BF, TEST: Fix handling of nReversals in data.StairHandler

`StairHandler.nReversals` should never be `None`, but be set to a
value equal to or greater than the length of the `stepSizes` parameter.
Otherwise, we will finish prematurely during `calculateNextIntensity()`.

This closes #1342.

psychopy/data.py

@@ -2936,11 +2936,12 @@ def __init__(self,
         self._variableStep = True if len(self.stepSizes) > 1 else False
         self.stepSizeCurrent = self.stepSizes[0]
 
-        if nReversals is not None and len(self.stepSizes) > nReversals:
-            logging.warn(
-                "Increasing number of minimum required reversals to "
-                "the number of step sizes (%i)." % len(self.stepSizes)
-            )
+        if nReversals is None:
+            self.nReversals = len(self.stepSizes)
+        elif len(self.stepSizes) > nReversals:
+            msg = ('Increasing number of minimum required reversals to the '
+                   'number of step sizes, (%i).' % len(self.stepSizes))
+            logging.warn(msg)
             self.nReversals = len(self.stepSizes)
         else:
             self.nReversals = nReversals

psychopy/tests/test_data/test_StairHandlers.py

@@ -73,8 +73,8 @@ def checkSimulationResults(self):
             # The first trial can never be a reversal.
             assert stairs.nReversals <= len(stairs.data) - 1
 
-            assert stairs.nReversals == len(stairs.reversalPoints)
-            assert stairs.nReversals == len(stairs.reversalIntensities)
+            assert len(stairs.reversalPoints) >= stairs.nReversals
+            assert len(stairs.reversalIntensities) >= stairs.nReversals
 
         if stairs.reversalPoints:
             assert stairs.reversalIntensities
@@ -307,6 +307,21 @@ def test_StairHandlerLinearScalarStepSizeReveralsNone(self):
         self.simulate()
         self.checkSimulationResults()
 
+    def test_nReversals(self):
+        start_val = 1
+        step_sizes = range(5)
+
+        staircase = data.StairHandler(startVal=start_val, stepSizes=step_sizes,
+                                      nReversals=None)
+        assert staircase.nReversals == len(step_sizes)
+
+        staircase = data.StairHandler(startVal=start_val, stepSizes=step_sizes,
+                                      nReversals=1)
+        assert staircase.nReversals == len(step_sizes)
+
+        staircase = data.StairHandler(startVal=start_val, stepSizes=step_sizes,
+                                      nReversals=10)
+        assert staircase.nReversals == 10
 
 class TestQuestHandler(_BaseTestStairHandler):
     """
@@ -330,6 +345,8 @@ def test_QuestHandler(self):
             minVal=minVal, maxVal=maxVal
         )
 
+        self.stairs.nReversals = None
+
         self.responses = makeBasicResponseCycles(
             cycles=3, nCorrect=2, nIncorrect=2, length=10
         )