Qiskit-terra: Users cannot name variables with the same names as gates

[rraymondhp]

I cannot define variables in python named as "x" due to conflict with gates of similar names. I think we should be able to let users use the names.

Expected Behavior

Current Behavior

#import library for drawing quantum circuit
from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister
from qiskit import QuantumProgram
from qiskit.backends import discover_local_backends, discover_remote_backends
from qiskit.tools.visualization import circuit_drawer

x = QuantumRegister("x", 6)
c = ClassicalRegister("c", 1) #classical registers is used as dummy
qc = QuantumCircuit(x, c)
qc.x(x[2])
qc.cx(x[1], x[5])
qc.cx(x[2], x[5])
qc.cx(x[3], x[5])
qc.ccx(x[1], x[2], x[4])
qc.ccx(x[3], x[4], x[5])
qc.ccx(x[1], x[2], x[4])
circuit_drawer(qc)

Running the above code results in QASM error:

QasmError                                 Traceback (most recent call last)
<ipython-input-2-3eec3d24b52c> in <module>()
     15 qc.ccx(x[3], x[4], x[5])
     16 qc.ccx(x[1], x[2], x[4])
---> 17 circuit_drawer(qc)

~/miniconda3/envs/QISKitenv/lib/python3.6/site-packages/qiskit/tools/visualization.py in circuit_drawer(circuit, basis, scale)
    714     with tempfile.TemporaryDirectory() as tmpdirname:
    715         latex_drawer(circuit, filename=os.path.join(tmpdirname, filename + '.tex'),
--> 716                      basis=basis, scale=scale)
    717         im = None
    718         try:

~/miniconda3/envs/QISKitenv/lib/python3.6/site-packages/qiskit/tools/visualization.py in latex_drawer(circuit, filename, basis, scale)
    785         str: Latex string appropriate for writing to file.
    786     """
--> 787     ast = qasm.Qasm(data=circuit.qasm()).parse()
    788     if basis:
    789         # Split basis only if it is not the empty string.

~/miniconda3/envs/QISKitenv/lib/python3.6/site-packages/qiskit/qasm/_qasm.py in parse(self)
     57         with QasmParser(self._filename) as qasm_p:
     58             qasm_p.parse_debug(False)
---> 59             return qasm_p.parse(self._data)

~/miniconda3/envs/QISKitenv/lib/python3.6/site-packages/qiskit/qasm/_qasmparser.py in parse(self, data)
   1073     def parse(self, data):
   1074         """Parse some data."""
-> 1075         self.parser.parse(data, lexer=self.lexer, debug=self.parse_deb)
   1076         if self.qasm is None:
   1077             raise QasmError("Uncaught exception in parser; "

~/miniconda3/envs/QISKitenv/lib/python3.6/site-packages/ply/yacc.py in parse(self, input, lexer, debug, tracking, tokenfunc)
    329             return self.parseopt(input, lexer, debug, tracking, tokenfunc)
    330         else:
--> 331             return self.parseopt_notrack(input, lexer, debug, tracking, tokenfunc)
    332 
    333 

~/miniconda3/envs/QISKitenv/lib/python3.6/site-packages/ply/yacc.py in parseopt_notrack(self, input, lexer, debug, tracking, tokenfunc)
   1116                             del symstack[-plen:]
   1117                             self.state = state
-> 1118                             p.callable(pslice)
   1119                             del statestack[-plen:]
   1120                             symstack.append(sym)

~/miniconda3/envs/QISKitenv/lib/python3.6/site-packages/qiskit/qasm/_qasmparser.py in p_qreg_decl(self, program)
    478         if program[2].index == 0:
    479             raise QasmError("QREG size must be positive")
--> 480         self.update_symtab(program[0])
    481 
    482     def p_qreg_decl_e(self, program):

~/miniconda3/envs/QISKitenv/lib/python3.6/site-packages/qiskit/qasm/_qasmparser.py in update_symtab(self, obj)
     81                             + ', file', obj.file
     82                             + '.\nPrevious occurence at line',
---> 83                             str(prev.line) + ', file', prev.file)
     84         self.current_symtab[obj.name] = obj
     85 

QasmError: "Duplicate declaration for qreg 'x' at line 3, file .\nPrevious occurence at line 22, file /Users/rraymondhp/miniconda3/envs/QISKitenv/lib/python3.6/site-packages/qiskit/qasm/libs/qelib1.inc"

Possible Solution

I have no ideas

Steps to Reproduce (for bugs)

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Context

Your Environment

• Version used:
• Environment name and version (e.g. Python 3.6.1):
• Operating System and version:

[ajavadia]

I'm gonna say this is not a bug, and necessary for correct parsing of OpenQASM. A parser builds a symbol table during parsing, and these symbols cannot clash. In OpenQASM, certain string tokens are keywords, like the "x" gate. For that reason, you cannot reuse that keyword in other places. Similar to how in C, you can't name a variable in "int".

@diego-plan9 @atilag @1ucian0 what do you guys think?

[jaygambetta]

I agree it’s not a bug. Better documentation and error handling is needed in the future.