Unexpected behavior of ``convert_effect`` in modelmember/povms/__init__.py.

[rileyjmurray]

I have a CPTP model, and I want to convert spam members in-place to only be TP. The following codepath gets hit in that conversion.

pyGSTi/pygsti/modelmembers/povms/__init__.py

Lines 579 to 582 in 962216e

	if not flatten_structure and isinstance(effect, ComposedPOVMEffect):
	return ComposedPOVMEffect(effect.effect_vec.copy(), # don't convert (usually static) effect vec
	_mm.operations.convert(effect.error_map, to_type, basis, "identity",
	flatten_structure))

I don't know if the code's current behavior is intended, but it doesn't return an effect that can be fed into a TPPOVM class.

Comments, @coreyostrove and @sserita?

[rileyjmurray]

Okay, I now see that I should have set flatten_structure if I wanted effects to be changed. So this isn't necessarily a bug. Can anyone think of a reason why we'd be okay with ComposedPOVMEffects inside a TPPOVM?

[coreyostrove]

I can't think of any instance where we'd expect that to be the case and enforcing the intuitive conversion behavior is fine by me.

[rileyjmurray]

Here's a minimal example, per @coreyostrove's request in chat:

from pygsti.modelpacks import smq1Q_XYI
model = smq1Q_XYI.target_model()
model.convert_members_inplace('full TP') # no error
model.convert_members_inplace('CPTPLND') # no error
model.convert_members_inplace('full TP') # error

The specific error is

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/rjmurr/Documents/pygsti-general/pyGSTi/pygsti/models/explicitmodel.py", line 359, in convert_members_inplace
    self._clean_paramvec()  # param indices were probabaly updated
  File "/Users/rjmurr/Documents/pygsti-general/pyGSTi/pygsti/models/model.py", line 754, in _clean_paramvec
    self._rebuild_paramvec()
  File "/Users/rjmurr/Documents/pygsti-general/pyGSTi/pygsti/models/model.py", line 1091, in _rebuild_paramvec
    w[obj.gpindices] = obj.to_vector()
  File "/Users/rjmurr/Documents/pygsti-general/pyGSTi/pygsti/modelmembers/povms/tppovm.py", line 98, in to_vector
    assert isinstance(effect, _FullPOVMEffect)
AssertionError