optimize the error information when the input for while op has a wron… (#19872)

* optimize the error information when the input for while op has a wrong shape test=develop

python/paddle/fluid/layers/control_flow.py

@@ -679,9 +679,11 @@ class While(object):
             raise TypeError("condition should be a variable")
         assert isinstance(cond, Variable)
         if cond.dtype != core.VarDesc.VarType.BOOL:
-            raise TypeError("condition should be a bool variable")
+            raise TypeError("condition should be a boolean variable")
         if reduce(lambda a, b: a * b, cond.shape, 1) != 1:
-            raise TypeError("condition should be a bool scalar")
+            raise TypeError(
+                "condition expected shape as [], but given shape as {0}.".
+                format(list(cond.shape)))
         self.cond_var = cond
         self.is_test = is_test
 

python/paddle/fluid/tests/unittests/test_while_op.py

@@ -96,6 +96,16 @@ class TestWhileOp(unittest.TestCase):
                        fetch_list=[sum_result])
         self.assertAlmostEqual(numpy.sum(d), numpy.sum(outs[0]), delta=0.01)
 
+    def test_exceptions(self):
+        i = layers.zeros(shape=[2], dtype='int64')
+        array_len = layers.fill_constant(shape=[2], dtype='int64', value=1)
+        cond = layers.less_than(x=i, y=array_len)
+        with self.assertRaises(TypeError):
+            layers.While(cond=cond)
+        cond = layers.cast(cond, dtype='float64')
+        with self.assertRaises(TypeError):
+            layers.While(cond=cond)
+
 
 if __name__ == '__main__':
     unittest.main()