make pipeline exceptions look more natural

beets/ui/pipeline.py

@@ -27,6 +27,7 @@ exception.
 from __future__ import with_statement # for Python 2.5
 import Queue
 from threading import Thread, Lock
+import sys
 
 BUBBLE = '__PIPELINE_BUBBLE__'
 POISON = '__PIPELINE_POISON__'
@@ -38,8 +39,8 @@ class PipelineError(object):
     object is passed through the pipeline to shut down all threads
     before it is raised again in the main thread.
     """
-    def __init__(self, exc):
-        self.exc = exc
+    def __init__(self, exc_info):
+        self.exc_info = exc_info
 
 class FirstPipelineThread(Thread):
     """The thread running the first stage in a parallel pipeline setup.
@@ -74,8 +75,8 @@ class FirstPipelineThread(Thread):
                 msg = self.coro.next()
             except StopIteration:
                 break
-            except Exception, exc:
-                self.out_queue.put(PipelineError(exc))
+            except:
+                self.out_queue.put(PipelineError(sys.exc_info()))
                 return
             
             # Send it to the next stage.
@@ -133,8 +134,8 @@ class MiddlePipelineThread(Thread):
             # Invoke the current stage.
             try:    
                 out = self.coro.send(msg)
-            except Exception, exc:
-                self.out_queue.put(PipelineError(exc))
+            except:
+                self.out_queue.put(PipelineError(sys.exc_info()))
                 return
             
             # Send message to next stage.
@@ -164,18 +165,18 @@ class LastPipelineThread(Thread):
             if msg is POISON:
                 break
             elif isinstance(msg, PipelineError):
-                self.exc = msg.exc
+                self.exc_info = msg.exc_info
                 return
             
             # Send to consumer.
             try:
                 self.coro.send(msg)
-            except Exception, exc:
-                self.exc = exc
+            except:
+                self.exc_info = sys.exc_info()
                 return
         
         # No exception raised in pipeline.
-        self.exc = None
+        self.exc_info = None
 
 class Pipeline(object):
     """Represents a staged pattern of work. Each stage in the pipeline
@@ -241,9 +242,10 @@ class Pipeline(object):
         for thread in threads[:-1]:
             thread.join()
 
-        exc = threads[-1].exc
-        if exc:
-            raise exc
+        exc_info = threads[-1].exc_info
+        if exc_info:
+            # Make the exception appear as it was raised originally.
+            raise exc_info[0], exc_info[1], exc_info[2]
 
 # Smoke test.
 if __name__ == '__main__':
@@ -295,4 +297,3 @@ if __name__ == '__main__':
             num = yield
             print 'received', num
     Pipeline([exc_produce(), exc_work(), exc_consume()]).run_parallel(1)
-