Add method is_locked(lock_name) to test whether lock is held.

Fixes #647

docs/api.rst

@@ -810,6 +810,11 @@ Huey object
                 with huey.lock_task('db-backup'):
                     do_db_backup()
 
+    .. py:method:: is_locked(lock_name)
+
+        :param str lock_name: Name of lock to check.
+        :returns: boolean value indicating whether lock is held or not.
+
     .. py:method:: flush_locks(*names)
 
         :param names: additional lock-names to flush.

huey/api.py

@@ -598,6 +598,9 @@ def flush(self):
     def lock_task(self, lock_name):
         return TaskLock(self, lock_name)
 
+    def is_locked(self, lock_name):
+        return TaskLock(self, lock_name).is_locked()
+
     def flush_locks(self, *names):
         flushed = set()
         locks = self._locks
@@ -852,6 +855,9 @@ def __init__(self, huey, name):
         self._key = '%s.lock.%s' % (self._huey.name, self._name)
         self._huey._locks.add(self._key)
 
+    def is_locked(self):
+        return self._huey.storage.has_data_for_key(self._key)
+
     def __call__(self, fn):
         @wraps(fn)
         def inner(*args, **kwargs):

huey/tests/test_api.py

@@ -1283,10 +1283,17 @@ def task_a(n):
 
         task_a(4)
         with self.huey.lock_task('lock_x'):
+            self.assertFalse(self.huey.is_locked('lock_a'))
+            self.assertTrue(self.huey.is_locked('lock_x'))
             self.assertEqual(self.execute_next(), 5)
 
+        # Ensure locks were cleared.
+        self.assertFalse(self.huey.is_locked('lock_a'))
+        self.assertFalse(self.huey.is_locked('lock_x'))
+
         r = task_a(5)
         with self.huey.lock_task('lock_a'):
+            self.assertTrue(self.huey.is_locked('lock_a'))
             self.assertTrue(self.execute_next() is None)
 
         exc = self.trap_exception(r)