python - 为什么 numpy 不允许数组乘以标量？

Question

我假设 @ 是 dot 方法的简写。是什么促使设计决定阻止形状为 () 的数组乘法？

In [6]: a = np.ones((2,1))

In [7]: a.dot(1)
Out[7]:
array([[ 1.],
       [ 1.]])

In [8]: a @ 1
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-8-398cff4c0ec9> in <module>()
----> 1 a @ 1

ValueError: Scalar operands are not allowed, use '*' instead

Answer 1

@ 是 matmul 的中缀运算符，而不是 dot(请注意，这两个函数对于高维数组(高于 2D)并不等价)。

文档中没有明确说明拒绝标量作为操作数的原因，但动机似乎可能源于 PEP 0465最初提议引入 @ 作为 Python 3.5 的中缀运算符。来自“语义”部分:

0d (scalar) inputs raise an error. Scalar * matrix multiplication is a mathematically and algorithmically distinct operation from matrix @ matrix multiplication, and is already covered by the elementwise * operator. Allowing scalar @ matrix would thus both require an unnecessary special case, and violate TOOWTDI ["There's Only One Way To Do It"].