Refactor a bit of uniformity.

Not-reiniting user_ns may also help fix issues on windows.

IPython/core/interactiveshell.py

@@ -270,6 +270,16 @@ def __repr__(self):
         return '<%s object at %x, execution_count=%s error_before_exec=%s error_in_exec=%s info=%s result=%s>' %\
                 (name, id(self), self.execution_count, self.error_before_exec, self.error_in_exec, repr(self.info), repr(self.result))
 
+@functools.wraps(io_open)
+def _modified_open(file, *args, **kwargs):
+    if file in {0, 1, 2}:
+        raise ValueError(
+            f"IPython won't let you open fd={file} by default "
+            "as it is likely to crash IPython. If you know what you are doing, "
+            "you can use builtins' open."
+        )
+
+    return io_open(file, *args, **kwargs)
 
 class InteractiveShell(SingletonConfigurable):
     """An enhanced, interactive shell for Python."""
@@ -1255,18 +1265,7 @@ def prepare_user_module(self, user_module=None, user_ns=None):
         if user_ns is None:
             user_ns = user_module.__dict__
 
-        @functools.wraps(io_open)
-        def modified_open(file, *args, **kwargs):
-            if file in {0, 1, 2}:
-                raise ValueError(
-                    f"IPython won't let you open fd={file} by default "
-                    "as it is likely to crash IPython. If you know what you are doing, "
-                    "you can use builtins' open."
-                )
-
-            return io_open(file, *args, **kwargs)
 
-        user_ns["open"] = modified_open
 
         return user_module, user_ns
 
@@ -1336,6 +1335,7 @@ def init_user_ns(self):
 
         ns['exit'] = self.exiter
         ns['quit'] = self.exiter
+        ns["open"] = _modified_open
 
         # Sync what we've added so far to user_ns_hidden so these aren't seen
         # by %who

IPython/core/tests/test_interactiveshell.py

@@ -104,17 +104,14 @@ def test_syntax_error(self):
         self.assertIsInstance(res.error_before_exec, SyntaxError)
 
     def test_open_standard_input_stream(self):
-        ip.init_create_namespaces()
         res = ip.run_cell("open(0)")
         self.assertIsInstance(res.error_in_exec, ValueError)
 
     def test_open_standard_output_stream(self):
-        ip.init_create_namespaces()
         res = ip.run_cell("open(1)")
         self.assertIsInstance(res.error_in_exec, ValueError)
 
     def test_open_standard_error_stream(self):
-        ip.init_create_namespaces()
         res = ip.run_cell("open(2)")
         self.assertIsInstance(res.error_in_exec, ValueError)