Instruction.params updates not reflected in definition (if definition already called)

Information

• Qiskit Terra version: master
• Python version: 3.5
• Operating system: osx 10.14

What is the current behavior?

.params values are fixed in gate definitions the first time ._define is called, and not updated, even if Instruction.params changes.

>>> from qiskit.extensions.standard import RZGate
>>> # Create a gate, and check the parameter values in its definition
...
>>> g = RZGate(0.1)
>>> g.definition
[(<qiskit.extensions.standard.u1.U1Gate object at 0x10b181208>, [Qubit(QuantumRegister(1, 'q'), 0)], [])]
>>> g.definition[0][0].params
[0.1]
>>> # Create a gate, change its parameters, then check parameter values in its definition
...
>>> g = RZGate(0.1)
>>> g.params[0] = 0.2
>>> g.definition[0][0].params
[0.2]
>>> # Create a gate, check its definition, updates its params, and see if the update is reflected in the definition
...
>>> g = RZGate(0.1)
>>> g.definition[0][0].params
[0.1]
>>> g.params[0] = 0.2
>>> g.definition[0][0].params
[0.1]

[Cryoris]

Personally, I’m not convinced params should be publically writeable at all. It might be both safer to users and easier for us to handle if gates define setters for mutable attributes. Parameterized gates, for instance, can be bound (or re-bound in future?) to certain values but covering the bind_parameters path and a direct access seems overcomplicated. ]

[jwoehr]

Still valid. Here’s a cleanup of the example:

from qiskit.circuit.library import RZGate
print('Create a gate, and check the parameter values in its definition')
g = RZGate(0.1)
print(g.definition)
print(g.definition[0][0].params)
print('Create a gate, change its parameters, then check parameter values in its definition')
g = RZGate(0.1)
g.params[0] = 0.2
print(g.definition[0][0].params)
print('Create a gate, check its definition, updates its params, and see if the update is reflected in the definition')
g = RZGate(0.1)
print(g.definition[0][0].params)
g.params[0] = 0.2
print(g.definition[0][0].params)