[Python-Dev] RE: Meaning of trailing comma?

[Chermside, Michael]

Hello Roman:

The list "python-dev" is not the correct place to ask questions like this. =
The list "python-list" (accessed as "python-list@python.org" is full of peo=
ple (including me!) who'd be willing to answer your question, but "python-d=
ev" is reserved for discussion of how to write the Python language itself, =
not how to USE python.

Now that I've chastised you for mis-using the list I'll assume that you sim=
ply didn't realize what the list was for, and that it was an honest mistake=
, so let me answer the question.

Have you ever (in any language) written something like this:

   myList =3D (
        a,
        b,
        c,
        d
   )

... then had to add a new item to the list:

   myList =3D (
        a,
        b,
        c,
        d
        e
   )

... and gotten a bug? (Look very carefully, and you'll find the bug.) Well,=
 that's why Python allows the
trailing comma. The only special case is a one-item tuple. Python has no go=
od way to tell a one-item
tuple apart from something that's just enclosed in parentheses (technically=
, that's because it's the
comma not the parentheses that make the tuple), so it punts and goes with a=
 not-so-good way:
using a trailing comma makes it a one-item tuple. That's the only time the =
trailing comma ever matters;
otherwise it's completely optional and used for cosmetic (and typo-avoidanc=
e) reasons only.

-- Michael Chermside



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