Don't expand empty [paths] so later interpolation can do the right thing.

Example:
[paths]
default =
default-push = %(default)s

mercurial/ui.py

@@ -1,224 +1,224
 # ui.py - user interface bits for mercurial
 #
 # Copyright 2005 Matt Mackall <[e-mail]>
 #
 # This software may be used and distributed according to the terms
 # of the GNU General Public License, incorporated herein by reference.
 
 import ConfigParser
 from i18n import gettext as _
 from demandload import *
 demandload(globals(), "os re socket sys util")
 
 class ui(object):
     def __init__(self, verbose=False, debug=False, quiet=False,
                  interactive=True, parentui=None):
         self.overlay = {}
         if parentui is None:
             # this is the parent of all ui children
             self.parentui = None
             self.cdata = ConfigParser.SafeConfigParser()
             self.readconfig(util.rcpath)
 
             self.quiet = self.configbool("ui", "quiet")
             self.verbose = self.configbool("ui", "verbose")
             self.debugflag = self.configbool("ui", "debug")
             self.interactive = self.configbool("ui", "interactive", True)
 
             self.updateopts(verbose, debug, quiet, interactive)
             self.diffcache = None
         else:
             # parentui may point to an ui object which is already a child
             self.parentui = parentui.parentui or parentui
             parent_cdata = self.parentui.cdata
             self.cdata = ConfigParser.SafeConfigParser(parent_cdata.defaults())
             # make interpolation work
             for section in parent_cdata.sections():
                 self.cdata.add_section(section)
                 for name, value in parent_cdata.items(section, raw=True):
                     self.cdata.set(section, name, value)
 
     def __getattr__(self, key):
         return getattr(self.parentui, key)
 
     def updateopts(self, verbose=False, debug=False, quiet=False,
                  interactive=True):
         self.quiet = (self.quiet or quiet) and not verbose and not debug
         self.verbose = (self.verbose or verbose) or debug
         self.debugflag = (self.debugflag or debug)
         self.interactive = (self.interactive and interactive)
 
     def readconfig(self, fn, root=None):
         if isinstance(fn, basestring):
             fn = [fn]
         for f in fn:
             try:
                 self.cdata.read(f)
             except ConfigParser.ParsingError, inst:
                 raise util.Abort(_("Failed to parse %s\n%s") % (f, inst))
         # translate paths relative to root (or home) into absolute paths
         if root is None:
             root = os.path.expanduser('~')
         for name, path in self.configitems("paths"):
-            if path.find("://") == -1 and not os.path.isabs(path):
+            if path and path.find("://") == -1 and not os.path.isabs(path):
                 self.cdata.set("paths", name, os.path.join(root, path))
 
     def setconfig(self, section, name, val):
         self.overlay[(section, name)] = val
 
     def config(self, section, name, default=None):
         if self.overlay.has_key((section, name)):
             return self.overlay[(section, name)]
         if self.cdata.has_option(section, name):
             try:
                 return self.cdata.get(section, name)
             except ConfigParser.InterpolationError, inst:
                 raise util.Abort(_("Error in configuration:\n%s") % inst)
         if self.parentui is None:
             return default
         else:
             return self.parentui.config(section, name, default)
 
     def configbool(self, section, name, default=False):
         if self.overlay.has_key((section, name)):
             return self.overlay[(section, name)]
         if self.cdata.has_option(section, name):
             try:
                 return self.cdata.getboolean(section, name)
             except ConfigParser.InterpolationError, inst:
                 raise util.Abort(_("Error in configuration:\n%s") % inst)
         if self.parentui is None:
             return default
         else:
             return self.parentui.configbool(section, name, default)
 
     def configitems(self, section):
         items = {}
         if self.parentui is not None:
             items = dict(self.parentui.configitems(section))
         if self.cdata.has_section(section):
             try:
                 items.update(dict(self.cdata.items(section)))
             except ConfigParser.InterpolationError, inst:
                 raise util.Abort(_("Error in configuration:\n%s") % inst)
         x = items.items()
         x.sort()
         return x
 
     def walkconfig(self, seen=None):
         if seen is None:
             seen = {}
         for (section, name), value in self.overlay.iteritems():
             yield section, name, value
             seen[section, name] = 1
         for section in self.cdata.sections():
             for name, value in self.cdata.items(section):
                 if (section, name) in seen: continue
                 yield section, name, value.replace('\n', '\\n')
                 seen[section, name] = 1
         if self.parentui is not None:
             for parent in self.parentui.walkconfig(seen):
                 yield parent
 
     def extensions(self):
         return self.configitems("extensions")
 
     def diffopts(self):
         if self.diffcache:
             return self.diffcache
         ret = { 'showfunc' : True, 'ignorews' : False}
         for x in self.configitems("diff"):
             k = x[0].lower()
             v = x[1]
             if v:
                 v = v.lower()
                 if v == 'true':
                     value = True
                 else:
                     value = False
                 ret[k] = value
         self.diffcache = ret
         return ret
 
     def username(self):
         return (os.environ.get("HGUSER") or
                 self.config("ui", "username") or
                 os.environ.get("EMAIL") or
                 (os.environ.get("LOGNAME",
                                 os.environ.get("USERNAME", "unknown"))
                  + '@' + socket.getfqdn()))
 
     def shortuser(self, user):
         """Return a short representation of a user name or email address."""
         if not self.verbose:
             f = user.find('@')
             if f >= 0:
                 user = user[:f]
             f = user.find('<')
             if f >= 0:
                 user = user[f+1:]
         return user
 
     def expandpath(self, loc):
         """Return repository location relative to cwd or from [paths]"""
         if loc.find("://") != -1 or os.path.exists(loc):
             return loc
 
         return self.config("paths", loc, loc)
 
     def write(self, *args):
         for a in args:
             sys.stdout.write(str(a))
 
     def write_err(self, *args):
         if not sys.stdout.closed: sys.stdout.flush()
         for a in args:
             sys.stderr.write(str(a))
 
     def flush(self):
         try:
             sys.stdout.flush()
         finally:
             sys.stderr.flush()
 
     def readline(self):
         return sys.stdin.readline()[:-1]
     def prompt(self, msg, pat, default="y"):
         if not self.interactive: return default
         while 1:
             self.write(msg, " ")
             r = self.readline()
             if re.match(pat, r):
                 return r
             else:
                 self.write(_("unrecognized response\n"))
     def status(self, *msg):
         if not self.quiet: self.write(*msg)
     def warn(self, *msg):
         self.write_err(*msg)
     def note(self, *msg):
         if self.verbose: self.write(*msg)
     def debug(self, *msg):
         if self.debugflag: self.write(*msg)
     def edit(self, text):
         import tempfile
         (fd, name) = tempfile.mkstemp("hg")
         f = os.fdopen(fd, "w")
         f.write(text)
         f.close()
 
         editor = (os.environ.get("HGEDITOR") or
                   self.config("ui", "editor") or
                   os.environ.get("EDITOR", "vi"))
 
         os.environ["HGUSER"] = self.username()
         util.system("%s \"%s\"" % (editor, name),
                     environ={'HGUSER': self.username()},
                     onerr=util.Abort, errprefix=_("edit failed"))
 
         t = open(name).read()
         t = re.sub("(?m)^HG:.*\n", "", t)
 
         os.unlink(name)
 
         return t