Inconsistent coordinate override in arithmetic expressions

[apfelix]

Based on the discussion in #256, I started using <variable>.loc[indices] in arithmetic expressions and observed some inconsistencies (or I still do not understand the behaviour to 100% ^^)

Following setup:

import xarray as xr

import linopy

m = linopy.Model()

# 2 days, hourly (48 hours)
t = [0, 1, 2, 3]
n = ["a", "b"]

level = m.add_variables(coords=[t, n], name="level", dims=["time", "node"])

alpha = xr.DataArray([[1, 2], [3, 4], [5, 6], [7, 8]], coords=level.coords)

other_times = [2, 0, 3, 1]
other_time_level = level.loc[dict(time=other_times)]

Some arithmetic expressions level + other_time_level works as expected, coordinates are overridden based on level

LinearExpression (time: 4, node: 2):
------------------------------------
[0, a]: +1 level[0, a] + 1 level[2, a]
[0, b]: +1 level[0, b] + 1 level[2, b]
[1, a]: +1 level[1, a] + 1 level[0, a]
[1, b]: +1 level[1, b] + 1 level[0, b]
[2, a]: +1 level[2, a] + 1 level[3, a]
[2, b]: +1 level[2, b] + 1 level[3, b]
[3, a]: +1 level[3, a] + 1 level[1, a]
[3, b]: +1 level[3, b] + 1 level[1, b]

When using level + alpha * other_time_level, I would have expected to get

LinearExpression (time: 4, node: 2):
------------------------------------
[0, a]: +1 level[0, a] + 1 level[2, a]
[0, b]: +1 level[0, b] + 2 level[2, b]
[1, a]: +1 level[1, a] + 3 level[0, a]
[1, b]: +1 level[1, b] + 4 level[0, b]
[2, a]: +1 level[2, a] + 5 level[3, a]
[2, b]: +1 level[2, b] + 6 level[3, b]
[3, a]: +1 level[3, a] + 7 level[1, a]
[3, b]: +1 level[3, b] + 8 level[1, b]

since I though that alpha would be term to define the coordinates in alpha * other_time_level

However, what I actually get is the following (coordinates of other_time_level are sorted according to the original time index again)

LinearExpression (time: 4, node: 2):
------------------------------------
[0, a]: +1 level[0, a] + 1 level[0, a]
[0, b]: +1 level[0, b] + 2 level[0, b]
[1, a]: +1 level[1, a] + 3 level[1, a]
[1, b]: +1 level[1, b] + 4 level[1, b]
[2, a]: +1 level[2, a] + 5 level[2, a]
[2, b]: +1 level[2, b] + 6 level[2, b]
[3, a]: +1 level[3, a] + 7 level[3, a]
[3, b]: +1 level[3, b] + 8 level[3, b]

Interestingly, the result changes when transforming other_time_level into a LinearExpression first, i.e., call level + alpha * other_time_level.to_linexpr()

LinearExpression (time: 4, node: 2):
------------------------------------
[0, a]: +1 level[0, a] + 5 level[2, a]
[0, b]: +1 level[0, b] + 6 level[2, b]
[1, a]: +1 level[1, a] + 1 level[0, a]
[1, b]: +1 level[1, b] + 2 level[0, b]
[2, a]: +1 level[2, a] + 7 level[3, a]
[2, b]: +1 level[2, b] + 8 level[3, b]
[3, a]: +1 level[3, a] + 3 level[1, a]
[3, b]: +1 level[3, b] + 4 level[1, b]

Finally, I tried to simply add alpha instead of multiplying it, i.e., level + alpha + other_time_level and this gives the expected result in terms of sorting the single terms into the expected rows:

LinearExpression (time: 4, node: 2):
------------------------------------
[0, a]: +1 level[0, a] + 1 level[2, a] + 1
[0, b]: +1 level[0, b] + 1 level[2, b] + 2
[1, a]: +1 level[1, a] + 1 level[0, a] + 3
[1, b]: +1 level[1, b] + 1 level[0, b] + 4
[2, a]: +1 level[2, a] + 1 level[3, a] + 5
[2, b]: +1 level[2, b] + 1 level[3, b] + 6
[3, a]: +1 level[3, a] + 1 level[1, a] + 7
[3, b]: +1 level[3, b] + 1 level[1, b] + 8

So my question would be: How to achieve the intended result and is all of the behaviour as it should be?

[FabianHofmann]

very good that you look into this @apfelix, there is some inconsistency indeed. However, I would not fully have the same expectation on the operations.

1. In my point of view overriding should only happen when adding and subtracting terms. works as intended. See level + other_time_level

2. multiplication should be aligned by coordinates, no overriding. Thus, when multiplying a variable with a coefficient, the first object in the multiplication should determine the coordinate alignment of the result . Meaning:

   alpha * other_time_level -> result has the same coordinates as alpha, other_time_level is sorted accordingly. works as intended

   other_time_level * alpha -> result has the same coordinates as other_time_level, alpha is sorted accordingly. :negative_squared_cross_mark: does not work.

3. If one wants to ignore the coefficient coords in the multiplication, i.e. not aligning the coefficient with the variable, then one should use

   other_time_level * alpha.values

what do you think?

[apfelix]

I though a little about this, and I think your expectation does make sense. However, I'm curious on what should happen if the multiplication does involve two variables. Currently level * other_time_level also works as coefficient multiplication, i.e., aligning the values by coordinate again

QuadraticExpression (time: 4, node: 2):
---------------------------------------
[0, a]: +1 level[0, a] level[0, a]
[0, b]: +1 level[0, b] level[0, b]
[1, a]: +1 level[1, a] level[1, a]
[1, b]: +1 level[1, b] level[1, b]
[2, a]: +1 level[2, a] level[2, a]
[2, b]: +1 level[2, b] level[2, b]
[3, a]: +1 level[3, a] level[3, a]
[3, b]: +1 level[3, b] level[3, b]

But then how would you achieve

QuadraticExpression (time: 4, node: 2):
---------------------------------------
[0, a]: +1 level[0, a] level[2, a]
[0, b]: +1 level[0, b] level[2, b]
[1, a]: +1 level[1, a] level[0, a]
[1, b]: +1 level[1, b] level[0, b]
[2, a]: +1 level[2, a] level[3, a]
[2, b]: +1 level[2, b] level[3, b]
[3, a]: +1 level[3, a] level[1, a]
[3, b]: +1 level[3, b] level[1, b]

?

If the multiplication of linopy objects should also ignore alignment, my suggestion for a consistent set of alignment rules would be the following (not sure how hard it would be to implement this):

• multiplication of linopy objects (Variable/Expression) with coefficients (xarray) works as in xarray (alignment of coordinates). If no alignment should be performed, us a numpy array via .values, which cannot be aligned due to lacking coordinates. (no change)
• to enables the combination of linopy objects of different coordinates in the same Expression, alignment is not performed when adding linopy objects (Variable/Expression) to other linopy objects. (no change)
• Also for multiplication of linopy objects with linopy objects no alignment should be performed, same reason as in the previous point (probably a change to the current behaviour)
• To be consistent to the previous three points, I would then also enable alignment for adding a constant term (xarray) to a linopy object, similar to the multiplication. To avoid this alignment, one could use the numpy array again. (probably a change to the current behaviour)

Order of the resulting coordinates should always be defined by the first term that actually has coordinates, so numpy arrays would be ignored even if they are first.

[FabianHofmann]

Interesting, let me think about it. I would like to have the API change as small as possible, as changes can lead to very much unexpected behaviour.