Bug with ConvertUnits Inplace

[Owen Arnold]

This is an issue raised by Tim Charlton. I've confirmed that this is an issue with the most recent builds.

This is Tim's original script:

RunNo=6224
w=Load(Filename='D:/data/cycle_12_1/POLREF0000%d.raw'%RunNo,OutputWorkspace='w%d'%RunNo,SpectrumMax='4')
ConvertUnits(InputWorkspace=w,OutputWorkspace=w,Target="Wavelength",AlignBins="1")
print w

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Mon 2. Jul 12:03:53 2012: Script execution started.

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RuntimeError: Variable invalidated, data has been deleted. at line 5 in

[Martyn Gigg]

After thinking about it I've realised what is actually happening and it's not a bug but more a feature of the underlying memory management & python's variable passing behaviour.

What's happening is:

• ConvertUnits asks for the input workspace named 'w' to be converted to the output workspace with the same name 'w';
• The python variable is a weak pointer to the shared_ptr in the ADS so that it doesn't hold on to memory when the workspace is deleted.
• The ConvertUnits call, while it looks like it is in place, is actually creating a new workspace, because of the AlignBins call, that is then stored in the ADS with the same name 'w'.
• The python reference is then invalidated because the original shared_ptr no longer exists.

The w value for the OutputWorkspace argument is immutable as far as Python is concerned so there is no way of changing it inside the ConvertUnits call.

I think we'll have to mark this:

ConvertUnits(InputWorkspace=w,OutputWorkspace=w,Target="Wavelength",AlignBins="1")

as not possible under the new API but this will work and should be encouraged,

w = ConvertUnits(InputWorkspace=w,Target="Wavelength",AlignBins="1")

or this would work too

w = ConvertUnits(InputWorkspace=w,OutputWorkspace=w,Target="Wavelength",AlignBins="1")

[Stuart Campbell]

Sounds good to me.

[Stuart Campbell]

This ticket has been transferred to github issue ​6411