Make IPythonPTCompleter(shell) backward-compatible

and declare that ptutils only contains private APIs

IPython/terminal/interactiveshell.py

@@ -238,7 +238,7 b' class TerminalInteractiveShell(InteractiveShell):'
                             editing_mode=editing_mode,
                             key_bindings_registry=kbmanager.registry,
                             history=history,
-                            completer=IPythonPTCompleter(self),
+                            completer=IPythonPTCompleter(shell=self),
                             enable_history_search=True,
                             style=style,
                             mouse_support=self.mouse_support,

IPython/terminal/ptutils.py

@@ -1,3 +1,12 b''
+"""prompt-toolkit utilities
+
+Everything in this module is a private API,
+not to be used outside IPython.
+"""
+
+# Copyright (c) IPython Development Team.
+# Distributed under the terms of the Modified BSD License.
+
 import unicodedata
 from wcwidth import wcwidth
 
@@ -13,15 +22,18 b' import pygments.lexers as pygments_lexers'
 
 class IPythonPTCompleter(Completer):
     """Adaptor to provide IPython completions to prompt_toolkit"""
-    def __init__(self, shell):
-        if isinstance(shell, IPCompleter):
-            raise TypeError("IPythonPTCompleter expects an InteractiveShell"
-                            " instance in IPython 5.1, not a Completer")
+    def __init__(self, ipy_completer=None, shell=None):
+        if shell is None and ipy_completer is None:
+            raise TypeError("Please pass shell=an InteractiveShell instance.")
+        self._ipy_completer = ipy_completer
         self.shell = shell
 
     @property
     def ipy_completer(self):
-        return self.shell.Completer
+        if self._ipy_completer:
+            return self._ipy_completer
+        else:
+            return self.shell.Completer
 
     def get_completions(self, document, complete_event):
         if not document.current_line.strip():