I am able to reproduce this behavior in the development version of qgis.
As a consequence, I opened two issues:
"Python call to --processing.run('qgis:fieldcalculator'-- fails to catch
divide by zero"
(https://github.com/qgis/QGIS/issues/37737)
"nonsense code causes infinite regress in error reports"
(https://github.com/qgis/QGIS/issues/37739). Both have since been
tagged as "Bug + Processing + Modeller".
I learned along the way from a comment in the qgis core source code that
the syntax of the language for the FORMULA argument, which I think is
called "Processing", is SQL or SQL-ish by design.
- JP
On 6/17/20 4:00 PM, James Powell wrote:
Hi there everybody,
I want to derive some floating point values using an expression in the
field calculator (see model snapshot below). I'm using qgis 3.10.6 on
Debian 10.
I am finding that the first field calculator call, which produces
'dbgfieldcalc',
gives only NULL answers.
The second field calculator call (at the bottom, which produces
'dbgfishnetFinal')
does almost the same thing and it works great.
At no time will running this model produce errors. In fact, even
changing attribute names to be invalid attribute names doesn't produce
an error at all.
What I expect:
- That the field calculator is able to consistently run simple
expressions through.
- That feeding invalid attribute names causes an error report and
aborts the model run.
I've read through the docs that show up for a search on "qgis-user
field calculator".
The python code produced for this model reads
: # Field calculator
: alg_params = {
: 'FIELD_LENGTH': 10,
: 'FIELD_NAME': 'emissionsAfterFishnetInGramPerHour',
: 'FIELD_PRECISION': 3,
: 'FIELD_TYPE': 0,
: 'FORMULA': 'attribute($currentfeature,
\'emissionsingramsperhour\') * attribute($currentfeature,
\'xlength_2\') / attribute($currentfeature, \'length\')',
: 'INPUT': outputs['AddGeometryAttributes']['OUTPUT'],
: 'NEW_FIELD': True,
: 'OUTPUT': parameters['Dbgfieldcalc']
: }
: outputs['FieldCalculator'] =
processing.run('qgis:fieldcalculator', alg_params, context=context,
feedback=feedback, is_child_algorithm=True)
: results['Dbgfieldcalc'] = outputs['FieldCalculator']['OUTPUT']
where "xlength_2" is a nonexistent attribute (but the script has the
same behavior when I use
"length_2", which is an existent attribute).
Is it usual to not detect errors like this? Is there some reason the
expression I
wrote (with the proper attribute name) wouldn't give a floating point
number, but produces NULL instead?
many thanks,
James P.
snapshot:
--
James E. Powell, MS
Pronouns: he/him/his
Applied Physics PhD Candidate
Department of Physics
Portland State University
Home page:http://web.pdx.edu/~powellj
Office: SRTC 409B Phone: +1-503-725-8515
--
James E. Powell, MS
Pronouns: he/him/his
Applied Physics PhD Candidate
Department of Physics
Portland State University
Home page: http://web.pdx.edu/~powellj
Office: SRTC 409B Phone: +1-503-725-8515
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