Is "*_" an acceptable way to ignore arguments in Python?

Question

If I have a function/method that is an implementation of a callback for some framework, and do not care for any further arguments, it seems to be syntactically correct, and to not have pylint/IDE complaints to use *_ to express no interest in any further arguments. The point I think is to express intent to both the tools, and other developers that these arguments are not currently relevant.

To clarify what I mean:

def my_callbacK_handler(a, b, *_):
    ...

I’ve not seen this idiom used in the wild – is it common, are there examples and are there known problems with this?

For those not familiar: _ expresses the intent that I am not interested in that symbol – it is the Python "dummy" that is recognized by IDE’s and linters.

Asked By: Danny Staple

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Answer 1

_ is just a variable like any other, but by convention it means
that you don’t intend to use that value, just declare and forget it.

[OrderedSet() for _ in xrange(n)]

builds a list of n empty ordered sets. _ is the index, but is unused; _ is commonly used as required, but unused, variable name (not only in Python).
This is just a tricky Python idiom because there is no built-in syntax to do this.

It’s not uncommon to see this in other languages (where _ is a valid identifier). _ often
means a variable one’s not interested in the value of, but which is
needed for syntactic reasons.

Note that _(...) as a macro call has another conventional meaning, which
comes from gettext, where one uses _("string literal") to indicate a
string that needs localization.

A thread to read from ActivesState

I can not say about any problems, Python doesn’t use it internally, it’s just a variable, it is us who have to be careful.

Regarding the syntax of * and **: The names *args and **kwargs are only by convention, but there’s no need not to use them.

def my_callbacK_handler(a, b, *_):
    ...

So what I understand if I see this function in your code by _ idiom, that this function only uses a and b arguments in its working and will ignore others.

As you state: "*_ to express no interest in any further arguments".

Answered By: Rahul Gautam

Answer 2

*_ means multiple placeholders,just like you write def my_callbacK_handler(a, b, _,_,_....,_): and will create a variable named _ which is a list of the extra arguments

It has two advantage:

It can handle the case where there is no extra argument

def f(): 
   return 1,2,3 

def g(): 
   return 1,2

def h(): 
   return 1,2,3,4

_ as a placeholder can only deal with exactly one value

x,y,_ = g() # ValueError: not enough values to unpack (expected 3, got 2)

x,y,_ = h() # ValueError: too many values to unpack (expected 3)

whereas *_ can deal with any amount of values

x,y,*_ = g()                                                                                                                  

print(_)                                                                                                                      
[]

x,y,*_ = h()                                                                                                                  

print(_)                                                                                                                      
[3, 4]

One word: Prefer *_ for more robust code.

Answered By: Rafael

Answer 3

Let me just add an example:

>>> *_, a = [1,2,3]
>>> _, a
([1, 2], 3)


>>> _, a = [1,2,3]
Traceback (most recent call last):
  File "<console>", line 1, in <module>
ValueError: too many values to unpack (expected 2)

Answered By: Denis

Answer 4

I belive that using _ as a "don’t care" variable name originates with Prolog.
In Prolog it not only means that you don’t care but it has a different semantics from other variables in that normally two instances of the same variable (eg Dummy) must match the same value, whereas two instances of _ can match different values.

I was surprised to see it being used in Python and confused when I first saw it. But it does make sense, and therefore so does *_ . Now I use it myself.

BTW It seems that _ does have a special semantics in Python as well but only in the context of it being used in the top level of the Python shell (I think), where
_ gets automatically assigned the value of the last returned function value.

Answered By: Brandon Bennett

Is "*_" an acceptable way to ignore arguments in Python?

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