Side Effects (Array Mutations) when Accessing Dataset Attributes

[adam-grant-hendry]

Describe the bug, what's wrong, and what you expected.

As revealed in PR #3170, accessing certain dataset attributes in function scopes causes side effects: mutations of arrays. Specifically, we saw:

1. Accessing volume.active_scalars in plotting.add_volume

2918:            scalars = volume.active_scalars

led to volume['Spatial Point Data'] from the examples.load_uniform() dataset getting mutated (it was rescaled to the 0-255 range)

and attempting to loop through

for name in volume.array_names:
    ...

led name to maintain a reference to the last element in volume.array_names, causing the last item of test_plot_helper_two_volumes to not be plotted:

Steps to reproduce the bug.

Please see the problem description.

System Information

--------------------------------------------------------------------------------
  Date: Wed Aug 17 17:09:20 2022 Pacific Daylight Time

                OS : Windows
            CPU(s) : 96
           Machine : AMD64
      Architecture : 64bit
               RAM : 190.7 GiB
       Environment : Python
        GPU Vendor : NVIDIA Corporation
      GPU Renderer : Quadro RTX 8000/PCIe/SSE2
       GPU Version : 4.5.0 NVIDIA 516.94

  Python 3.8.10 (tags/v3.8.10:3d8993a, May  3 2021, 11:48:03) [MSC v.1928 64    
  bit (AMD64)]

           pyvista : 0.36.1
               vtk : 9.1.0
             numpy : 1.23.2
           imageio : 2.21.1
           appdirs : 1.4.4
            scooby : 0.5.12
        matplotlib : 3.5.3
         pyvistaqt : 0.9.0
           IPython : 8.4.0
              tqdm : 4.64.0
            meshio : 5.3.4
--------------------------------------------------------------------------------

Screenshots

Please see the problem description.

NB: I highly recommend watching:

1. Python Oddities Explained, by Trey Hunner (PyCon 2022)
2. Facts and Myths about Python names and values, by Ned Batchelder (PyCon 2015)

to best understand what is going on here.

In essence though, Python variables are C-references (e.g. int& var) and they are "pass by assignment", whereas in C variables copy by default and are pass by copy (value) by default.

[adam-grant-hendry]

Adding this here so I don't forget: I wonder if we use weakref anywhere in the code base...?