New issue 114 by dubn...@gmail.com: MultiOpener doesn't has ability to set  
writefs [patch]
http://code.google.com/p/pyfilesystem/issues/detail?id=114

To create writable milti://... filesystems, the following patch should be  
applied to fs/opener.py:

      @classmethod
      def get_fs(cls, registry, fs_name, fs_name_params, fs_path, writeable,  
create_dir):

          from fs.multifs import MultiFS
          from ConfigParser import ConfigParser
          cfg = ConfigParser()

          if '#' in fs_path:
              path, section = fs_path.split('#', 1)
          else:
              path = fs_path
              section = 'fs'

          cfg.readfp(registry.open(path))

          multi_fs = MultiFS()
          for name, fs_url in cfg.items(section):
-            multi_fs.addfs(name, registry.opendir(fs_url,  
create_dir=create_dir))
+            multi_fs.addfs(name, registry.opendir(fs_url,  
create_dir=create_dir), name=='writefs')
          return multi_fs, ''

Comment #1 on issue 114 by gc...@loowis.durge.org: MultiOpener doesn't has  
ability to set writefs [patch]
http://code.google.com/p/pyfilesystem/issues/detail?id=114

I'm -1 on the suggested patch. MultiFS can only have one writefs, so your  
patch would be constantly switching the writefs to the last-added FS. And  
maybe you don't even want a writefs?! Or maybe you simply don't want to  
have the writefs as the very last line of the ini file.

IMHO a better solution would be to add some kind of extra (optional) syntax  
to the end of the FS URL that you want to be the writefs in the ini file  
used by MultiOpener (I'm not familiar enough with fs.opener to suggest what  
the syntax might be) and then have MultiOpener raise an exception if  
there's more than one writefs specified in the ini file.