add get_fill_value Variable method and fill_value='default' option

src/netCDF4/_netCDF4.pyx

@@ -4035,11 +4035,15 @@ behavior is similar to Fortran or Matlab, but different than numpy.
         Ignored if `significant_digts` not specified. If 'BitRound' is used, then
         `significant_digits` is interpreted as binary (not decimal) digits.
 
-        **`fill_value`**:  If specified, the default netCDF `_FillValue` (the
+        **`fill_value`**:  If specified, the default netCDF fill value (the
         value that the variable gets filled with before any data is written to it)
-        is replaced with this value.  If fill_value is set to `False`, then
-        the variable is not pre-filled. The default netCDF fill values can be found
-        in the dictionary `netCDF4.default_fillvals`.
+        is replaced with this value, and the `_FillValue` attribute is set.
+        If fill_value is set to `False`, then the variable is not pre-filled.
+        The default netCDF fill values can be found in the dictionary `netCDF4.default_fillvals`.
+        If not set, the default fill value will be used but no `_FillValue` attribute will be created
+        (this is the default behavior of the netcdf-c library). `Variable.get_fill_value`
+        can be used to retrieve the fill value, even if the `_FillValue` attribute is
+        not set.
 
         **`chunk_cache`**: If specified, sets the chunk cache size for this variable.
         Persists as long as Dataset is open. Use `set_var_chunk_cache` to
@@ -4638,6 +4642,36 @@ behavior is similar to Fortran or Matlab, but different than numpy.
 return the group that this `Variable` is a member of."""
         return self._grp
 
+    def get_fill_value(self):
+        """
+**`get_fill_value(self)`**
+
+return the fill value associated with this `Variable` (None if data is not
+pre-filled). Works even if default fill value was used, and `_FillValue` attribute
+does not exist."""
+        cdef int ierr, no_fill
+        with nogil:
+            ierr = nc_inq_var_fill(self._grpid,self._varid,&no_fill,NULL)
+        _ensure_nc_success(ierr)
+        if no_fill == 1: # no filling for this variable
+            return None
+        else:
+            try:
+                fillval = self._FillValue
+                return fillval
+            except AttributeError:
+                # _FillValue attribute not set, see if we can retrieve _FillValue.
+                # for primitive data types.
+                if self._isprimitive:
+                    #return numpy.array(default_fillvals[self.dtype.str[1:]],self.dtype)
+                    fillval = numpy.empty((),self.dtype)
+                    ierr=nc_inq_var_fill(self._grpid,self._varid,&no_fill,PyArray_DATA(fillval))
+                    _ensure_nc_success(ierr)
+                    return fillval
+                else:
+                    # no default filling for non-primitive data types.
+                    return None
+
     def ncattrs(self):
         """
 **`ncattrs(self)`**

test/test_get_fill_value.py

@@ -0,0 +1,38 @@
+import unittest, os, tempfile
+import netCDF4
+from numpy.testing import assert_array_equal
+import numpy as np
+
+fill_val = np.array(9.9e31)
+
+# test Variable.get_fill_value
+
+class TestGetFillValue(unittest.TestCase):
+    def setUp(self):
+        self.testfile = tempfile.NamedTemporaryFile(suffix='.nc', delete=False).name
+        f = netCDF4.Dataset(self.testfile, 'w')
+        dim = f.createDimension('x',10)
+        for dt in netCDF4.default_fillvals.keys():
+            if not dt.startswith('c'):
+                v = f.createVariable(dt+'_var',dt,dim)
+        v = f.createVariable('float_var',np.float64,dim,fill_value=fill_val)
+        f.close()
+
+    def tearDown(self):
+        os.remove(self.testfile)
+
+    def runTest(self):
+        f = netCDF4.Dataset(self.testfile, "r")
+        # no _FillValue set, test that default fill value returned
+        for dt in netCDF4.default_fillvals.keys():
+            if not dt.startswith('c'):
+                fillval = np.array(netCDF4.default_fillvals[dt])
+                if dt == 'S1': fillval = fillval.astype(dt)
+                v = f[dt+'_var']
+                assert_array_equal(fillval, v.get_fill_value())
+        # _FillValue attribute is set.
+        v = f['float_var']
+        assert_array_equal(fill_val, v.get_fill_value())
+
+if __name__ == '__main__':
+    unittest.main()