Python: useful error when initialize_warpx not called before creating ParticleContainerWrapper

[archermarx]

Currently, WarpX is initialized when sim.step is called, or when the user calls initialize_warpx. However, if the user tries to create a ParticleContainerWrapper before this point, they get an error along the following lines:

 File "/home/marksta/projects/warpx-ionization/picmi.py", line 185, in <module>
    sim.run()
  File "/home/marksta/projects/warpx-ionization/picmi.py", line 179, in run
    self.elec_wrapper = particle_containers.ParticleContainerWrapper(self.electrons.name)
  File "/home/marksta/.local/lib/python3.10/site-packages/pywarpx/particle_containers.py", line 29, in __init__
    mypc = libwarpx.warpx.multi_particle_container()
  File "/home/marksta/.local/lib/python3.10/site-packages/pywarpx/_libwarpx.py", line 46, in __getattr__
    return self.__getattribute__(attribute)
AttributeError: 'LibWarpX' object has no attribute 'warpx'

This is confusing. When I got this, I assumed I had maybe installed WarpX wrong. I added a catch for this exception that re-raises it with some additional context that should help direct the user to call initialize_warpx

[dpgrote]

I would suggest handling this in a different way. Now, the issue is that the libwarpx.warpx is accessed in the __init__ routine. A change would be to only access libwarpx.warpx when it is needed. Perhaps make particle_container a property that gets the underlying particle container when needed. The user can then create the ParticleContainerWrapper at any time. The message this PR adds would then go in the particle_container property get method. This is how the field wrappers are setup.

[archermarx]

Got it, thanks for the feedback!

[archermarx]

Ok, I've modified the PR to now lazy initialize the particle container (and boundary buffers) only when they're first needed instead of in __init__. Only then is an error thrown.

[archermarx]

Changes made. That's definitely a lot cleaner.

[dpgrote]

Thanks! This looks good to me.

[ax3l]

Thank you for this PR! Sorry for just seeing it now, I was just on parental leave when the PR was opened.

I thought we could also add one more fallback, right in the library loader, where we assign the libwarpx.warpx attribute.

In essence, we could assign an initial attribute that throws when accessed with a more distinct error message ("initialize_warpx was not yet called!"), to catch all. More specific exceptions like this one are great to add and keep.

One initialization, we then just overwrite the attribute.

[archermarx]

Oh neat idea! I can open a new PR shortly!