Lock Mailbox when used in a context manager

Author: sblondon

Doc/library/mailbox.rst

@@ -77,8 +77,8 @@ Supported mailbox formats are Maildir, mbox, MH, Babyl, and MMDF.
       corrupting the entire mailbox.
 
    The :class:`Mailbox` class supports the :keyword:`with` statement.  When used
-   like this, the Mailbox :meth:`close` method is called automatically when the
-   :keyword:`with` statement exits.
+   like this, the Mailbox get a lock when the :keyword:`with` statement enters and
+   release it and :meth:`close` when the :keyword:`with` statement exits.
 
    .. versionchanged:: 3.7
       Support for the :keyword:`with` statement was added.

Lib/mailbox.py

@@ -39,6 +39,7 @@ def __init__(self, path, factory=None, create=True):
         self._factory = factory
 
     def __enter__(self):
+        self.lock()
         return self
 
     def __exit__(self, type, value, traceback):