Defer Traits import in ipy_traits_completer.

Please note that this extension is for now in 'deathrow', we need to check
it and update it to any changes made, so we can bring it back into mainline.

Closes https://bugs.launchpad.net/ipython/+bug/416174

IPython/deathrow/ipy_traits_completer.py

@@ -44,13 +44,14 b' Notes'
 =====
 
   - This requires Python 2.4 to work (I use sets).  I don't think anyone is
-  using traits with 2.3 anyway, so that's OK.
+    using traits with 2.3 anyway, so that's OK.
+
+  - Imports from enthought.traits are deferred until an object with a class that
+    looks like it subclasses from HasTraits comes along. This test is done by
+    looking at the name of the class and its superclasses.
 """
 
 #############################################################################
-# External imports
-from enthought.traits import api as T
-
 # IPython imports
 from IPython.core.error import TryNext
 from IPython.core.ipapi import get as ipget
@@ -71,11 +72,37 b' COMPLETE_THRESHOLD = 3'
 
 # Set of names that Traits automatically adds to ANY traits-inheriting object.
 # These are the names we'll filter out.
-TRAIT_NAMES = set( dir2(T.HasTraits()) ) - set( dir2(object()) )
+TRAIT_NAMES = None
+def get_trait_names():
+    global TRAIT_NAMES
+    from enthought.traits.api import HasTraits
+    if TRAIT_NAMES is None:
+        TRAIT_NAMES = set( dir2(HasTraits()) ) - set( dir2(object()) )
+    else:
+        return TRAIT_NAMES
 
 #############################################################################
 # Code begins
 
+def looks_like_isinstance(obj, classname):
+    """ Return True if the object has a class or superclass with the given class
+    name.
+
+    Ignores old-style classes.
+    """
+    from types import InstanceType
+
+    t = type(obj)
+    if t is InstanceType:
+        # Old-style classes.
+        return False
+    elif t.__name__ == classname:
+        return True
+    for klass in t.__mro__:
+        if klass.__name__ == classname:
+            return True
+    return False
+
 def trait_completer(self,event):
     """A custom IPython tab-completer that is traits-aware.
 
@@ -94,7 +121,13 b' def trait_completer(self,event):'
 
     obj = oinfo['obj']
     # OK, we got the object.  See if it's traits, else punt
-    if not isinstance(obj,T.HasTraits):
+    if not looks_like_isinstance(obj, 'HasTraits'):
+        raise TryNext
+
+    # Defer import until here so as not to require Traits until we get something
+    # that looks like it might be a HasTraits instance.
+    from enthought.traits.api import HasTraits
+    if not isinstance(obj, HasTraits):
         raise TryNext
 
     # it's a traits object, don't show the tr* attributes unless the completion
@@ -115,7 +148,7 b' def trait_completer(self,event):'
     #print '\nastart:<%r>' % attr_start  # dbg
 
     if len(attr_start)<COMPLETE_THRESHOLD:
-        attrs = list(set(attrs) - TRAIT_NAMES)
+        attrs = list(set(attrs) - get_trait_names())
         
     # The base of the completion, so we can form the final results list
     bdot = base+'.'
@@ -150,13 +183,14 b' def activate(complete_threshold = COMPLETE_THRESHOLD):'
 #############################################################################
 if __name__ == '__main__':
     # Testing/debugging
+    from enthought.traits.api import HasTraits
 
     # A sorted list of the names we'll filter out
-    TNL = list(TRAIT_NAMES)
+    TNL = list(get_trait_names())
     TNL.sort()
 
     # Make a few objects for testing
-    class TClean(T.HasTraits): pass
+    class TClean(HasTraits): pass
     class Bunch(object): pass
     # A clean traits object
     t = TClean()