Issue #13355: Raise ValueError on random.triangular call with invalid params.

Initial patch by Yuriy Senko.

Lib/random.py

@@ -341,6 +341,16 @@ def triangular(self, low=0.0, high=1.0, mode=None):
         http://en.wikipedia.org/wiki/Triangular_distribution
 
         """
+        # Sanity check. According to the doc low must be less or equal to
+        # high. And mode should be somewhere between these bounds.
+        if low > high:
+            raise ValueError('high cannot be less then low.')
+        if mode is not None and (mode < low or mode > high):
+            raise ValueError('mode must be between low and high.')
+
+        if high == low:
+            return low
+
         u = self.random()
         c = 0.5 if mode is None else (mode - low) / (high - low)
         if u > c:

Lib/test/test_random.py

@@ -46,6 +46,36 @@ def __hash__(self):
         self.assertRaises(TypeError, self.gen.seed, 1, 2, 3, 4)
         self.assertRaises(TypeError, type(self.gen), [])
 
+    def test_triangular(self):
+        # Check that triangular() correctly handles bad input. See issue 13355.
+
+        # mode > high.
+        with self.assertRaises(ValueError):
+            random.triangular(mode=2)
+        with self.assertRaises(ValueError):
+            random.triangular(low=1, high=10, mode=11)
+        with self.assertRaises(ValueError):
+            random.triangular(low=1, high=1, mode=11)
+
+        # mode < low.
+        with self.assertRaises(ValueError):
+            random.triangular(mode=-1)
+        with self.assertRaises(ValueError):
+            random.triangular(low=1, high=10, mode=0)
+        with self.assertRaises(ValueError):
+            random.triangular(low=1, high=1, mode=0)
+
+        # low > high
+        with self.assertRaises(ValueError):
+            random.triangular(low=5, high=2)
+        with self.assertRaises(ValueError):
+            random.triangular(low=5, high=2, mode=1)
+        with self.assertRaises(ValueError):
+           random.triangular(low=-2, high=-5)
+
+        self.assertEqual(random.triangular(low=10, high=10), 10)
+        self.assertEqual(random.triangular(low=10, high=10, mode=10), 10)
+
     def test_choice(self):
         choice = self.gen.choice
         with self.assertRaises(IndexError):
@@ -489,7 +519,7 @@ def test_constant(self):
         for variate, args, expected in [
                 (g.uniform, (10.0, 10.0), 10.0),
                 (g.triangular, (10.0, 10.0), 10.0),
-                #(g.triangular, (10.0, 10.0, 10.0), 10.0),
+                (g.triangular, (10.0, 10.0, 10.0), 10.0),
                 (g.expovariate, (float('inf'),), 0.0),
                 (g.vonmisesvariate, (3.0, float('inf')), 3.0),
                 (g.gauss, (10.0, 0.0), 10.0),

Misc/ACKS

@@ -1088,6 +1088,7 @@ Nick Seidenman
 Žiga Seilnacht
 Yury Selivanov
 Fred Sells
+Yuriy Senko
 Jiwon Seo
 Iñigo Serna
 Joakim Sernbrant

Misc/NEWS

@@ -23,6 +23,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #13355: Raise ValueError on random.triangular call with invalid params.
+  Initial patch by Yuriy Senko.
+
 - Issue #16658: add missing return to HTTPConnection.send()
   Patch by Jeff Knupp.