rhat now checks that there are two chains. Fixes #533.

Author: MichaelClerx

pints/_diagnostics.py

@@ -210,13 +210,14 @@ def rhat(chains, warm_up=0.0):
             ' [0,1].')
     if chains.shape[0] < 2:
         raise ValueError('Number of chains needs to be 2 or higher')
-    if chains.shape[1] < 1:
-        raise ValueError(
-            'Number of samples per chain after warm-up and chain splitting is '
-            f'{n}. Method needs at least 1 sample per chain.')
 
     # Get number of samples
     n = chains.shape[1]
+    if n < 2:
+        raise ValueError(
+            'Number of samples per chain after warm-up and chain splitting is '
+            f'{n}. Method needs at least 2 samples per chain.')
+
 
     # Exclude warm-up
     chains = chains[:, int(n * warm_up):]

pints/tests/test_diagnostics.py

@@ -137,7 +137,13 @@ def test_bad_rhat_inputs(self):
 
         # Pass only a single chain
         chains = np.empty(shape=(1, 5))
-        self.assertRaisesRegex(ValueError, '2 or higher', pints.rhat, chains)
+        self.assertRaisesRegex(
+            ValueError, '2 or higher', pints.rhat, chains)
+
+        # Pass only a single sample
+        chains = np.empty(shape=(5, 1))
+        self.assertRaisesRegex(
+            ValueError, 'at least 2 samples', pints.rhat, chains)
 
         # Pass bad warm-up arguments
         chains = np.empty(shape=(2, 4))