apdavison · Feb 13, 2020 · Jan 28, 2020 · Feb 13, 2020
diff --git a/quantities/quantity.py b/quantities/quantity.py
@@ -13,6 +13,10 @@
 from .registry import unit_registry
 from .decorators import with_doc
 
+PREFERRED = []  # List of preferred quanitities for each symbol,
+                # e.g. PREFERRED = [pq.mV, pq.pA, pq.UnitQuantity('femtocoulomb', 1e-15*pq.C, 'fC')]
+                # Intended to be overwritten in down-stream packages
+
 if sys.version.startswith('3'):
     unicode = str
 
@@ -195,10 +199,17 @@ def units(self, units):
         mag *= cf
         self._dimensionality = to_u.dimensionality
 
-    def rescale(self, units):
+    def rescale(self, units=None):
         """
-        Return a copy of the quantity converted to the specified units
+        Return a copy of the quantity converted to the specified units.
+        If `units` is `None`, an attempt will be made to rescale the quantity
+        to preferred units (see `rescale_preferred`).  
         """
+        if units is None:
+            try:
+                return self.rescale_preferred()
+            except Exception as e:
+                raise Exception('No argument passed to `.rescale` and %s' % e)
         to_dims = validate_dimensionality(units)
         if self.dimensionality == to_dims:
             return self.astype(self.dtype)
@@ -212,7 +223,27 @@ def rescale(self, units):
                 %(from_u._dimensionality, to_u._dimensionality)
             )
         return Quantity(cf*self.magnitude, to_u)
-
+
+    def rescale_preferred(self):
+        """
+        Return a copy of the quantity converted to the preferred units and scale.
+        These will be identified from among the compatible units specified in the
+        list PREFERRED in this module. For example, a voltage quantity might be
+        converted to `mV`:
+        ```
+        import quantities as pq
+        pq.quantity.PREFERRED = [pq.mV, pq.pA]
+        old = 3.1415 * pq.V
+        new = old.rescale_preferred() # `new` will be 3141.5 mV.
+        ```
+        """
+        units_str = str(self.simplified.dimensionality)
+        for preferred in PREFERRED:
+            if units_str == str(preferred.simplified.dimensionality):
+                return self.rescale(preferred)
+        raise Exception(("Preferred units for '%s' (or equivalent) not specified in "
+                        "quantites.quantity.PREFERRED." % self.dimensionality))
+
     @with_doc(np.ndarray.astype)
     def astype(self, dtype=None, **kwargs):
         '''Scalars are returned as scalar Quantity arrays.'''

diff --git a/quantities/tests/test_conversion.py b/quantities/tests/test_conversion.py
@@ -1,6 +1,8 @@
 # -*- coding: utf-8 -*-
 
+import unittest
 from .. import units as pq
+from .. import quantity
 from .common import TestCase
 
 
@@ -15,6 +17,44 @@ def test_inplace_conversion(self):
     def test_rescale(self):
         for u in ('ft', 'feet', pq.ft):
             self.assertQuantityEqual((10*pq.m).rescale(u), 32.80839895 * pq.ft)
+
+    def test_rescale_preferred(self):
+        quantity.PREFERRED = [pq.mV, pq.pA]
+        q = 10*pq.V
+        self.assertQuantityEqual(q.rescale_preferred(), q.rescale(pq.mV))
+        q = 5*pq.A
+        self.assertQuantityEqual(q.rescale_preferred(), q.rescale(pq.pA))
+        quantity.PREFERRED = []
+
+    def test_rescale_preferred_failure(self):
+        quantity.PREFERRED = [pq.pA]
+        q = 10*pq.V
+        try:
+            self.assertQuantityEqual(q.rescale_preferred(), q.rescale(pq.mV))
+        except:
+            self.assertTrue(True)
+        else:
+            self.assertTrue(False)
+        quantity.PREFERRED = []
+
+    def test_rescale_noargs(self):
+        quantity.PREFERRED = [pq.mV, pq.pA]
+        q = 10*pq.V
+        self.assertQuantityEqual(q.rescale(), q.rescale(pq.mV))
+        q = 5*pq.A
+        self.assertQuantityEqual(q.rescale(), q.rescale(pq.pA))
+        quantity.PREFERRED = []
+
+    def test_rescale_noargs_failure(self):
+        quantity.PREFERRED = [pq.pA]
+        q = 10*pq.V
+        try:
+            self.assertQuantityEqual(q.rescale_preferred(), q.rescale(pq.mV))
+        except:
+            self.assertTrue(True)
+        else:
+            self.assertTrue(False)
+        quantity.PREFERRED = []
 
     def test_compound_reduction(self):
         pc_per_cc = pq.CompoundUnit("pc/cm**3")