Misleading error messages from quantum circuits

[pennylaneai]

Misleading error messages from quantum circuits

[cgogolin]

I had the same problem. If a gate in the circuit throws an error, the same thing happens. The actual error is preserved, but the last error in the output is the "A qfunc must always conclude with a classical expectation value." error.

[josh146]

Preliminarily fixed in a42497b. The issue was the context manager Device. If any exception was raised within the Device context manager, any corresponding exceptions within __exit__() were also raised; to fix this, I removed all circuit validations from __exit__(). Instead, they now take place within QNode.construct().

A side of effect of this is that circuit validation now requires the use of the return statement, in order to check the form of the expectations required.

[co9olguy]

Sounds like a good side effect :)

[josh146]

Needs more checking/testing to ensure that unexpected exceptions propagate as expected! I'll add exception handling tests to my todo list