python - 为什么这个 Numpy 函数链中需要命名表达式？

Question

这段代码:

a = np.arange(12, 0, -1).reshape(3, 4)
(a := a.flatten()).sort()
print(a)

产生

[ 1  2  3  4  5  6  7  8  9 10 11 12]

正如预期的那样。没有命名表达式:

a = np.arange(12, 0, -1).reshape(3, 4)
a = a.flatten().sort()
print(a)

我得到无。这是为什么？

Answer 1

引用自PEP 572 – Assignment Expressions

Syntax and semantics

In most contexts where arbitrary Python expressions can be used, a named expression can appear. This is of the form NAME := expr where expr is any valid Python expression other than an unparenthesized tuple, and NAME is an identifier.

The value of such a named expression is the same as the incorporated expression, with the additional side-effect that the target is assigned that value:

所以这段代码，

(a := a.flatten()).sort()
print(a)

大致相当于

a = a.flatten()
a.sort()
print(a)

这意味着您没有将 sort 的结果分配到任何地方。您让 a.sort() 完成并稍后打印 a(这将产生预期的输出)

记住sort是一个就地操作，它返回None，所以在你的第二个代码中你分配了的返回值>排序(即无)。

a = a.flatten().sort()