Made merge brick apply application

[dmitriy-serdyuk]

No description provided.

[dmitriy-serdyuk]

There is something wrong with Merge brick. This code

z = merge.apply(x, y)
cg = ComputationGraph(z)
VariableFilter(bricks=[merge], roles=[OUTPUT])(cg.variables)

returns [merge_apply_x, merge_apply_y].

[dmitriy-serdyuk]

I added a test for this case. Do you have any ideas why is is happening?

[rizar]

I think this weird behavior is due to the fact that Merge inherits from Parallel and call the parent's apply method in its apply method. However, I would then expect three OUTPUT variables: two of Parallel.apply and one of Merge.apply.

[rizar]

As for the main body of your PR, it looks all right to me.

[dmitriy-serdyuk]

Yes, you are right, there were 3 outputs.

[rizar]

But then it is completely expected behavior, isn't it? Should your new test be fixed to check that 3 variables are found?

[dmitriy-serdyuk]

It is still returning all three variables. I don't think that it is expected. Why filtering by applications gives only one output, but filtering by bricks returns outputs of all predecessors?

[rizar]

Because when you call merge.apply it calls super(Merge, self).apply, that is two different application methods of the same brick merge are called. That's why what you get when you filter outputs of the merge brick are outputs of both application methods, two of Parallel.apply and one of Merge.apply.

[dmitriy-serdyuk]

Ok, I see. But it doesn't seem intuitive.

[rizar]

That is an inevitable consequence of having Merge as a descendant of Parallel. Whether or not this was a good design decision can be discussed.

[rizar]

Thank for this small clean-up anyway :)