fix paddle.where torch diff (#38870)

* fix paddle.where torch diff

* update

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

@@ -305,6 +305,36 @@ class TestWhereDygraphAPI(unittest.TestCase):
         b_shape = [2, 2, 1]
         self.__test_where_with_broadcast_dygraph(cond_shape, a_shape, b_shape)
 
+    def test_where_condition(self):
+        data = np.array([[True, False], [False, True]])
+        with program_guard(Program(), Program()):
+            x = fluid.layers.data(name='x', shape=[-1, 2])
+            y = paddle.where(x)
+            self.assertEqual(type(y), tuple)
+            self.assertEqual(len(y), 2)
+            z = fluid.layers.concat(list(y), axis=1)
+            exe = fluid.Executor(fluid.CPUPlace())
+
+            res, = exe.run(feed={'x': data},
+                           fetch_list=[z.name],
+                           return_numpy=False)
+        expect_out = np.array([[0, 0], [1, 1]])
+        self.assertTrue(np.allclose(expect_out, np.array(res)))
+
+        data = np.array([True, True, False])
+        with program_guard(Program(), Program()):
+            x = fluid.layers.data(name='x', shape=[-1])
+            y = paddle.where(x)
+            self.assertEqual(type(y), tuple)
+            self.assertEqual(len(y), 1)
+            z = fluid.layers.concat(list(y), axis=1)
+            exe = fluid.Executor(fluid.CPUPlace())
+            res, = exe.run(feed={'x': data},
+                           fetch_list=[z.name],
+                           return_numpy=False)
+        expect_out = np.array([[0], [1]])
+        self.assertTrue(np.allclose(expect_out, np.array(res)))
+
 
 class TestWhereOpError(unittest.TestCase):
     def test_errors(self):
@@ -326,6 +356,14 @@ class TestWhereOpError(unittest.TestCase):
 
             self.assertRaises(TypeError, test_type)
 
+    def test_value_error(self):
+        with fluid.dygraph.guard():
+            cond_shape = [2, 2, 4]
+            cond_tmp = paddle.rand(cond_shape)
+            cond = cond_tmp < 0.3
+            a = paddle.rand(cond_shape)
+            self.assertRaises(ValueError, paddle.where, cond, a)
+
 
 if __name__ == '__main__':
     unittest.main()

python/paddle/tensor/search.py

@@ -523,23 +523,26 @@ def mode(x, axis=-1, keepdim=False, name=None):
     return values, indices
 
 
-def where(condition, x, y, name=None):
+def where(condition, x=None, y=None, name=None):
     r"""
     Return a tensor of elements selected from either $x$ or $y$, depending on $condition$.
 
+    **Note**:
+        ``paddle.where(condition)`` is identical to ``paddle.nonzero(condition, as_tuple=True)``.
+
     .. math::
 
       out_i =
-      \\begin{cases}
-      x_i, \quad  \\text{if}  \\ condition_i \\  is \\ True \\\\
-      y_i, \quad  \\text{if}  \\ condition_i \\  is \\ False \\\\
-      \\end{cases}
+      \begin{cases}
+      x_i, \quad  \text{if}  \ condition_i \  is \ True \\
+      y_i, \quad  \text{if}  \ condition_i \  is \ False \\
+      \end{cases}
 
 
     Args:
         condition(Tensor): The condition to choose x or y.
-        x(Tensor): x is a Tensor with data type float32, float64, int32, int64.
-        y(Tensor): y is a Tensor with data type float32, float64, int32, int64.
+        x(Tensor, optional): x is a Tensor with data type float32, float64, int32, int64. Either both or neither of x and y should be given.
+        y(Tensor, optional): y is a Tensor with data type float32, float64, int32, int64. Either both or neither of x and y should be given.
 
         name(str, optional): The default value is None. Normally there is no
             need for user to set this property. For more information, please
@@ -559,7 +562,19 @@ def where(condition, x, y, name=None):
 
           print(out)
           #out: [1.0, 1.0, 3.2, 1.2]
+
+          out = paddle.where(x>1)
+          print(out)
+          #out: (Tensor(shape=[2, 1], dtype=int64, place=CPUPlace, stop_gradient=True,
+          #            [[2],
+          #             [3]]),)
     """
+    if x is None and y is None:
+        return nonzero(condition, as_tuple=True)
+
+    if x is None or y is None:
+        raise ValueError("either both or neither of x and y should be given")
+
     if not in_dygraph_mode():
         check_variable_and_dtype(condition, 'condition', ['bool'], 'where')
         check_variable_and_dtype(