Add failing test for issue gh-1456

IPython/core/tests/test_ultratb.py

@@ -0,0 +1,51 b''
+"""Tests for IPython.core.ultratb
+"""
+
+import os.path
+import unittest
+
+from IPython.testing import tools as tt
+from IPython.utils.syspathcontext import prepended_to_syspath
+from IPython.utils.tempdir import TemporaryDirectory
+
+ip = get_ipython()
+
+file_1 = """1
+2
+3
+def f():
+  1/0
+"""
+
+file_2 = """def f():
+  1/0
+"""
+
+class ChangedPyFileTest(unittest.TestCase):
+    def test_changing_py_file(self):
+        """Traceback produced if the line where the error occurred is missing?
+        
+        https://github.com/ipython/ipython/issues/1456
+        """
+        with TemporaryDirectory() as td:
+            fname = os.path.join(td, "foo.py")
+            with open(fname, "w") as f:
+                f.write(file_1)
+            
+            with prepended_to_syspath(td):
+                ip.run_cell("import foo")
+            
+            with tt.AssertPrints("ZeroDivisionError"):
+                ip.run_cell("foo.f()")
+            
+            # Make the file shorter, so the line of the error is missing.
+            with open(fname, "w") as f:
+                f.write(file_2)
+            
+            # For some reason, this was failing on the *second* call after
+            # changing the file, so we call f() twice.
+            with tt.AssertNotPrints("Internal Python error", channel='stderr'):
+                with tt.AssertPrints("ZeroDivisionError"):
+                    ip.run_cell("foo.f()")
+                with tt.AssertPrints("ZeroDivisionError"):
+                    ip.run_cell("foo.f()")

IPython/testing/tools.py

@@ -306,7 +306,10 b' else:'
             super(MyStringIO, self).write(s)
 
 notprinted_msg = """Did not find {0!r} in printed output (on {1}):
-{2!r}"""
+-------
+{2!s}
+-------
+"""
 
 class AssertPrints(object):
     """Context manager for testing that code prints certain text.
@@ -338,6 +341,12 b' class AssertPrints(object):'
         assert self.s in printed, notprinted_msg.format(self.s, self.channel, printed)
         return False
     
+printed_msg = """Found {0!r} in printed output (on {1}):
+-------
+{2!s}
+-------
+"""
+
 class AssertNotPrints(AssertPrints):
     """Context manager for checking that certain output *isn't* produced.
     
@@ -346,7 +355,7 b' class AssertNotPrints(AssertPrints):'
         self.tee.flush()
         setattr(sys, self.channel, self.orig_stream)
         printed = self.buffer.getvalue()
-        assert self.s not in printed, notprinted_msg.format(self.s, self.channel, printed)
+        assert self.s not in printed, printed_msg.format(self.s, self.channel, printed)
         return False
 
 @contextmanager