Enforce Astropy compliant unit string

The Schema.addField method now first attempts to construct
an astropy.units.Unit object from the provided unit string.
This will raise a ValueError if the string is non-compliant.
The optional keyword argument parse_strict from
astropy.units.Unit is also added to addField.
It has three levels: 'raise' (default), 'warn' and 'silent'.
The latter causes any unit string to be accepted.
Also add Schema.checkUnits to allow a full Shema to be validated.

python/lsst/afw/table/Base.i

@@ -117,6 +117,7 @@ template <> struct NumpyTraits<lsst::afw::geom::Angle> : public NumpyTraits<doub
 
 %pythoncode %{
 from . import _syntax
+import astropy.units
 %}
 
 %include "lsst/afw/table/misc.h"
@@ -321,7 +322,13 @@ def find(self, k):
              pass
     raise KeyError("Field '%s' not found in Schema." % k)
 
-def addField(self, field, type=None, doc="", units="", size=None, doReplace=False):
+def checkUnits(self, parse_strict='raise'):
+    for schemaItem in self.asList():
+        astropy.units.Unit(schemaItem.getField().getUnits(), parse_strict=parse_strict)
+
+def addField(self, field, type=None, doc="", units="", size=None, doReplace=False, parse_strict='raise'):
+    # Check for astropy compatible unit string
+    astropy.units.Unit(units, parse_strict=parse_strict)
     if type is None:
         try:
             prefix, suffix = __builtins__['type'](field).__name__.split("_")

tests/testSchema.py

@@ -106,6 +106,52 @@ def testKey(name, key):
         keys2 = map(lambda x: x.key, schema4.asList())
         self.assertEqual(keys, keys2)
 
+    def testUnits(self):
+        schema = lsst.afw.table.Schema();
+	# first insert some valid units
+        try:
+            schema.addField("a", type="I", units="pixel")
+        except ValueError:
+            self.fail("addField raised ValueError unexpectedly!")
+        try:
+            schema.addField("b", type="I", units="m2")
+        except ValueError:
+            self.fail("addField raised ValueError unexpectedly!")
+        try:
+            schema.addField("c", type="I", units="electron / adu")
+        except ValueError:
+            self.fail("addField raised ValueError unexpectedly!")
+        try:
+            schema.addField("d", type="I", units="kg m s^(-2)")
+        except ValueError:
+            self.fail("addField raised ValueError unexpectedly!")
+        try:
+            schema.addField("e", type="I", units="GHz / Mpc")
+        except ValueError:
+            self.fail("addField raised ValueError unexpectedly!")
+        try:
+            schema.addField("f", type="Angle", units="deg")
+        except ValueError:
+            self.fail("addField raised ValueError unexpectedly!")
+        try:
+            schema.addField("g", type="Angle", units="rad")
+        except ValueError:
+            self.fail("addField raised ValueError unexpectedly!")
+        try:
+            schema.checkUnits()
+        except ValueError:
+            self.fail("checkUnits raised ValueError unexpectedly!")
+	# now try inserting invalid units
+        self.assertRaises(ValueError, schema.addField, "a", type="I", units="camel")
+        self.assertRaises(ValueError, schema.addField, "b", type="I", units="pixels^2^2")
+	# add invalid units in silent mode, should work fine
+        try:
+            schema.addField("h", type="I", units="lala", parse_strict='silent')
+        except ValueError:
+            self.fail("addField raised ValueError unexpectedly!")
+	# Now this check should raise because there is an invalid unit
+	self.assertRaises(ValueError, schema.checkUnits)
+
     def testInspection(self):
         schema = lsst.afw.table.Schema()
         keys = []