Do not emit single completions trailing with space.

Workaround  #9658, that can thus be delayed post 5.0

IPython/core/completer.py

@@ -77,6 +77,8 b' from IPython.utils.process import arg_split'
 from IPython.utils.py3compat import builtin_mod, string_types, PY3, cast_unicode_py2
 from traitlets import Bool, Enum, observe
 
+from functools import wraps
+
 #-----------------------------------------------------------------------------
 # Globals
 #-----------------------------------------------------------------------------
@@ -91,6 +93,44 b' else:'
 
 
 #-----------------------------------------------------------------------------
+# Work around BUG decorators.
+#-----------------------------------------------------------------------------
+
+def _strip_single_trailing_space(complete):
+    """
+    This is a workaround for a weird IPython/Prompt_toolkit behavior,
+    that can be removed once we rely on a slightly more recent prompt_toolkit
+    version (likely > 1.0.3). So this can likely be removed in IPython 6.0
+
+    cf https://github.com/ipython/ipython/issues/9658
+    and https://github.com/jonathanslenders/python-prompt-toolkit/pull/328
+
+    The bug is due to the fact that in PTK the completer will reinvoke itself
+    after trying to completer to the longuest common prefix of all the
+    completions, unless only one completion is available.
+
+    This logic is faulty if the completion ends with space, which can happen in
+    case like::
+
+        from foo import im<ta>
+
+    which only matching completion is `import `. Note the leading space at the
+    end. So leaving a space at the end is a reasonable request, but for now
+    we'll strip it.
+    """
+
+    @wraps(complete)
+    def comp(*args, **kwargs):
+        text, matches =  complete(*args, **kwargs)
+        if len(matches) == 1:
+            return text, [matches[0].rstrip()]
+        return text, matches
+
+    return comp
+
+
+
+#-----------------------------------------------------------------------------
 # Main functions and classes
 #-----------------------------------------------------------------------------
 
@@ -1121,6 +1161,7 b' class IPCompleter(Completer):'
 
         return None
 
+    @_strip_single_trailing_space
     def complete(self, text=None, line_buffer=None, cursor_pos=None):
         """Find completions for the given text and line context.