Catch errors raised by user objects when accessing attributes.

When analyzing the line with prefilter, we look into whether objects
have a 'rewrite' attribute.  While this is off-spec, we've seen in the
wild objects that raise something other than AttributeError on
attribute access.  Now we catch all exceptions in this codepath.

Closes  #988.

IPython/core/prefilter.py

@@ -812,7 +812,14 b' class AutoHandler(PrefilterHandler):'
             return line
 
         force_auto = isinstance(obj, IPyAutocall)
-        auto_rewrite = getattr(obj, 'rewrite', True)
+
+        # User objects sometimes raise exceptions on attribute access other
+        # than AttributeError (we've seen it in the past), so it's safest to be
+        # ultra-conservative here and catch all.
+        try:
+            auto_rewrite = obj.rewrite
+        except Exception:
+            auto_rewrite = True
 
         if esc == ESC_QUOTE:
             # Auto-quote splitting on whitespace

IPython/core/tests/test_prefilter.py

@@ -35,6 +35,7 b' def test_prefilter():'
     for raw, correct in pairs:
         yield nt.assert_equals(ip.prefilter(raw), correct)
 
+
 @dec.parametric
 def test_autocall_binops():
     """See https://github.com/ipython/ipython/issues/81"""
@@ -49,10 +50,11 b' def test_autocall_binops():'
         ip.magic('autocall 0')
         del ip.user_ns['f']
 
+
 @dec.parametric
-def test_issue114():
+def test_issue_114():
     """Check that multiline string literals don't expand as magic
-    see http://github.com/ipython/ipython/issues/#issue/114"""
+    see http://github.com/ipython/ipython/issues/114"""
 
     template = '"""\n%s\n"""'
     # Store the current value of multi_line_specials and turn it off before
@@ -66,3 +68,27 b' def test_issue114():'
             yield nt.assert_equals(ip.prefilter(raw), raw)
     finally:
         ip.prefilter_manager.multi_line_specials = msp
+
+
+def test_prefilter_attribute_errors():
+    """Capture exceptions thrown by user objects on attribute access.
+
+    See http://github.com/ipython/ipython/issues/988."""
+
+    class X(object):
+        def __getattr__(self, k):
+            raise ValueError('broken object')
+        def __call__(self, x):
+            return x
+
+    # Create a callable broken object
+    ip.user_ns['x'] = X()
+    ip.magic('autocall 2')
+    try:
+        # Even if x throws an attribute error when looking at its rewrite
+        # attribute, we should not crash.  So the test here is simply making
+        # the prefilter call and not having an exception.
+        ip.prefilter('x 1')
+    finally:
+        del ip.user_ns['x']
+        ip.magic('autocall 0')