[ncl-talk] variable metadata getting overwritten when variable is written to a NetCDF4 file

[David Brown]

Hi Keith,
I think that what you are seeing is actually documented (although
poorly apparently) behavior. At least, if, as I am assuming from your
test script, the only attribute that gets passed through the
value-only assignment is the _FillValue. The NCL Reference Manual says
this about value-only assignment:

"No dimension names, coordinate variables or attributes other than
_FillValue are assigned. "

In fact the _FillValue is a special attribute that even values (as
opposed to variables) in NCL carry around. And it is propagated even
through value-only assignment. That is because it is considered an
essential component of the value.

If in fact you are seeing other attributes get propagated let us know.
Your test script only showed the _FillValue propagation as far as I
could tell.
 -dave






On Mon, Nov 2, 2015 at 9:03 AM, Keith Lindsay <klindsay at ucar.edu> wrote:
> Hi,
> Thanks Dennis. For other users that run into this behavior, a workaround to
> avoid it is to ensure that the variable on the rhs of the problematic
> assignment does not have the _FillValue attribute. I.e.
>       if (isatt(tmpvar, "_FillValue")) then
>         delete(tmpvar at _FillValue)
>       end if
> Keith
>
>
> On Mon, Nov 2, 2015 at 8:44 AM, Dennis Shea <shea at ucar.edu> wrote:
>>
>> A JIRA ticket [  NCL-2302 ] has been opened.  THX
>>
>> On Mon, Nov 2, 2015 at 8:06 AM, Keith Lindsay <klindsay at ucar.edu> wrote:
>> > Hi,
>> >
>> > I have come across an instance where I'm writing a variable to a file
>> > and
>> > the variable metadata in the file is getting overwritten, even though
>> > I'm
>> > using (/ ... /) on the right-hand side of the write to file statement.
>> > That
>> > is, the statement
>> > f->$varname$ = (/ tmpvar /)
>> > is overwriting the metadata for the file variable $varname$ in the file
>> > pointed to by f. The attached script demonstrates the problem. This
>> > seems
>> > like an ncl bug to me.
>> >
>> > Details of my script that seem to matter are:
>> > 1) the file is NetCDF4
>> > 2) the file variable whose metadata is getting overwritten has been
>> > defined
>> > in the file but not written to yet
>> > 3) the file variable whose metadata is getting overwritten does not have
>> > the
>> > _FillValue attribute
>> > 4) the variable on rhs of write to file statement has _FillValue
>> > attribute
>> >
>> > The problem happens with ncl 6.3.0 on yellowstone. It does not happen
>> > with
>> > ncl 6.2.1, though with 6.2.1 the _FillValue gets added to the file
>> > variable's metadata, which is unexpected to me.
>> >
>> > Keith
>> >
>> > _______________________________________________
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>
>
>
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