Subgroups and linked variables -- weak reference problem

[mstimberg]

As reported in the discourse forum, using a VariableView of a subgroup in linked_var will lead to a ReferenceError. The following example reproduces the issue:

group1 = NeuronGroup(10, 'v : 1')
group2 = NeuronGroup(5, 'v : 1 (linked)')

group2.v = linked_var(group1[3:8].v)

The problem is that group1[3:8].v creates a VariableView object which only holds a weak reference to its group, and group1[3:8] therefore disappears. In linked_var, we are trying to access the group attribute which leads to the error.

I think in general the approach to only store a weak reference is the right one, because VariableView objects were previously a major source of memory problems. We can get away with this normally even if its group argument disappears because we hold strong references to the Variable object (which has the full data of v) and the Indexing object (which allows us to deduce that we need values 3–8 of v). This should be fixable by moving a bit around the logic of index interpretation between linked_var and LinkedVariable, but I'll have to have a closer look.

I'll mark this as low priority, though, because there are a number of workarounds. All the following variants will work:

# named subgroup
subgroup = group1[3:8]
group2.v = linked_var(subgroup.v)
# avoid VariableView by separating group and variable name
group2.v = linked_var(group1[3:8], 'v')
# link to full group and use absolute indices
group2.v = linked_var(group1, 'v', index=np.arange(3, 8))

[rohithvarma3000]

Hello @mstimberg ,
I am Rohith Varma (@rohithvarma3000) and I would like to help to solve this issue. Could you guide me on how to help you solve this issue?

[mstimberg]

I wouldn't recommend to tackle this issue right now, I am not 100% sure what the correct fix is myself :-) !

[rohithvarma3000]

Ok