Report impossible constraints in SIMULATE.

Should make this mistake a little easier to understand than a
divide-by-zero error.

Author: riastradh-probcomp

src/bqlfn.py

@@ -449,6 +449,9 @@ def simulate(generator_id, metamodel, n):
     loglikelihoods = map(loglikelihood, generator_ids, metamodels)
     likelihoods = map(math.exp, loglikelihoods)
     total_likelihood = sum(likelihoods)
+    if total_likelihood == 0:
+        # XXX Show the constraints with symbolic names.
+        raise BQLError(bdb, 'Impossible constraints: %r' % (constraints,))
     probabilities = [likelihood/total_likelihood for likelihood in likelihoods]
     countses = bdb.np_prng.multinomial(numpredictions, probabilities, size=1)
     counts = countses[0]

tests/test_codebook.py

@@ -20,6 +20,8 @@
 import crosscat.LocalEngine
 
 import bayeslite
+
+from bayeslite.exception import BQLError
 from bayeslite.metamodels.crosscat import CrosscatMetamodel
 
 
@@ -80,7 +82,7 @@ def test_codebook_value_map():
                 ('jackie', 18, 'LA'), ('rocker', 22, 'DC')
         ''')
         bdb.execute('ANALYZE dummy_cc FOR 20 ITERATIONS WAIT')
-        with pytest.raises(KeyError):
+        with pytest.raises(BQLError):
             bdb.execute('SIMULATE age FROM dummy_pop GIVEN city = LA LIMIT 5')
 
 def test_empty_codebook():