returning to the topydo prompt (issue #33).

Also, other changes done while the prompt is running will be picked up.

A safety measure was taken to refuse executing a command when the
todo.txt file was modified while the prompt was visible (i.e. the user
was busy typing a command). By doing so, subcommands such as 'do' and
'del' will only operate on the actual state of the todo.txt file, not
some previous state still in memory.

topydo/cli/Prompt.py

@@ -16,6 +16,7 @@
 
 """ Entry file for the topydo Prompt interface (CLI). """
 
+import os.path
 import sys
 
 from topydo.cli.CLIApplicationBase import CLIApplicationBase, error, usage
@@ -39,6 +40,12 @@ from topydo.commands.SortCommand import SortCommand
 from topydo.lib import TodoFile
 from topydo.lib import TodoList
 
+def _todotxt_mtime():
+    """
+    Returns the mtime for the configured todo.txt file.
+    """
+    return os.path.getmtime(config().todotxt())
+
 class PromptApplication(CLIApplicationBase):
     """
     This class implements a variant of topydo's CLI showing a shell and
@@ -47,24 +54,50 @@ class PromptApplication(CLIApplicationBase):
     def __init__(self):
         super(PromptApplication, self).__init__()
 
-    def run(self):
-        """ Main entry function. """
-        args = self._process_flags()
+        self._process_flags()
+        self.mtime = None
+
+    def _load_file(self):
+        """
+        Reads the configured todo.txt file and loads it into the todo list
+        instance.
+
+        If the modification time of the todo.txt file is equal to the last time
+        it was checked, nothing will be done.
+        """
 
-        self.todofile = TodoFile.TodoFile(config().todotxt())
-        self.todolist = TodoList.TodoList(self.todofile.read())
+        current_mtime = _todotxt_mtime()
 
+        if self.mtime != current_mtime:
+            self.todofile = TodoFile.TodoFile(config().todotxt())
+            self.todolist = TodoList.TodoList(self.todofile.read())
+            self.mtime = current_mtime
+
+    def run(self):
+        """ Main entry function. """
         # suppress upstream issue with Python 2.7
         # pylint: disable=no-value-for-parameter
         completer = TopydoCompleter(self.todolist)
         history = History()
 
         while True:
+            # (re)load the todo.txt file (only if it has been modified)
+            self._load_file()
+
             try:
-                user_input = get_input(u'topydo> ', history=history, completer=completer).split()
+                user_input = get_input(u'topydo> ', history=history,
+                                       completer=completer).split()
             except (EOFError, KeyboardInterrupt):
                 sys.exit(0)
 
+            mtime_after = _todotxt_mtime()
+
+            if self.mtime != mtime_after:
+                # refuse to perform operations such as 'del' and 'do' if the
+                # todo.txt file has been changed in the background.
+                error("WARNING: todo.txt file was modified by another application.\nTo prevent unintended changes, this operation was not executed.")
+                continue
+
             (subcommand, args) = get_subcommand(user_input)
 
             try:
@@ -80,8 +113,9 @@ class PromptApplication(CLIApplicationBase):
                 usage()
 
 def main():
-    """ Main entry point of the CLI. """
+    """ Main entry point of the prompt interface. """
     PromptApplication().run()
 
 if __name__ == '__main__':
     main()
+