diff --git a/mercurial/demandimport.py b/mercurial/demandimport.py
new file mode 100644
--- /dev/null
+++ b/mercurial/demandimport.py
@@ -0,0 +1,104 @@
+# demandimport.py - global demand-loading of modules for Mercurial
+#
+# Copyright 2006 Matt Mackall <mpm@selenic.com>
+#
+# This software may be used and distributed according to the terms
+# of the GNU General Public License, incorporated herein by reference.
+
+'''
+demandimport - automatic demandloading of modules
+
+To enable this module, do:
+
+  import demandimport; demandimport.enable()
+
+Imports of the following forms will be demand-loaded:
+
+  import a, b.c
+  import a.b as c
+  from a import b,c # a will be loaded immediately
+
+These imports will not be delayed:
+
+  from a import *
+  b = __import__(a)
+'''
+
+_origimport = __import__
+
+class _demandmod(object):
+    """module demand-loader and proxy"""
+    def __init__(self, name, globals, locals):
+        if '.' in name:
+            head, rest = name.split('.', 1)
+            after = [rest]
+        else:
+            head = name
+            after = []
+        self.__dict__["_data"] = (head, globals, locals, after)
+        self.__dict__["_module"] = None
+    def _extend(self, name):
+        """add to the list of submodules to load"""
+        self._data[3].append(name)
+    def _load(self):
+        if not self._module:
+            head, globals, locals, after = self._data
+            mod = _origimport(head, globals, locals)
+            # load submodules
+            for x in after:
+                hx = x
+                if '.' in x:
+                    hx = x.split('.')[0]
+                if not hasattr(mod, hx):
+                    setattr(mod, hx, _demandmod(x, mod.__dict__, mod.__dict__))
+            # are we in the locals dictionary still?
+            if locals and locals.get(head) == self:
+                locals[head] = mod
+            self.__dict__["_module"] = mod
+    def __repr__(self):
+        return "<unloaded module '%s'>" % self._data[0]
+    def __call__(self, *args, **kwargs):
+        raise TypeError("'unloaded module' object is not callable")
+    def __getattr__(self, attr):
+        self._load()
+        return getattr(self._module, attr)
+    def __setattr__(self, attr, val):
+        self._load()
+        setattr(self._module, attr, val)
+
+def _demandimport(name, globals=None, locals=None, fromlist=None):
+    if not locals or name in ignore or fromlist == ('*',):
+        # these cases we can't really delay
+        return _origimport(name, globals, locals, fromlist)
+    elif not fromlist:
+        # import a [as b]
+        if '.' in name: # a.b
+            base, rest = name.split('.', 1)
+            # if a is already demand-loaded, add b to its submodule list
+            if base in locals:
+                if isinstance(locals[base], _demandmod):
+                    locals[base]._extend(rest)
+                return locals[base]
+        return _demandmod(name, globals, locals)
+    else:
+        # from a import b,c,d
+        mod = _origimport(name, globals, locals)
+        # recurse down the module chain
+        for comp in name.split('.')[1:]:
+            mod = getattr(mod, comp)
+        for x in fromlist:
+            # set requested submodules for demand load
+            if not(hasattr(mod, x)):
+                setattr(mod, x, _demandmod(x, mod.__dict__, mod.__dict__))
+        return mod
+
+ignore = []
+
+def enable():
+    "enable global demand-loading of modules"
+    __builtins__["__import__"] = _demandimport
+
+def disable():
+    "disable global demand-loading of modules"
+    __builtins__["__import__"] = _origimport
+