move scalar_weight functionality to DomainTuple

nifty5/domain_tuple.py

@@ -127,6 +127,34 @@ class DomainTuple(object):
         """
         return self._size
 
+    def scalar_weight(self, spaces=None):
+        """Returns the uniform volume element for a sub-domain of `self`.
+
+        Parameters
+        ----------
+        spaces : int, tuple of int or None
+            Indices of the sub-domains to be considered.
+            If `None`, the entire domain is used.
+
+        Returns
+        -------
+        float or None
+            If the requested sub-domain has a uniform volume element, it is
+            returned. Otherwise, `None` is returned.
+        """
+        if np.isscalar(spaces):
+            return self._dom[spaces].scalar_dvol
+
+        if spaces is None:
+            spaces = range(len(self._dom))
+        res = 1.
+        for i in spaces:
+            tmp = self._dom[i].scalar_dvol
+            if tmp is None:
+                return None
+            res *= tmp
+        return res
+
     def total_volume(self, spaces=None):
         """Returns the total volume of `self` or of a subspace of it.
 

nifty5/field.py

@@ -246,18 +246,7 @@ class Field(object):
             If the requested sub-domain has a uniform volume element, it is
             returned. Otherwise, `None` is returned.
         """
-        if np.isscalar(spaces):
-            return self._domain[spaces].scalar_dvol
-
-        if spaces is None:
-            spaces = range(len(self._domain))
-        res = 1.
-        for i in spaces:
-            tmp = self._domain[i].scalar_dvol
-            if tmp is None:
-                return None
-            res *= tmp
-        return res
+        return self._domain.scalar_weight(spaces)
 
     def total_volume(self, spaces=None):
         """Returns the total volume of the field's domain or of a subspace of it.