[Tutor] reason(s) for trailing comma in dict declarations

[akleider at sonic.net]

Thanks for the clarification. Now it is clear. ak

> On Sat, Aug 25, 2012 at 11:53 AM,  <akleider at sonic.net> wrote:
>>
>> Put each entry on its own line, indented by two spaces, and leave a
>> trailing comma on the last entry. The latter is especially important
>> in sequences of strings to prevent them from being
>> "silently"<=>"concatenated" if you were to add an entry and forget the
>> comma.
>> """
>>
>> When I first saw this I thought it would lead to a syntax error so tried
>> it out..
>> Then played with it to try to recreate the '"silently"<=>"concatenated"'
>> problem but couldn't.  I like this syntax because it avoids the syntax
>> error if the comma is omitted when adding an entry but I don't
>> understand
>> the (potential) concatenation problem.
>>
>> Could you explain please?
>
> Sure, I said the problem is with "sequences", not mappings. The syntax
> for a dictionary will catch the mistake. But try it with a list.
>
> x = [
>   "string1",
>   "string2",
>   "string3"   # no comma
> ]
>
> Later you decide to add "string4" but forget to add the comma:
>
> x = [
>   "string1",
>   "string2",
>   "string3"   # no comma
>   "string4"
> ]
>
> Result:
>
> ['string1', 'string2', 'string3string4']
>
>