docs: def get_uri(expnum, ccd=None,
              version='p', ext=FITS_EXT,
              subdir=None, prefix=None):
      """
      Build the uri for an OSSOS image stored in the dbimages
      containerNode.
  
      :rtype : basestring
      expnum: CFHT exposure number
      ccd: CCD in the mosaic [0-35]
      version: one of p,s,o etc.
      dbimages: dbimages containerNode.
      @type subdir: str
      @param expnum: int
      @param ccd:
      @param version:
      @param ext:
      @param subdir:
      @param prefix:
      """
      if subdir is None:
          subdir = str(expnum)
      if prefix is None:
          prefix = ''
      uri = os.path.join(DBIMAGES, subdir)
  
      if ext is None:
          ext = ''
      elif len(ext) > 0 and ext[0] != '.':
          ext = '.' + ext
  
      if version is None:
          version = ''
  
      if ccd is None:
          uri = os.path.join(uri,
                             '%s%s%s%s' % (prefix, str(expnum),
                                           version,
                                           ext))
      else:
          ccd = str(ccd).zfill(2)
          uri = os.path.join(uri,
                             'ccd{}'.format(ccd),
                             '%s%s%s%s%s' % (prefix, str(expnum),
                                             version,
                                             ccd,
                                             ext))
  
      return uri