Provide a workaround to #422, Sampler failing when no measurements

[garrison]

Temporary workaround to #422.

Needs

• a test

[CLAassistant]

All committers have signed the CLA.

[ibrahim-shehzad]

I added a test, @garrison, in test_cutting_roundtrip.py, called test_sampler_fail.

[caleb-johnson]

Will these circuits with 1 measurement be ignored during reconstruction, since they don't actually have non-trivial observables associated with them?

[caleb-johnson]

What can pauli_indices be here? Should we just explicitly state what we expect? (e.g. if pauli_indices == [])

Or we could just assert pauli_indices == [] inside the if? (assuming an empty list is what is returned in I observable case)

[garrison]

What can pauli_indices be here? Should we just explicitly state what we expect? (e.g. if pauli_indices == [])

It's a sequence of ints. It contains the qubit indices that need to be measured.

The choice of style here is influenced by PEP 8:

For sequences, (strings, lists, tuples), use the fact that empty sequences are false:

# Correct:
if not seq:
if seq:

# Wrong:
if len(seq):
if not len(seq):

The "nice" thing about doing it this way is it doesn't matter what type the sequence is. For instance, if it were to be a tuple of indices, then it will still behave correctly with the current code, but it would not if we were to compare with [] because () != [].

[caleb-johnson]

nice, thanks for explanation

[ibrahim-shehzad]

Wow, this is more profound than I thought. Nice!

[garrison]

Will these circuits with 1 measurement be ignored during reconstruction, since they don't actually have non-trivial observables associated with them?

Yes

[caleb-johnson]

LGTM 👍

Thanks @ibrahimshehzad and @garrison !

[garrison]

I am leaning toward waiting first until #434 is merged and then updating this branch accordingly.

EDIT: Actually, I might have changed my mind already. 😂

[ibrahim-shehzad]

Everything now looks good to me. Thanks.