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POEM ID: 054
Title: Specifying source array as flat or non-flat when setting src_indices or dv or constraint indices

Authors: [naylor-b]

Competing POEMs: N/A

Related POEMs: [https://github.com/OpenMDAO/POEMs/blob/master/POEM_053.md]

Associated implementation PR: #2279

Status:

  • Active
  • Requesting decision
  • Accepted
  • Rejected
  • Integrated

Motivation

In POEM 53, we modified the behavior of indexing to mimic the way indexing behaves when applied to numpy arrays, but in the process of doing this we accidentally left out certain behaviors related to indexing into a non-flat source array. In the existing implementation, if an index is flat, for example, [1], or [2, 4, 6], the source array is assumed to be flat, but indexing into a numpy array doesn't behave that way. For example, when indexing into a 3x3 numpy source array, an index of [1] would result in a 1x3 array, but using that same [1] to index into an openmdao 3x3 array would result in a size 1 array.

Description

Indexing in openmdao appears in 2 groups of methods. The first group, Group.connect, Group.promotes, and Component.add_input, all have a src_indices argument that specifies how an input variable is connected to its source variable. These methods also have a flat_src_indices argument that, if True, will specify that the indices are assumed to index into a flattened source. By setting this argument appropriately the user can get either the old openmdao behavior (flat_src_indices=True), or the more numpy-like behavior (flat_src_indices=False). Note that specifying src_indices in Component.add_input is deprecated and will be prohibited in some future release.

The second group, System.add_design_var, System.add_constraint, and System.add_objective, all specify which part of a given output variable should be used by openmdao as a design variable, constraint, or objective. These methods have an indices argument (or an index argument for add_objective) to specify which part, and a new argument, flat_indices, will be added to allow the user to have the same flexibility as in the first group of methods mentioned above.

This POEM will be implemented in two parts. The first part will we a transitional release that will not break backwards compatability. It will add the new flat_indices argument and set its default value, as well as the default value of flat_src_indices to None. If the user does not set the flat_* flag value and the number of dimensions of the source array are greater then the number of dimensions indicated by the indices, a warning will be issued describing the proper use of the flat_src_indices and flat_indices flags. If the flag value is set to True, there will be no warning and the source array will be treated as flat. If the flat is set to False, an exception will be raised because internally the source array will still be treated as flat so setting flat_* to False isn't a valid option.

The next part will be a release that removes the deprecation warnings and changes the internal behavior to no longer assume a flat source regardless of the number of dimensions of the indices. The default values of flat_src_indices and flat_indices will be changed to False. If a model ran under the previous release without any deprecation warnings related to assuming a flat source, then it should run without modification in this release.

Note also that flat_src_indices and flat_indices only have meaning for indices that appear flat. For example, setting flat_src_indices=True for src_indices=om.slicer[:, 2] doesn't make any sense because the source array in that case would have to have at least 2 dimensions in order for the index to be valid. In cases like this, the flag values will be ignored and a warning will be generated.