only raise errors when converting MKS E&M units to CGS E&M units

unyt/tests/test_unyt_array.py

@@ -17,6 +17,7 @@
 import copy
 from six.moves import cPickle as pickle
 import itertools
+import math
 import numpy as np
 import operator
 import os
@@ -1611,14 +1612,8 @@ def test_electromagnetic():
     assert_equal(u_mks.units.dimensions, dimensions.pressure)
     u_cgs = B_cgs*B_cgs/(8*np.pi)
     assert_equal(u_cgs.units.dimensions, dimensions.pressure)
-    with pytest.raises(UnitConversionError):
-        u_cgs.to(u_mks.units)
-    with pytest.raises(UnitConversionError):
-        u_mks.to(u_cgs.units)
-    with pytest.raises(UnitsNotReducible):
-        u_mks.in_cgs()
-    with pytest.raises(UnitsNotReducible):
-        u_cgs.in_mks()
+    assert_equal(u_cgs.to(u_mks.units), u_mks)
+    assert_equal(u_mks.to(u_cgs.units), u_cgs)
 
     current = 1.0*u.A
     I_cgs = current.in_units("statA")
@@ -1667,6 +1662,10 @@ def test_electromagnetic():
     with pytest.raises(UnitsNotReducible):
         data.in_mks()
 
+    mu_0 = 4.0e-7*math.pi*u.N/u.A**2
+    eps_0 = 8.85418781782e-12*u.m**-3/u.kg*u.s**4*u.A**2
+    assert_almost_equal((1.0/(u.clight**2*mu_0)).in_units(eps_0.units), eps_0)
+
 
 def test_ytarray_coercion():
     a = unyt_array([1, 2, 3], 'cm')

unyt/unit_object.py

@@ -799,9 +799,16 @@ def _check_em_conversion(unit_expr, to_unit_expr=None):
     for unit in unit_expr.atoms():
         if unit.is_Number:
             pass
-        base_unit = _get_em_base_unit(str(unit))
-        if base_unit in em_conversions:
-            raise MKSCGSConversionError(unit_expr)
+        bu = _get_em_base_unit(str(unit))
+        if bu in em_conversions:
+            conv_unit = em_conversions[bu]
+            if to_unit_expr is not None:
+                for ounit in to_unit_expr.atoms():
+                    bou = _get_em_base_unit(str(ounit))
+                    if bou == conv_unit:
+                        raise MKSCGSConversionError(unit_expr)
+            else:
+                raise MKSCGSConversionError(unit_expr)
     return em_map