python: cleanup the __nonzero__ implementations

Cleanup the code, reword the docstring and use the same implementation
in the Threads, Tags and Messages classes.

__nonzero__ implements truth value testing. If __nonzero__ is not
implemented, the python runtime would fall back to `len(..) > 0` thus
exhausting the iterator.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>

bindings/python/notmuch/messages.py

@@ -172,11 +172,15 @@ class Messages(object):
     next = __next__ # python2.x iterator protocol compatibility
 
     def __nonzero__(self):
-        """
-        :return: True if there is at least one more thread in the
-            Iterator, False if not."""
-        return self._msgs is not None and \
-            self._valid(self._msgs) > 0
+        '''
+        Implement truth value testing. If __nonzero__ is not
+        implemented, the python runtime would fall back to `len(..) >
+        0` thus exhausting the iterator.
+
+        :returns: True if the wrapped iterator has at least one more object
+                  left.
+        '''
+        return self._msgs and self._valid(self._msgs)
 
     _destroy = nmlib.notmuch_messages_destroy
     _destroy.argtypes = [NotmuchMessagesP]

bindings/python/notmuch/tag.py

@@ -109,15 +109,15 @@ class Tags(Python3StringMixIn):
     next = __next__ # python2.x iterator protocol compatibility
 
     def __nonzero__(self):
-        """Implement bool(Tags) check that can be repeatedly used
+        '''
+        Implement truth value testing. If __nonzero__ is not
+        implemented, the python runtime would fall back to `len(..) >
+        0` thus exhausting the iterator.
 
-        If __nonzero__ is not implemented, "if Tags()"
-        will implicitly call __len__, using up our iterator, so it is
-        important that this function is defined.
-
-        :returns: True if the Tags() iterator has at least one more Tag
-            left."""
-        return self._valid(self._tags) > 0
+        :returns: True if the wrapped iterator has at least one more object
+                  left.
+        '''
+        return self._tags and self._valid(self._tags)
 
     def __unicode__(self):
         """string representation of :class:`Tags`: a space separated list of tags

bindings/python/notmuch/threads.py

@@ -157,18 +157,15 @@ class Threads(Python3StringMixIn):
         return i
 
     def __nonzero__(self):
-        """Check if :class:`Threads` contains at least one more valid thread
+        '''
+        Implement truth value testing. If __nonzero__ is not
+        implemented, the python runtime would fall back to `len(..) >
+        0` thus exhausting the iterator.
 
-        The existence of this function makes 'if Threads: foo' work, as
-        that will implicitely call len() exhausting the iterator if
-        __nonzero__ does not exist. This function makes `bool(Threads())`
-        work repeatedly.
-
-        :return: True if there is at least one more thread in the
-           Iterator, False if not. None on a "Out-of-memory" error.
-        """
-        return self._threads is not None and \
-            self._valid(self._threads) > 0
+        :returns: True if the wrapped iterator has at least one more object
+                  left.
+        '''
+        return self._threads and self._valid(self._threads)
 
     _destroy = nmlib.notmuch_threads_destroy
     _destroy.argtypes = [NotmuchThreadsP]