Don't calculate default value when setting traitlet for the first time

IPython/core/application.py

@@ -199,11 +199,12 b' class BaseIPythonApplication(Application):'
             return crashhandler.crash_handler_lite(etype, evalue, tb)
     
     def _ipython_dir_changed(self, name, old, new):
-        str_old = py3compat.cast_bytes_py2(os.path.abspath(old),
-            sys.getfilesystemencoding()
-        )
-        if str_old in sys.path:
-            sys.path.remove(str_old)
+        if old is not None:
+            str_old = py3compat.cast_bytes_py2(os.path.abspath(old),
+                sys.getfilesystemencoding()
+            )
+            if str_old in sys.path:
+                sys.path.remove(str_old)
         str_path = py3compat.cast_bytes_py2(os.path.abspath(new),
             sys.getfilesystemencoding()
         )

IPython/kernel/zmq/pylab/config.py

@@ -13,6 +13,7 b' This module does not import anything from matplotlib.'
 # Imports
 #-----------------------------------------------------------------------------
 
+from IPython.config import Config
 from IPython.config.configurable import SingletonConfigurable
 from IPython.utils.traitlets import (
     Dict, Instance, CaselessStrEnum, Set, Bool, Int, TraitError, Unicode
@@ -42,7 +43,7 b' class InlineBackend(InlineBackendConfig):'
 
     def _config_changed(self, name, old, new):
         # warn on change of renamed config section
-        if new.InlineBackendConfig != old.InlineBackendConfig:
+        if new.InlineBackendConfig != getattr(old, 'InlineBackendConfig', Config()):
             warn("InlineBackendConfig has been renamed to InlineBackend")
         super(InlineBackend, self)._config_changed(name, old, new)
 

IPython/utils/traitlets.py

@@ -415,7 +415,11 b' class TraitType(object):'
 
     def __set__(self, obj, value):
         new_value = self._validate(obj, value)
-        old_value = self.__get__(obj)
+        try:
+            old_value = obj._trait_values[self.name]
+        except KeyError:
+            old_value = None
+
         obj._trait_values[self.name] = new_value
         try:
             silent = bool(old_value == new_value)