Allow to reshape unknown tensors. (keras-team#7960)

* Allow to reshape unknown tensors.

* Add test for reshaping unknown shapes.

keras/layers/core.py

@@ -372,24 +372,17 @@ def _fix_unknown_dimension(self, input_shape, output_shape):
         return tuple(output_shape)
 
     def compute_output_shape(self, input_shape):
-        return (input_shape[0],) + self._fix_unknown_dimension(
-            input_shape[1:], self.target_shape)
+        if None in input_shape[1:]:
+            # input shape (partially) unknown? replace -1's with None's
+            return ((input_shape[0],) +
+                    tuple(s if s != -1 else None for s in self.target_shape))
+        else:
+            # input shape known? then we can compute the output shape
+            return (input_shape[0],) + self._fix_unknown_dimension(
+                input_shape[1:], self.target_shape)
 
     def call(self, inputs):
-        # In case the target shape is not fully defined,
-        # we need access to the shape of `inputs`.
-        # solution: rely on `K.int_shape`.
-        target_shape = self.target_shape
-        if -1 in target_shape:
-            # Target shape not fully defined.
-            input_shape = None
-            try:
-                input_shape = K.int_shape(inputs)
-            except TypeError:
-                pass
-            if input_shape is not None:
-                target_shape = self.compute_output_shape(input_shape)[1:]
-        return K.reshape(inputs, (-1,) + target_shape)
+        return K.reshape(inputs, (K.shape(inputs)[0],) + self.target_shape)
 
     def get_config(self):
         config = {'target_shape': self.target_shape}

tests/keras/layers/core_test.py

@@ -79,6 +79,10 @@ def test_reshape():
                kwargs={'target_shape': (1, -1)},
                input_shape=(3, 2, 4))
 
+    layer_test(layers.Reshape,
+               kwargs={'target_shape': (-1, 1)},
+               input_shape=(None, None, 4))
+
 
 @keras_test
 def test_permute():