VarParsing default doesn't work for lists?

[gpetruc]

Hi,

The behaviour of VarParsing for lists doesn't seem right to me,

    def register (self, name,
                  default = "",
                  mult    = multiplicity.singleton,
                  mytype  = varType.int,
                  info    = "",
                  **kwargs):
            [....]
            # if it's a list, we only want to use the default if it
            # does exist.
            if len (default):
                self._lists[name].append (default)

The first line checks whether the provided default is a non-empty list, but then the second appends the whole list as entry in the default. As a result, what one gets as default value is not what is passed as default to register, but rather a list of lists, with one single entry that is equal of lists.

What would make more sense to me if the line were to be self._lists[name] += default.

As is I can't call register with as default a non-empty list (except in the case where it's a list of strings containing just one entry, as in that case I can pass the entry value as default, and len() will act on the string)

I can make a PR, but git blame records the code as being 11 years old, so maybe this is the intended behaviour and I'm not understanding the logic for it?

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[cmsbuild]

A new Issue was created by @gpetruc.

@Dr15Jones, @antoniovilela, @makortel, @mandrenguyen, @rappoccio, @sextonkennedy, @smuzaffar can you please review it and eventually sign/assign? Thanks.

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[Dr15Jones]

assign core

[cmsbuild]

New categories assigned: core

@Dr15Jones,@makortel,@smuzaffar you have been requested to review this Pull request/Issue and eventually sign? Thanks

[gpetruc]

Code https://github.com/cms-sw/cmssw/blob/master/FWCore/ParameterSet/python/VarParsing.py#L416

Reproducer code

import FWCore.ParameterSet.Config as cms
import FWCore.ParameterSet.VarParsing as VarParsing

options = VarParsing.VarParsing ('analysis')
options.register ('buNumStreams',
                  [1], # default value
                  VarParsing.VarParsing.multiplicity.list,
                  VarParsing.VarParsing.varType.int,          # string, int, or float
                  "Number of input streams (i.e. files) used simultaneously for each BU directory")

options.parseArguments()
print(f"numStreams set to '{options.buNumStreams}', len {len(options.buNumStreams)}, type of first element is {type(options.buNumStreams[0])}")

process = cms.Process("NULL")

if run with
python3 demo.py buNumStreams=4,5
it works fine printing
numStreams set to '[4, 5]', len 2, type of first element is <class 'int'>,
but if I don't give a command line argument it prints out the incorrect
numStreams set to '[[1]]', len 1, type of first element is <class 'list'>
while the expected behaviour would be
numStreams set to '[1]', len 1, type of first element is <class 'int'>

[Dr15Jones]

Although this code is in FWCore.ParameterSet, it really isn't maintained anymore. Instead, we suggest people migrate the the python standard argparse as work was done to make that easier to use with cmsRun.

That being said, we do accept changes done to VarParsing committed by others.

[makortel]

This looks like the same issue reported already in #44630, and for which there is a PR open #45640 to try to improve the situation (see especially #45640 (comment)).

I can make a PR, but git blame records the code as being 11 years old, so maybe this is the intended behaviour and I'm not understanding the logic for it?

I believe the knowledge on "intended behavior" of VarParsing is long gone.