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Comment by iainmerrick | Hacker News Reader

It works the way you want if you add a trailing comma:

  important_numbers = {
    "x": 3,
    "y": 42,  # Answer to the Ultimate Question!
    "z": 2,
  }

You might complain that that seems a bit obscure, but it only took me 10 or 20 seconds to discover it after pasting the original code snippet into an editor.

The trailing comma is an improvement as it makes the diff clearer on future edits.

Edit to add: occurs to me that I oversimplified my position earlier and it probably looks like I'm trying to have it both ways. I do advocate aiming for clean and clear formatting; I'm just against doing this manually. You should instead use automation, and steer it lightly only when you have to.

For example, I explicitly don't want people to manually "tab-align" columns in their code. It looks nice, sure, but it'll inevitably get messed up in future edits. Better to do something simpler and more robust.

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The trailing comma communicates an intent of possibly adding more things in the future. I actually use it quite a lot -- when I have that intent.

In the above example, if I think I have listed all of the `important_numbers`, there is a certain point of not having the trailing comma there.

Here's another terrible example from `black`:

From this:

    my_print(f"This string has two parameters, `a` which is equal to {a} and `b` which is equal to {b}", 
        a=1, b=2)

To this:

    my_print(
        f"This string has two parameters, `a` which is equal to {a} and `b` which is equal to {b}",
        a=1,
        b=2,
    )

The trailing comma it added makes no sense whatsoever because I can not have an intent of adding more things -- I've already exhausted the parameters in the string!

On the top of it, I don't quite get why I need to change the way I write in order to please the machine. Who should be serving whom?

Edit: changed "print" to "my_print" to not have to argue about named parameters of print ("sep", "file" etc.).

Edit 2: here's a variant that `black` has no issues with whatsoever. It does not suggest a trailing comma or any other change:

    my_print(f"This string has two params, `a` which is {a} and `b` which is {b}", a=1, b=2)

So an existence of a trailing comma is a product of string length?

Yes, it gets a trailing comma if it's on it's own line. That way when you add/remove arguments in a multi-line call it's only a one-line diff. This doesn't apply when the diff is only one line anyway.

Who's to say you don't add a new argument to the function in the future, like

    my_print(
        "This string has two parameters, `a` which is equal to {a} and `b` which is equal to {b}",
        a=1,
        b=2,
        color_negative_red=True,
    )

> it gets a trailing comma if it's on it's own line.

Sorry but it doesn't make any sense to me. If your argument is "a trailing comma is a good thing," it should go into any and all function calls/list declarations/etc. Who's to say I won't add this in the future:

    my_print("a={a}, b={b}", a=1, b=2, color_negative_red=True)

So do I need to have this now?

    my_print("a={a}, b={b}", a=1, b=2,)

There's a very responsive playground at https://black.vercel.app/ and whatever it does looks strange to me, because the underlying assumptions look inconsistent one with the other (to my eye at least.) Specifically, "the length of the string should decide whether there is a trailing comma or there isn't" makes zero sense.

>Sorry but it doesn't make any sense to me. If your argument is "a trailing comma is a good thing," it should go into any and all function calls/list declarations/et

No, the argument is quite specifically that a one line diff to add a new argument/element to the end of a list is preferable to a two line diff to do the same thing. The presence of the trailing comma is necessary to achieve that only when elements are on their own line.

Ok, we're then back to `print` example:

    print(
        'Hello there from a very long line abcdefghijklmnopqrstuvwxyz',
        sep=' ', 
        end='\n', 
        file=None, 
        flush=False,
    )

All of the existing named parameters to `print()` function are already provided, and that standard function is highly unlikely to change. Should I add another string to `print`, I will have to do it before the named parameters anyway. There is no sense in the trailing comma here however you look at it.

Edit: sorry for using single quotes, in my 20 years of writing Python it was never an issue, but now with `black` it apparently is.

I think this boils it down to the essence. Whether you use a trailing comma here, and whether you use single or double quotes, is just bike-shedding. If there's an automated tool that can make a consistent choice everywhere, that's worthwhile.

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