Flag "is_executed" is defined but not implemented

[mlazzarin]

The "is_executed" flag in the AbstractCircuit class is defined here

qibo/src/qibo/abstractions/circuit.py

Lines 91 to 93 in 14bb454

	# Flag to keep track if the circuit was executed
	# We do not allow adding gates in an executed circuit
	self.is_executed = False

but it seems like it is not accessed nor modified anywhere in the source code of Qibo.
Indeed, it always returns False. The "add" method ignores it and simply checks for "_final_state"

qibo/src/qibo/abstractions/circuit.py

Lines 390 to 392 in 4073ce9

	if self._final_state is not None:
	raise_error(RuntimeError, "Cannot add gates to a circuit after it is "
	"executed.")

This is a minor bug but it may cause issues if such flag is used by unaware users.
Should we implement it properly (or simply remove it)?

[scarrazza]

@mlazzarin thanks for pointing this out. I think we forgot to remove this attribute after some cleanup, nowadays we check this features in

qibo/src/qibo/abstractions/circuit.py

Lines 390 to 392 in 14bb454

	if self._final_state is not None:
	raise_error(RuntimeError, "Cannot add gates to a circuit after it is "
	"executed.")

@stavros11 do you agree, right?

[stavros11]

@mlazzarin thank you for checking the code. Indeed you may find some attributes around the code that are not used anymore because we forgot to remove them during a cleanup or refactoring. This particular attribute seems redundant to me as any if self.is_executed could be replaced by if self._final_state is not None since, if I am not mistaken, the final state attribute is assigned after a circuit execution.

I don't think such attributes would cause problems in every day usage but may confuse people that look at the code and should be removed. @mlazzarin you can remove this if you like.