Add numpy compatible dot funciton and override __matmul__ operator

python/src/nnabla/_arithmetic_ops.pyx

@@ -209,3 +209,26 @@ cdef object pow(object x, object y, object z):
             return F.r_pow_scalar(y, x)
         else:
             return x ** y
+
+
+cdef object matmul(object x, object y):
+    """
+    Matlix multiplication
+
+    Implements the matmul operator expression ``x @ y``.
+    When both of ``x`` and ``y`` are either :obj:`~nnabla.Variable` or
+    :obj:`~nnabla.NdArray`, :func:`~nnabla.functions.affine`` is
+    internally called.
+
+    Args:
+        x (~nnabla.Variable or ~nnabla.NdArray): Left operand. It must be 2-dimensional.
+        y (~nnabla.Variable or ~nnabla.NdArray): Right operand. It must be 2-dimensional.
+
+    Returns: :class:`~nnabla.Variable` or :class:`~nnabla.NdArray`.
+
+    """
+    import nnabla.functions as F
+    assert x.ndim == 2 and y.ndim == 2, "Both of x and y must be matrices."
+    assert isinstance(x, (NdArray, Variable))
+    assert isinstance(y, (NdArray, Variable))
+    return F.affine(x, y)

python/src/nnabla/_variable.pyx

@@ -1062,6 +1062,9 @@ cdef class Variable:
     def __pow__(x, y, z):
         return AOP.pow(x, y, z)
 
+    def __matmul__(x, y):
+        return AOP.matmul(x, y)
+
     def __iadd__(self, x):
         import nnabla.functions as F
         if isinstance(x, (NdArray, Variable)):

python/src/nnabla/functions.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2017 Sony Corporation. All Rights Reserved.
+# Copyright (c) 2017-2021 Sony Corporation. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -18,6 +18,7 @@
 import nnabla as nn
 import numpy as np
 from .normalization_functions import *
+from .numpy_compat_functions import *
 
 
 def sum(x, axis=None, keepdims=False):

python/src/nnabla/numpy_compat_functions.py

@@ -0,0 +1,53 @@
+# Copyright (c) 2021 Sony Corporation. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import numpy as np
+
+
+def dot(a, b, out=None):
+    '''
+    A compatible operation with ``numpy.dot``.
+
+    Note:
+        Any operation between nnabla's Variable/NdArray and numpy array is not supported.
+
+    Args:
+        a (Variable, NdArray or scalar): Left input array.
+        b (Variable, NdArray or scalar): Right input array.
+        out: Not supported so far.
+
+    Returns:
+        ~nnabla.Variable: N-D array.
+
+    '''
+    import nnabla as nn
+    import nnabla.fucntions as F
+    assert out is None, "The `out` option is not supported."
+
+    def _chk(x):
+        return isinstance(x, (nn.NdArray, nn.Variable))
+
+    if _chk(a) and _chk(b):
+        if a.ndim == 1 and b.ndim == 1:
+            return return F.sum(a * b)
+        if a.ndim == 2 and b.ndim >= 2:
+            return F.affine(a, b)
+        if a.ndim == 0 or b.ndim == 0:
+            return a * b
+        if a.ndim > 2 and b.ndim == 1:
+            h = F.affine(x, F.reshape(y, (-1, 1)), base_axis=x.ndim - 1)
+            return F.reshape(h, h.shape[:-1])
+        raise ValueError(f'Undefined configuration: a.ndim={a.ndim}, b.ndim:{b.ndim}')
+
+    return x * y