Improve error message for empty models (#2293)

* Improve error message for empty models

* Fix model.ndim

Author: aseyboldt

pymc3/model.py

@@ -425,7 +425,7 @@ def dict_to_array(self):
 
     @property
     def ndim(self):
-        return self.dict_to_array(self.test_point).shape[0]
+        return sum(var.dsize for var in self.free_RVs)
 
     @property
     @memoize

pymc3/sampling.py

@@ -228,6 +228,9 @@ def sample(draws=500, step=None, init='auto', n_init=200000, start=None,
             raise ValueError("Specify only one of step_kwargs and nuts_kwargs")
         step_kwargs = {'nuts': nuts_kwargs}
 
+    if model.ndim == 0:
+        raise ValueError('The model does not contain any free variables.')
+
     if step is None and init is not None and pm.model.all_continuous(model.vars):
         # By default, use NUTS sampler
         pm._log.info('Auto-assigning NUTS sampler...')

pymc3/tests/test_sampling.py

@@ -117,10 +117,18 @@ def test_sample_tune_len(self):
             assert len(trace) == 100
 
 
+def test_empty_model():
+    with pm.Model():
+        pm.Normal('a', observed=1)
+        with pytest.raises(ValueError) as error:
+            pm.sample()
+        error.match('any free variables')
+
+
 class TestSoftUpdate(SeededTest):
     def setup_method(self):
         super(TestSoftUpdate, self).setup_method()
-        
+
     def test_soft_update_all_present(self):
         start = {'a': 1, 'b': 2}
         test_point = {'a': 3, 'b': 4}