Backport PR #2305: RemoteError._render_traceback_ calls self.render_t…

…raceback

rather than an alias

There are two options here:

1. `_render_traceback_`  should *call* render_traceback
2. any subclass that redefines render_traceback must also redefine `_render_traceback_`

I went with 1., which is more efficient code-wise when subclassing RemoteError
(would prevent future cases of this same mistake),
but less efficient execution-wise, because it involves an extra function call.

closes #2303

IPython/parallel/error.py

@@ -194,8 +194,14 @@ def render_traceback(self):
         """render traceback to a list of lines"""
         return (self.traceback or "No traceback available").splitlines()
 
-    # Special method for custom tracebacks within IPython
-    _render_traceback_ = render_traceback
+    def _render_traceback_(self):
+        """Special method for custom tracebacks within IPython.
+
+        This will be called by IPython instead of displaying the local traceback.
+
+        It should return a traceback rendered as a list of lines.
+        """
+        return self.render_traceback()
 
     def print_traceback(self, excid=None):
         """print my traceback"""

IPython/parallel/tests/test_view.py

@@ -27,6 +27,7 @@
 
 from IPython.testing import decorators as dec
 from IPython.testing.ipunittest import ParametricTestCase
+from IPython.utils.io import capture_output
 
 from IPython import parallel  as pmod
 from IPython.parallel import error
@@ -578,6 +579,30 @@ def test_execute_raises(self):
         ar = view.execute("1/0")
         self.assertRaisesRemote(ZeroDivisionError, ar.get, 2)
 
+    def test_remoteerror_render_exception(self):
+        """RemoteErrors get nice tracebacks"""
+        view = self.client[-1]
+        ar = view.execute("1/0")
+        ip = get_ipython()
+        ip.user_ns['ar'] = ar
+        with capture_output() as io:
+            ip.run_cell("ar.get(2)")
+
+        self.assertTrue('ZeroDivisionError' in io.stdout, io.stdout)
+
+    def test_compositeerror_render_exception(self):
+        """CompositeErrors get nice tracebacks"""
+        view = self.client[:]
+        ar = view.execute("1/0")
+        ip = get_ipython()
+        ip.user_ns['ar'] = ar
+        with capture_output() as io:
+            ip.run_cell("ar.get(2)")
+
+        self.assertEqual(io.stdout.count('ZeroDivisionError'), len(view) * 2, io.stdout)
+        self.assertEqual(io.stdout.count('integer division'), len(view), io.stdout)
+        self.assertEqual(io.stdout.count(':execute'), len(view), io.stdout)
+
     @dec.skipif_not_matplotlib
     def test_magic_pylab(self):
         """%pylab works on engines"""