Add good __repr__ methods to all public classes

What should we add?

Informative reprs are very helpful, especially for debugging purposes. Ideally, repr(obj) should yield executable code that reconstructs obj, and we could test this. Good reprs are defined inconsistently across the library. For example,

from qiskit import QuantumCircuit, QuantumRegister

print(repr(QuantumRegister(1)))
print(repr(QuantumCircuit(1)))

QuantumRegister(1, 'q0')
<qiskit.circuit.quantumcircuit.QuantumCircuit object at 0x7f372c1dfac0>

The following classes require attention:

• Gate
• QuantumCircuit
• Sampler

[jakelishman]

Given that it’s an alternate class method constructor, I’m still not sure it would be appropriate for a repr (and Instruction, Qubit and Clbit don’t general have a evalable reprs either, which would be required). The point of repr is to give a human-readable indication of what the object is, rather than to implement a complete serialisation to text - it’s just that for small, simple objects, an evalable string is a very legible description of the object.

For QuantumCircuit, there’s loads of internal attributes like global phase, metadata, scheduling information, stored internal layout, etc etc that would make a “complete” textual serialisation super long and really hard to read. The simplified version I suggested above I think is still good.

[jakelishman]

I’ve tagged this “good first issue”, though we wouldn’t want one PR to add reprs to every class - that would be too much work, and a good repr needs a little bit of thought, so it’s not an entirely mechanical process. Instead, a good first community PR would be to add a __repr__ method to QuantumCircuit that returns a string like

<QuantumCircuit '<name>' with <x> qubits, <y> clbits and <z> instructions>