下划线在Python中代表什么？

Question

I am kind of new to Python, so I am trying to read over existing code. I am a little confused on the syntax of this though.

For example:

rlist, _, _ = select.select(sockets, [], [])

I understand that select.select() takes 3 lists (and I assume [] just means empty list), but is the _ used to denote a placeholder of some sort?

Answer 1

It's just the name of a variable! Usually people use _ for variables that are temporary or insignificant.

As other people have stated, _ is a common alias for gettext, a translation library. You can identify when it's being used as gettext if you see it called as a function, eg. _('Hello, world!') .

In the python console it can be used to retrieve the result of the last statement.

>>> 3 + 4
7
>>> a = _
>>> print a
7

Answer 2

It's just an anonymous variable, and has no special meaning to python. Compare it with using i as a loop counter.

You generally use it to document that the surrounding code is going to ignore the value of that variable.

In the python interactive console, the result of the last expression is assigned to _ , but that does not carry through in python programs.

Answer 3

Despite what the other answers say, _ does have a special meaning in Python. It's the last result printed at the interactive prompt.

>>> 2+2
4
>>> _+2
6

(Of course if there is no interactive prompt, eg, because you're running a Python script from the shell, then it doesn't have a special meaning.)

Answer 4

It represents an anonymous variable . It's used because the variable is required but the value can be ignored.

Answer 5

Generally, you name a variable with a single underscore when you never need to refer to the variable again. For example, something like this:

for _ in range(10):
    print "hello"

This just prints "hello" 10 times, and we never need to refer to the loop control variable ( _ in this case).

In your example, select.select(sockets, [], []) returns a tuple (or list or set) from which you seemingly only need the first item, hence you the use of the underscores.