Fixing subtle bug in expanduser(expandvars(path)) on Windows.

On Windows, some paths can have the string $\ in them:

\\server\users$\%username%.

The "$" means it is a hidden share. Unfortunately, os.path.expandvars
turns that into:

\\server\users\%username% (it deletes the '$').

I have implemented a simple function (expand_path) that calls
os.expanduser and os.expandvars in a way that preserves th '$' on 
Windows in this case.

IPython/kernel/clusterdir.py

@@ -29,6 +29,7 b' from IPython.core.application import Application'
 from IPython.core.component import Component
 from IPython.config.loader import ArgParseConfigLoader, NoConfigDefault
 from IPython.utils.traitlets import Unicode, Bool
+from IPython.utils import genutils
 
 #-----------------------------------------------------------------------------
 # Imports
@@ -216,9 +217,9 b' class ClusterDir(Component):'
         ----------
         cluster_dir : unicode or str
             The path of the cluster directory.  This is expanded using
-            :func:`os.path.expandvars` and :func:`os.path.expanduser`.
+            :func:`IPython.utils.genutils.expand_path`.
         """
-        cluster_dir = os.path.expandvars(os.path.expanduser(cluster_dir))
+        cluster_dir = genutils.expand_path(cluster_dir)
         if not os.path.isdir(cluster_dir):
             raise ClusterDirError('Cluster directory not found: %s' % cluster_dir)
         return ClusterDir(cluster_dir)
@@ -324,7 +325,7 b' class ApplicationWithClusterDir(Application):'
             cluster_dir = self.command_line_config.Global.cluster_dir
         except AttributeError:
             cluster_dir = self.default_config.Global.cluster_dir
-        cluster_dir = os.path.expandvars(os.path.expanduser(cluster_dir))
+        cluster_dir = genutils.expand_path(cluster_dir)
         try:
             self.cluster_dir_obj = ClusterDir.find_cluster_dir(cluster_dir)
         except ClusterDirError:

IPython/utils/genutils.py

@@ -559,9 +559,7 b' def filefind(filename, path_dirs=None):'
         path_dirs = (path_dirs,)
     for path in path_dirs:
         if path == '.': path = os.getcwd()
-        testname = os.path.expandvars(
-                       os.path.expanduser(
-                           os.path.join(path, filename)))
+        testname = expand_path(os.path.join(path, filename))
         if os.path.isfile(testname):
             return os.path.abspath(testname)
     raise IOError("File does not exist in any "
@@ -1740,7 +1738,7 b' def extract_vars_above(*names):'
     callerNS = sys._getframe(2).f_locals
     return dict((k,callerNS[k]) for k in names)
 
-def shexp(s):
+def expand_path(s):
     """Expand $VARS and ~names in a string, like a shell
 
     :Examples:
@@ -1750,8 +1748,17 b' def shexp(s):'
        In [3]: shexp('variable FOO is $FOO')
        Out[3]: 'variable FOO is test'
     """
-    return os.path.expandvars(os.path.expanduser(s))
-    
+    # This is a pretty subtle hack. When expand user is given a UNC path
+    # on Windows (\\server\share$\%username%), os.path.expandvars, removes
+    # the $ to get (\\server\share\%username%). I think it considered $
+    # alone an empty var. But, we need the $ to remains there (it indicates
+    # a hidden share).
+    if os.name=='nt':
+        s.replace('$\\', 'IPYTHON_TEMP')
+    s2 = os.path.expandvars(os.path.expanduser(s))
+    if os.name=='nt':
+        s2.replace('IPYTHON_TEMP', '$\\')
+    return s2
 
 def list_strings(arg):
     """Always return a list of strings, given a string or list of strings