Misleading error message of the day

[Benjamin Kaplan]

On Thu, Dec 8, 2011 at 1:42 PM, Roy Smith <roy at panix.com> wrote:
> (some,
>  very,
>  long,
>  list,
>  of,
>  variable,
>  names,
>  to,
>  get,
>  the,
>  stuff,
>  unpacked,
>  into) = function_that_should_return_a_14_tuple()
>
> raises
>
> ValueError: too many values to unpack
>
> Quick, what's the bug?  Did I forget a variable on the LHS, or is my function returning more things than it should?  I know it's supposed to be 14, but I don't know which side is wrong.  Had it said "... expected 13, got 14", I would know immediately.
>

If the RHS was a tuple or a list, yes you could know immediately. But
unpacking works with any iterable, so it probably doesn't special-case
lists and tuples. Iterables don't have a size- they just keep going
until StopIteration is raised. So in EVERY SINGLE CASE, you would get
"expected n args, got n+1" even if the iterable would return 24 items
instead of 14, or would never stop returning items.


> Error messages should be as explicit as possible.  It's just like bug reports.  The basic mantra of a bug report is:
>
> 1) This is what I did
>
> 2) This is what I expected to happen
>
> 3) This is what I observed happen
>
> 4) This is how what I observed differed from what I expected
>
> Saying, "expected X, got Y" is more explicit than "got too many"
>
>
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