Implement a cancel() method on jobs.

This fixes #29.

rq/job.py

@@ -179,6 +179,20 @@ class Job(object):
 
         conn.hmset(key, obj)
 
+    def cancel(self):
+        """Cancels the given job, which will prevent the job from ever being
+        ran (or inspected).
+
+        This method merely exists as a high-level API call to cancel jobs
+        without worrying about the internals required to implement job
+        cancellation.  Technically, this call is (currently) the same as just
+        deleting the job hash.
+        """
+        self.delete()
+
+    def delete(self):
+        """Deletes the job hash from Redis."""
+        conn.delete(self.key)
 
     # Job execution
     def perform(self):  # noqa

tests/test_worker.py

@@ -1,3 +1,4 @@
+import os
 from tests import RQTestCase
 from tests import testjob, failing_job
 from tests.helpers import strip_milliseconds
@@ -5,6 +6,15 @@ from rq import Queue, Worker
 from rq.job import Job
 
 
+SENTINEL_FILE = '/tmp/rq-tests.txt'
+
+
+def create_sentinel():
+    # Create some evidence that the job ran
+    with open(SENTINEL_FILE, 'w') as f:
+        f.write('Just a sentinel.')
+
+
 class TestWorker(RQTestCase):
     def test_create_worker(self):
         """Worker creation."""
@@ -83,3 +93,28 @@ class TestWorker(RQTestCase):
         self.assertEquals(job.enqueued_at, enqueued_at_date)
         self.assertIsNotNone(job.exc_info)  # should contain exc_info
 
+
+    def test_cancelled_jobs_arent_executed(self):
+        """Cancelling jobs."""
+        try:
+            # Remove the sentinel if it is leftover from a previous test run
+            os.remove(SENTINEL_FILE)
+        except OSError as e:
+            if e.errno != 2:
+                raise
+
+        q = Queue()
+        result = q.enqueue(create_sentinel)
+
+        # Here, we cancel the job, so the sentinel file may not be created
+        assert q.count == 1
+        result.cancel()
+        assert q.count == 1
+
+        w = Worker([q])
+        w.work(burst=True)
+        assert q.count == 0
+
+        # Should not have created evidence of execution
+        self.assertEquals(os.path.exists(SENTINEL_FILE), False)
+