“ _”在这里做什么？

Question

我使用了生成器表达式，然后使用了列表推导，'_'在这里做什么？

x = (i for i in [1, 2, 3])
[_ for i in x]

它给出了这样的输出

[]

[[], [], []]

[[[], [], []], [[], [], []], [[], [], []]]

[[[[], [], []], [[], [], []], [[], [], []]],
 [[[], [], []], [[], [], []], [[], [], []]],
 [[[], [], []], [[], [], []], [[], [], []]]]

多次运行两行

Answer 1

如果您正在Python REPL上运行代码（我想是这样），那么下划线是一个内置变量，该变量保存上一次计算的值（在计算值时由REPL更新）。

例如：

>>> x = 5   #  this is not _
>>> _ ** 2  #  so referencing it will raise a NameError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '_' is not defined
>>> 5  #  the result of this will be the value referenced by _
5
>>> _ ** 2  #  now referencing it is okay
25
>>> _ ** 2  #  note that now the value of _ is 25 (the last value calculated)
625
>>>  #  and here its value is 625 (and so on)