rewrite demandload module to be more flexible.

performance not affected.

old demandload module could only import top-level modules like os and
sys.

with rewrite, all of following syntax is emulated perfectly:

import foo, bar                    foo bar
import foo.bar.quux                foo.bar.quux
from foo.bar import quux, blurfl   foo.bar:quux,blurfl

only thing missing is "import foo as bar".

also, namespace clashes between placeholder object and demand loaded
module are no longer possible.

mercurial/demandload.py

@@ -1,15 +1,125 b''
-def demandload(scope, modules):
-    class d:
-        def __getattr__(self, name):
-            mod = self.__dict__["mod"]
-            scope = self.__dict__["scope"]
-            scope[mod] = __import__(mod, scope, scope, [])
-            return getattr(scope[mod], name)
+'''Demand load modules when used, not when imported.'''
+
+__author__ = '''Copyright 2006 Vadim Gelfer <[e-mail]>.
+This software may be used and distributed according to the terms
+of the GNU General Public License, incorporated herein by reference.'''
+
+# this is based on matt's original demandload module.  it is a
+# complete rewrite.  some time, we may need to support syntax of
+# "import foo as bar".
+
+class _importer(object):
+    '''import a module.  it is not imported until needed, and is
+    imported at most once per scope.'''
+
+    def __init__(self, scope, modname, fromlist):
+        '''scope is context (globals() or locals()) in which import
+        should be made.  modname is name of module to import.
+        fromlist is list of modules for "from foo import ..."
+        emulation.'''
+
+        self.scope = scope
+        self.modname = modname
+        self.fromlist = fromlist
+        self.mod = None
+
+    def module(self):
+        '''import the module if needed, and return.'''
+        if self.mod is None:
+            self.mod = __import__(self.modname, self.scope, self.scope,
+                                  self.fromlist)
+            del self.modname, self.fromlist
+        return self.mod
+
+class _replacer(object):
+    '''placeholder for a demand loaded module. demandload puts this in
+    a target scope.  when an attribute of this object is looked up,
+    this object is replaced in the target scope with the actual
+    module.
+
+    we use __getattribute__ to avoid namespace clashes between
+    placeholder object and real module.'''
+
+    def __init__(self, importer, target):
+        self.importer = importer
+        self.target = target
+        # consider case where we do this:
+        #   demandload(globals(), 'foo.bar foo.quux')
+        # foo will already exist in target scope when we get to
+        # foo.quux.  so we remember that we will need to demandload
+        # quux into foo's scope when we really load it.
+        self.later = []
+
+    def module(self):
+        return object.__getattribute__(self, 'importer').module()
+
+    def __getattribute__(self, key):
+        '''look up an attribute in a module and return it. replace the
+        name of the module in the caller\'s dict with the actual
+        module.'''
 
-    for m in modules.split():
-        dl = d()
-        dl.mod = m
-        dl.scope = scope
-        scope[m] = dl
+        module = object.__getattribute__(self, 'module')()
+        target = object.__getattribute__(self, 'target')
+        importer = object.__getattribute__(self, 'importer')
+        later = object.__getattribute__(self, 'later')
+
+        if later:
+            demandload(module.__dict__, ' '.join(later))
+
+        importer.scope[target] = module
+
+        return getattr(module, key)
+
+class _replacer_from(_replacer):
+    '''placeholder for a demand loaded module.  used for "from foo
+    import ..." emulation. semantics of this are different than
+    regular import, so different implementation needed.'''
+
+    def module(self):
+        importer = object.__getattribute__(self, 'importer')
+        target = object.__getattribute__(self, 'target')
+
+        return getattr(importer.module(), target)
+
+def demandload(scope, modules):
+    '''import modules into scope when each is first used.
+
+    scope should be the value of globals() in the module calling this
+    function, or locals() in the calling function.
+
+    modules is a string listing module names, separated by white
+    space.  names are handled like this:
 
+    foo            import foo
+    foo bar        import foo, bar
+    foo.bar        import foo.bar
+    foo:bar        from foo import bar
+    foo:bar,quux   from foo import bar, quux
+    foo.bar:quux   from foo.bar import quux'''
 
+    for mod in modules.split():
+        col = mod.find(':')
+        if col >= 0:
+            fromlist = mod[col+1:].split(',')
+            mod = mod[:col]
+        else:
+            fromlist = []
+        importer = _importer(scope, mod, fromlist)
+        if fromlist:
+            for name in fromlist:
+                scope[name] = _replacer_from(importer, name)
+        else:
+            dot = mod.find('.')
+            if dot >= 0:
+                basemod = mod[:dot]
+                val = scope.get(basemod)
+                # if base module has already been demandload()ed,
+                # remember to load this submodule into its namespace
+                # when needed.
+                if isinstance(val, _replacer):
+                    later = object.__getattribute__(val, 'later')
+                    later.append(mod[dot+1:])
+                    continue
+            else:
+                basemod = mod
+            scope[basemod] = _replacer(importer, basemod)