Save objects imported with an alias and top level modules by referenc…

…e in `dump_session(byref=TRUE)`

Currently, `dump_session(byref=True)` misses some imported objects. For example:
- If the session had a statement `import numpy as np`, it may find a reference to the `numpy` named
as `np` in some internal module listed in `sys.resources`.  But if the module was imported with a
non-canonical name, like `import numpy as nump`, it won't find it at all. Mapping the objects by id
in `modmap` solves the issue.  Note that just types of objects usually imported under an alias must
be looked up by id, otherwise common objects like singletons may be wrongly attributed to a module,
and such reference in the module could change to a different object depending on its initialization
and state.
- If a object in the global scope is a top level module, like `math`, again `save_session` may find
a reference to it in another module and it works. But if this module isn't referenced anywhere else,
it won't be found because the function only looks for objects inside the `sys.resources` modules and
not for the modules themselves.

This commit introduces two new attributes to session modules saved by reference:
- `__dill_imported_as`: a list with (module name, object name, object alias in session)
- `__dill_imported_top_level`: a list with (module name, module alias in session)

I did it this way for forwards (complete) and backwards (partial) compatibility.

Oh, and I got rid of that nasty `exec()` call in `_restore_modules()`! ;)

dill/_dill.py

@@ -397,50 +397,75 @@ def loads(str, ignore=None, **kwds):
 ### End: Shorthands ###
 
 ### Pickle the Interpreter Session
+SESSION_IMPORTED_AS_TYPES = (ModuleType, ClassType, TypeType, Exception,
+                             FunctionType, MethodType, BuiltinMethodType)
+
 def _module_map():
     """get map of imported modules"""
-    from collections import defaultdict
-    modmap = defaultdict(list)
+    from collections import defaultdict, namedtuple
+    modmap = namedtuple('Modmap', ['by_module', 'by_name', 'top_level'])
+    modmap = modmap(defaultdict(list), defaultdict(list), {})
     items = 'items' if PY3 else 'iteritems'
-    for name, module in getattr(sys.modules, items)():
+    for modname, module in getattr(sys.modules, items)():
         if module is None:
             continue
-        for objname, obj in module.__dict__.items():
-            modmap[objname].append((obj, name))
-    return modmap
-
-def _lookup_module(modmap, name, obj, main_module): #FIXME: needs work
-    """lookup name if module is imported"""
-    for modobj, modname in modmap[name]:
+        if '.' not in modname:
+            modmap.top_level[id(module)] = modname
+        for objname, modobj in module.__dict__.items():
+            modmap.by_name[objname].append((modobj, modname))
+            modmap.by_id[id(modobj)].append((modobj, objname, modname))
+    return modmap, modmap_byid
+
+def _lookup_module(modmap, name, obj, main_module):
+    """lookup name or id of obj if module is imported"""
+    for modobj, modname in modmap.by_name[name]:
         if modobj is obj and sys.modules[modname] is not main_module:
-            return modname
+            return modname, name
+    if isinstance(obj, SESSION_IMPORTED_AS_TYPES):
+        for modobj, objname, modname in modmap.by_id[id(obj)]:
+            if sys.modules[modname] is not main_module:
+                return modname, objname
+    return None, None
 
 def _stash_modules(main_module):
     modmap = _module_map()
     imported = []
+    imported_as = []
+    imported_top_level = []  # backwards compatibility
     original = {}
     items = 'items' if PY3 else 'iteritems'
     for name, obj in getattr(main_module.__dict__, items)():
-        source_module = _lookup_module(modmap, name, obj, main_module)
+        source_module, objname = _lookup_module(modmap, name, obj, main_module)
         if source_module:
-            imported.append((source_module, name))
+            if objname == name:
+                imported.append((source_module, name))
+            else:
+                imported_as.append((source_module, objname, name))
         else:
-            original[name] = obj
+            try:
+                imported_top_level.append((modmap.top_level[id(obj)], name))
+            except KeyError:
+                original[name] = obj
     if len(imported):
-        import types
-        newmod = types.ModuleType(main_module.__name__)
+        newmod = ModuleType(main_module.__name__)
         newmod.__dict__.update(original)
         newmod.__dill_imported = imported
+        newmod.__dill_imported_as = imported_as
+        newmod.__dill_imported_top_level = imported_top_level
         return newmod
     else:
         return main_module
 
 def _restore_modules(main_module):
-    if '__dill_imported' not in main_module.__dict__:
-        return
-    imports = main_module.__dict__.pop('__dill_imported')
-    for module, name in imports:
-        exec("from %s import %s" % (module, name), main_module.__dict__)
+    try:
+        for modname, name in main_module.__dict__.pop('__dill_imported'):
+            main_module.__dict__[name] = __import__(modname, None, None, [name]).__dict__[name]
+        for modname, objname, name in main_module.__dict__.pop('__dill_imported_as'):
+            main_module.__dict__[name] = __import__(modname, None, None, [objname]).__dict__[objname]
+        for modname, name in main_module.__dict__.pop('__dill_imported_top_level'):
+            main_module.__dict__[name] = __import__(modname)
+    except KeyError:
+        pass
 
 #NOTE: 06/03/15 renamed main_module to main
 def dump_session(filename='/tmp/session.pkl', main=None, byref=False, **kwds):