[Python-ideas] Treat underscores specially in argument lists

[Andrew Barnert]

On Feb 16, 2015, at 18:45, Ryan Gonzalez <rymg19 at gmail.com> wrote:

> It's usually in callbacks where you only care about a few arguments.

A perfect example is using Tk variable tracing in tkinter (for validation or post-change triggering on widgets). The signature of the trace callback is callback(name, index, mode). You almost never need the index, you rarely need the mode, so you write:

    def callback(self, name, _, mode):

In fact, you usually don't need the mode either (in fact, you often don't even need the name), but you can't write this:

   def callback(self, name, _, _):

So instead you usually write something like:

   def callback(self, name, *_):

Personally, I find *_ ugly, so I use *dummy instead. But whatever.

So anyway, what if you needed the index, but not the name or mode? You can't use *_ to get around that; you have to come up with two dummy names. That's a bit annoying.

The question is, how often do you really need to ignore multiple parameters, but not all of them (or all of them from this point on)? I can't remember any callback API that I've used where that came up, but I can imagine someone else has one:

> On Mon, Feb 16, 2015 at 8:41 PM, Steven D'Aprano <steve at pearwood.info> wrote:
>> On Mon, Feb 16, 2015 at 07:23:30PM -0600, Ryan Gonzalez wrote:
>> > Often, underscores are used in function argument lists:
>> >
>> > def f(_): pass
>> >
>> > However, sometimes you want to ignore more than one argument, in which case
>> > this doesn't work:
>> 
>> Why are you ignoring *any* arguments?
>> 
>> 
>> --
>> Steve
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> 
> 
> -- 
> Ryan
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