Gate instances (without a definition) are not qasm2-exported as opaque

[special-kay]

Information

• Qiskit Terra version: 0.15.1
• Python version: 3.8.5
• Operating system: Mac OS Darwin

What is the current behavior?

Parsing a qasm string including a custom gate definiton - as given by Gate.qasm() - using QuantumCircuit.from_qasm_str() returns QasmError: "Cannot find gate definition for 'test', line 6 file ".

This issue was reported in #3066, which was closed, but the issue persists.

Steps to reproduce the problem

from qiskit.circuit import Gate
from qiskit.circuit import QuantumCircuit

test_gate = Gate('test',num_qubits=1,params=[])
test_qasm = 'OPENQASM 2.0;\ninclude "qelib1.inc";\n\nqreg q[2];\ncreg cr[2];\ntest q[0];\n'

test_circ = QuantumCircuit.from_qasm_str(test_qasm)

What is the expected behavior?

Generate quantum circuit with custom gate without throwing error.

Suggested solutions

[1ucian0]

Custom gates should be opaque.

[special-kay]

So does the functionality to create a circuit with custom gates from a qasm string exist?

[kdk]

In QASM, you can declare a custom opaque gate using the opaque statement, like

>>> qasm = '''
... OPENQASM 2.0;
... include "qelib1.inc";
... opaque test q0;
... qreg q[1];
... test q[0];
... '''
>>> qk.QuantumCircuit.from_qasm_str(qasm).draw()
     ┌──────┐
q_0: ┤ test ├
     └──────┘

or a non-opaque gate using the gate statement like

>>> qasm = '''
OPENQASM 2.0;
include "qelib1.inc";
gate circuit8 q0,q1 {cx q0,q1; cx q1,q0; }
qreg q[2];
h q[0];
h q[1];
circuit8 q[0],q[1];
'''
>>> qc.draw()
     ┌───┐┌───────────┐
q_0: ┤ H ├┤0          ├
     ├───┤│  circuit8 │
q_1: ┤ H ├┤1          ├
     └───┘└───────────┘

In your example, you are defining a python Gate object in qiskit, but the definition for that gate is not included the qasm string passed to QuantumCircuit.from_qasm_str, leading to the error.

I did find a related bug in that user-defined Gate objects are not properly being exported to qasm as opaque gates. Is this the issue you're seeing?

>>> from qiskit.circuit import Gate
>>> g = Gate('foo', 1, [])
>>> qc.append(g, [0])
>>> print(qc.qasm())
OPENQASM 2.0;
include "qelib1.inc";
qreg q[2];
foo q[0];
>>> qk.QuantumCircuit.from_qasm_str((qc.qasm())
... )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/kevin.krsulichibm.com/q/qiskit-terra/qiskit/circuit/quantumcircuit.py", line 1730, in from_qasm_str
    return _circuit_from_qasm(qasm)
  File "/Users/kevin.krsulichibm.com/q/qiskit-terra/qiskit/circuit/quantumcircuit.py", line 2293, in _circuit_from_qasm
    ast = qasm.parse()
  File "/Users/kevin.krsulichibm.com/q/qiskit-terra/qiskit/qasm/qasm.py", line 54, in parse
    return qasm_p.parse(self._data)
  File "/Users/kevin.krsulichibm.com/q/qiskit-terra/qiskit/qasm/qasmparser.py", line 1070, in parse
    self.parser.parse(data, lexer=self.lexer, debug=self.parse_deb)
  File "/Users/kevin.krsulichibm.com/q/qiskit-terra/lib/python3.5/site-packages/ply/yacc.py", line 333, in parse
    return self.parseopt_notrack(input, lexer, debug, tracking, tokenfunc)
  File "/Users/kevin.krsulichibm.com/q/qiskit-terra/lib/python3.5/site-packages/ply/yacc.py", line 1120, in parseopt_notrack
    p.callable(pslice)
  File "/Users/kevin.krsulichibm.com/q/qiskit-terra/qiskit/qasm/qasmparser.py", line 638, in p_unitary_op_2
    self.verify_as_gate(program[1], program[2])
  File "/Users/kevin.krsulichibm.com/q/qiskit-terra/qiskit/qasm/qasmparser.py", line 130, in verify_as_gate
    + "', line", str(obj.line), 'file', obj.file)
qiskit.qasm.exceptions.QasmError: "Cannot find gate definition for 'foo', line 8 file "

[special-kay]

Yes that's the issue I'm having Kevin - thanks.

[jwoehr]

@kdk 's example:

>>> from qiskit import QuantumCircuit
>>> from qiskit.circuit import Gate
>>> qc = QuantumCircuit(1)
>>> g = Gate('foo', 1, [])
>>> qc.append(g, [0])
>>> print(qc.qasm())

does not work because QuantumCircuit._add_sub_instruction_to_existing_composite_circuits() raises on the empty instruction list.

[1ucian0]

Indeed. Renaming the issue.

@kdk says:

the bug is that the qasm that’s generated is invalid (and thus fails during parsing). It’s a QASM2 exporter bug, not a from_qasm_str bug.