Fix UnitaryGate.repeat() method

[timmintam]

Summary

Closes #11990 : error when calling the UnitaryGate.repeat() method (inherited from the Gate class).

Details and comments

The method repeat() internally calls the private method _return_repeat() which is inherited from the Gate class.
It creates a new Gate object with arguments params=self.params, which is expected to be of type ParameterExpression or float. However the parameters are of type numpy.ndarray for a UnitaryGate.
A simple fix is to omit these parameters (params=[]) as they are not directly used by the returned Gate. Actually these parameters are still stored in the repeat() method and used only there.

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One or more of the following people are relevant to this code:

• @Cryoris
• @Qiskit/terra-core
• @ajavadia

[ElePT]

Thanks a lot @timmintam! The PR looks pretty good, I just added a small suggestion

[Cryoris]

This would also happen for a set of other gates that have special params attributes, right? E.g. Isometry (also array), UCGate (list of arrays), PermutationGate (matrix of ints)... If it does, it would be nice to fix this generally 🙂

These would fail because the custom validate_parameter functions are not being used, since _return_repeat constructs a generic Gate. Maybe we could pass that function along?

[timmintam]

This would also happen for a set of other gates that have special params attributes, right? E.g. Isometry (also array), UCGate (list of arrays), PermutationGate (matrix of ints)... If it does, it would be nice to fix this generally 🙂

These would fail because the custom validate_parameter functions are not being used, since _return_repeat constructs a generic Gate. Maybe we could pass that function along?

You are right @Cryoris, this would fail for other gates as well actually. Do you mean something along these lines this in the Gate class ?

def _return_repeat(self, exponent: float) -> "Gate":
   gate = Gate(name=f"{self.name}*{exponent}", num_qubits=self.num_qubits, params=[])
   gate.validate_parameter = self.validate_parameter
   gate.params = self.params
   return gate

It seems to do the trick.

[Cryoris]

Yeah exactly! 🙂 An alternative solution would be to extend the Gate.__init__ to accept a custom validate_parameter, but in Python we can just override this function and it should be fine.

Could you also add tests for these other gate types that fail?

[coveralls]

Pull Request Test Coverage Report for Build 10525187951

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 89.59%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
crates/qasm2/src/lex.rs	6	91.73%

Totals
Change from base Build 10525107270:	0.01%
Covered Lines:	67787
Relevant Lines:	75664

---

💛 - Coveralls

[ElePT]

Thanks a lot timmintam, just to err on the side of caution, could you also add a test for the PermutationGate case in test/python/circuit/library/test_permutation.py?

[ElePT]

Nice! The changes look good to me, but let's wait to see if @Cryoris has any final comment before merging.

[Cryoris]

I think this needs a trailing underscore (but @ElePT might know this better than me 🙂)

Suggested change

	Refer to `#11990 <https://github.com/Qiskit/qiskit/issues/11990>` for more
	Refer to `#11990 <https://github.com/Qiskit/qiskit/issues/11990>`__ for more

[Cryoris]

Nice, thanks for the update! Sorry I wasn't explicit about all gates that would need this kind of testing, this should be the complete list:

• Initialize (though this is an Instruction so this might need the same fix on the Instruction class)
• StatePreparation
• DiagonalGate
• LinearFunction (not called gate but is a gate 🙂 )
• PauliGate
• UCPauliRotGate
• HamiltonianGate
  Could you add a small test for these as well? They can all also be on 1 or 2 qubits max like what you already have 🙂

[Cryoris]

LGTM thanks for the updates 🙂

[ElePT]

LGTM to me too!