add LazyConfigValue

exposes container methods on container traits, to be lazily evaluated.

Specifically:

- append, extend, insert on lists
- update on dicts
- update, add on sets

IPython/config/configurable.py

@@ -28,7 +28,7 b' Authors:'
 import datetime
 from copy import deepcopy
 
-from loader import Config
+from .loader import Config, LazyConfigValue
 from IPython.utils.traitlets import HasTraits, Instance
 from IPython.utils.text import indent, wrap_paragraphs
 
@@ -149,6 +149,11 b' class Configurable(HasTraits):'
         my_config = self._find_my_config(cfg)
         for name, config_value in my_config.iteritems():
             if name in traits:
+                if isinstance(config_value, LazyConfigValue):
+                    # ConfigValue is a wrapper for using append / update on containers
+                    # without having to copy the 
+                    initial = getattr(self, name)
+                    config_value = config_value.get_value(initial)
                 # We have to do a deepcopy here if we don't deepcopy the entire
                 # config object. If we don't, a mutable config_value will be
                 # shared by all instances, effectively making it a class attribute.

IPython/config/loader.py

@@ -25,6 +25,7 b' Authors'
 
 import __builtin__ as builtin_mod
 import argparse
+import copy
 import os
 import re
 import sys
@@ -32,6 +33,7 b' import sys'
 from IPython.utils.path import filefind, get_ipython_dir
 from IPython.utils import py3compat, warn
 from IPython.utils.encoding import DEFAULT_ENCODING
+from IPython.utils.traitlets import HasTraits, List, Any, TraitError
 
 #-----------------------------------------------------------------------------
 # Exceptions
@@ -74,6 +76,83 b' class ArgumentParser(argparse.ArgumentParser):'
 # Config class for holding config information
 #-----------------------------------------------------------------------------
 
+class LazyConfigValue(HasTraits):
+    """Proxy object for exposing methods on configurable containers
+    
+    Exposes:
+    
+    - append, extend, insert on lists
+    - update on dicts
+    - update, add on sets
+    """
+    
+    _value = None
+    
+    # list methods
+    _extend = List()
+    
+    def append(self, obj):
+        self._extend.append(obj)
+    
+    def extend(self, other):
+        self._extend.extend(other)
+    
+    _inserts = List()
+    def insert(self, index, other):
+        if not isinstance(index, int):
+            raise TypeError("An integer is required")
+        self._inserts.append((index, other))
+    
+    # dict methods
+    # update is used for both dict and set
+    _update = Any()
+    def update(self, other):
+        if self._update is None:
+            if isinstance(other, dict):
+                self._update = {}
+            else:
+                self._update = set()
+        self._update.update(other)
+    
+    # set methods
+    def add(self, obj):
+        self.update({obj})
+    
+    def get_value(self, initial):
+        """construct the value from the initial one
+        
+        after applying any insert / extend / update changes
+        """
+        if self._value is not None:
+            return self._value
+        value = copy.deepcopy(initial)
+        if isinstance(value, list):
+            for idx, obj in self._inserts:
+                value.insert(idx, obj)
+            value.extend(self._extend)
+        elif isinstance(value, dict):
+            if self._update:
+                value.update(self._update)
+        elif isinstance(value, set):
+            if self._update:
+                value.update(self._update)
+        self._value = value
+        return value
+    
+    def to_dict(self):
+        """return JSONable dict form of my data
+        
+        Currently update as dict or set, extend as list, and inserts as list of tuples.
+        """
+        d = {}
+        if self._update:
+            d['update'] = self._update
+        if self._extend:
+            d['extend'] = self._extend
+        elif self._inserts:
+            d['inserts'] = self._inserts
+        return d
+
 
 class Config(dict):
     """An attribute based dict that can do smart merges."""
@@ -125,6 +204,14 b' class Config(dict):'
             return False
 
     def __contains__(self, key):
+        # allow nested contains of the form `Section.key in config`
+        if '.' in key:
+            first, remainder = key.split('.', 1)
+            if first not in self:
+                return False
+            return remainder in self[first]
+        
+        # we always have Sections
         if self._is_section_key(key):
             return True
         else:
@@ -170,7 +257,14 b' class Config(dict):'
                 dict.__setitem__(self, key, c)
                 return c
         else:
+            try:
             return dict.__getitem__(self, key)
+            except KeyError:
+                # undefined
+                v = LazyConfigValue()
+                dict.__setitem__(self, key, v)
+                return v
+            
 
     def __setitem__(self, key, value):
         if self._is_section_key(key):