Merge pull request #319 from davidhassell/dask-bool

dask: `Data.__bool__`

cf/data/data.py

@@ -908,23 +908,26 @@ def __bool__(self):
 
         x.__bool__() <==> bool(x)
 
-        **Examples:**
+        **Performance**
+
+        `__bool__` causes all delayed operations to be computed.
+
+        **Examples**
 
-        >>> bool(Data(1))
+        >>> bool(cf.Data(1.5))
         True
-        >>> bool(Data([[False]]))
+        >>> bool(cf.Data([[False]]))
         False
-        >>> bool(Data([1, 2]))
-        ValueError: The truth value of Data with more than one element is ambiguous. Use d.any() or d.all()
 
         """
-        if self._size == 1:
-            return bool(self.array)
+        size = self.size
+        if size != 1:
+            raise ValueError(
+                f"The truth value of a {self.__class__.__name__} with {size} "
+                "elements is ambiguous. Use d.any() or d.all()"
+            )
 
-        raise ValueError(
-            "The truth value of Data with more than one element is "
-            "ambiguous. Use d.any() or d.all()"
-        )
+        return bool(self.array)
 
     def __repr__(self):
         """Called by the `repr` built-in function.

cf/test/test_Data.py

@@ -3776,6 +3776,21 @@ def test_Data_zeros(self):
             self.assertEqual(d.dtype, dtype_out)
             self.assertTrue((d.array == np.zeros(shape, dtype=dtype_in)).all())
 
+    def test_Data__bool__(self):
+        for x in (1, 1.5, True, "x"):
+            self.assertTrue(bool(cf.Data(x)))
+            self.assertTrue(bool(cf.Data([[x]])))
+
+        for x in (0, 0.0, False, ""):
+            self.assertFalse(bool(cf.Data(x)))
+            self.assertFalse(bool(cf.Data([[x]])))
+
+        with self.assertRaises(ValueError):
+            bool(cf.Data([]))
+
+        with self.assertRaises(ValueError):
+            bool(cf.Data([1, 2]))
+
 
 if __name__ == "__main__":
     print("Run date:", datetime.datetime.now())