allow _FillValue to be set using fill_value createVariable kwarg for vlen string vars

Changelog

@@ -11,6 +11,7 @@
    `'standard'` or `'gregorian'`. If `None` (the default), check that the calendar
    attribute is present on each variable and values are unique across files raising
    a `ValueError` otherwise.
+ * Allow _FillValue to be set for vlen string variables (issue #730).
 
  version 1.3.0 (tag v1.3.0rel)
 ==============================

netCDF4/_netCDF4.pyx

@@ -3469,14 +3469,16 @@ behavior is similar to Fortran or Matlab, but different than numpy.
                         if grp.data_model != 'NETCDF4': grp._enddef()
                         _ensure_nc_success(ierr)
                 else:
-                    # cast fill_value to type of variable.
-                    # also make sure it is written in native byte order
-                    # (the same as the data)
-                    if self._isprimitive or self._isenum:
-                        fillval = numpy.array(fill_value, self.dtype)
-                        if not fillval.dtype.isnative: fillval.byteswap(True)
-                        _set_att(self._grp, self._varid, '_FillValue',\
-                                 fillval, xtype=xtype)
+                    if self._isprimitive or self._isenum or \
+                       (self._isvlen and self.dtype == str):
+                        if self._isvlen and self.dtype == str:
+                            _set_att(self._grp, self._varid, '_FillValue',\
+                               _tostr(fill_value), xtype=xtype, force_ncstring=True)
+                        else:
+                            fillval = numpy.array(fill_value, self.dtype)
+                            if not fillval.dtype.isnative: fillval.byteswap(True)
+                            _set_att(self._grp, self._varid, '_FillValue',\
+                                     fillval, xtype=xtype)
                     else:
                         raise AttributeError("cannot set _FillValue attribute for VLEN or compound variable")
             if least_significant_digit is not None:

test/tst_masked5.py

@@ -23,9 +23,12 @@ def setUp(self):
         f = Dataset(self.testfile, 'w')
         d = f.createDimension('x',6)
         v = f.createVariable('v', "i2", 'x')
+        # issue 730: set fill_value for vlen str vars
+        v2 = f.createVariable('v2',str,'x',fill_value=u'<missing>')
 
         v.missing_value = self.missing_values
         v[:] = self.v
+        v2[0]='first'
 
         f.close()
 
@@ -41,13 +44,20 @@ def test_scaled(self):
 
         f = Dataset(self.testfile)
         v = f.variables["v"]
+        v2 = f.variables["v2"]
         self.assertTrue(isinstance(v[:], ma.core.MaskedArray))
         assert_array_equal(v[:], self.v_ma)
         assert_array_equal(v[2],self.v[2]) # issue #624.
         v.set_auto_mask(False)
         self.assertTrue(isinstance(v[:], np.ndarray))
         assert_array_equal(v[:], self.v)
 
+        # issue 730
+        assert (v2[0]=='first')
+        print v2[1]
+        assert (v2[1]==u'<missing>')
+
+
         f.close()