如何表示未使用的 function arguments？

Question

When "deconstructing" a tuple, I can use _ to denote tuple elements I'm not interested in, eg

>>> a,_,_ = (1,2,3)
>>> a
1

Using Python 2.x, how can I express the same with function arguments? I tried to use underscores:

>>> def f(a,_,_): return a
...
  File "<stdin>", line 1
SyntaxError: duplicate argument '_' in function definition

I also tried to just omit the argument altogether:

>>> def f(a,,): return a
  File "<stdin>", line 1
    def f(a,,): return a
        ^
SyntaxError: invalid syntax

Is there another way to achieve the same?

Answer 1

A funny way I just thought of is to delete the variable:

def f(foo, unused1, unused2, unused3):
    del unused1, unused2, unused3
    return foo

This has numerous advantages:

• The unused variable can still be used when calling the function both as a positional argument and as a keyword argument.
• If you start to use it later, you can't since it's deleted, so there is less risk of mistakes.
• It's standard python syntax.
• PyCharm does the right thing, (As of 2020: PyCharm no longer does the right thing :( tracking this at https://youtrack.jetbrains.com/issue/PY-39889 )
• PyLint won't complain and using del is the solution recommended in the PyLint manual .

Answer 2

The underscore is used for things we don't care about and the * in *args denotes a list of arguments. Therefore, we can use *_ to denote a list of things we don't care about:

def foo(bar, *_):
    return bar

It even passes PyCharm's checks.

Answer 3

You can use '_' as prefix, so that pylint will ignore these parameters:

def f(a, _b, _c):

Answer 4

Here's what I do with unused arguments:

def f(a, *unused):
    return a

Answer 5

In order to avoid "unused variable" inspection messages for unused *args and/or **kwargs, I replace args and kwargs by _ and __ :

def f(a, b, *_, **__):
    ...

In addition to remove messages, it clearly shows that you don't care about these arguments.

I can't say if it is a really universal solution, but it worked everywhere I've used it until now.

Answer 6

If you have both args and keyword arg you should use

def f(a, *args, **kwargs):
    return a

Answer 7

I think the accepted answer is bad, but it can still work, if you use what I should call "Perl way" of dealing with arguments (I don't know Perl really, but I quit trying to learn it after seeing the sub syntax, with manual argument unpacking):

Your function has 3 arguments - this is what it gets called with (Duck typing, remember?). So you get:

def funfun(a, b, c):
    return b * 2

2 unused parameters. But now, enter improved larsmans' approach:

def funfun(*args):
    return args[1] * 2

And there go the warnings...

However, I still enjoy more the boxed's way:

def funfun(a, b, c):
    del a, c
    return b * 2

It keeps the self-documenting quality of parameter names. They're a feature, not a bug.

But, the language itself doesn't force you there - you could also go the other way around, and just let all your function have the signature (*args, **kwargs) , and do the argument parsing manually every time. Imagine the level of control that gives you. And no more exceptions when being called in a deprecated way after changing your "signature" (argument count and meaning). This is something worth considering;)

Answer 8

You can use "*_" to use multiple unused arguments

def test(x, *_):
    return x