one-element tuples

[BartC]

On 11/04/2016 01:48, Fillmore wrote:
> On 04/10/2016 08:31 PM, Ben Finney wrote:
>> Can you describe explicitly what that “discontinuation point” is? I'm
>> not seeing it.
>
> Here you go:
>
>  >>> a = '"string1"'
>  >>> b = '"string1","string2"'
>  >>> c = '"string1","string2","string3"'
>  >>> ea = eval(a)
>  >>> eb = eval(b)
>  >>> ec = eval(c)
>  >>> type(ea)
> <class 'str'>   <--- HERE !!!!
>  >>> type(eb)
> <class 'tuple'>
>  >>> type(ec)
> <class 'tuple'>
>
> I can tell you that it exists because it bit me in the butt today...
>
> and mind you, I am not saying that this is wrong. I'm just saying that
> it surprised me.

I think this shows more clearly what you mean:

a = 10             # int
b = 10,            # tuple
c = 10,20          # tuple
d = 10,20,30       # tuple

The difference between a and b is that trailing comma. A bit of a 
kludge, but it's enough to distinguish between a single value (x), and a 
one-element tuple (x,).

In this case, you might call it a discontinuity in the syntax as, when 
you go from d to c, you remove ",30", including the comma, but when 
going from c to b, you remove only "20".

But this can be fixed if tuples are written like this:

a = 10             # int
b = 10,            # tuple
c = 10,20,         # tuple
d = 10,20,30,      # tuple

Now, you remove "30," then "20,". No exception.

Of course this doesn't help you parsing typical input which uses commas 
as separators, not terminators!


-- 
Bartc