behaviour of qasmparser is unclear when parse is called multiple times

[dlyongemallo]

It's unclear how to use qasmparser if there are multiple strings or circuits to be parsed. In particular, parse has behaviour which is surprising to me. I expect each call to basically be independent and return a circuit based on the input string. What actually happens is parse builds on top of the circuit processed in previous calls.

In particular, it looks to me like this line is intended to clear the previous state, but is bugged because gates rather than self.gates is used.

Repro case:

from pyzx.circuit.qasmparser import QASMParser
s = """
OPENQASM 2;
include "qelib1.inc";
qreg q[1];
h q[0];
"""
p = QASMParser()
c1 = p.parse(s)
c2 = p.parse(s)
print(c1)
print(c1.gates)
print(c2)
print(c2.gates)

Expected output:

Circuit(1 qubits, 0 bits, 1 gates)
[HAD(0)]
Circuit(1 qubits, 0 bits, 1 gates)
[HAD(0)]

Actual output:

Circuit(1 qubits, 0 bits, 2 gates)
[HAD(0), HAD(1)]
Circuit(2 qubits, 0 bits, 2 gates)
[HAD(0), HAD(1)]

In particular, it is surprising that a subsequent call to parse alters the value of c1.

[jvdwetering]

Yes, I agree that that is weird and unexpected. I would call that a bug.