How do I fix the UnboundLocalError?

Question

I have read about the global and nonlocal variable and I know they may solve the problem, but as I'm still naive at this I couldn't figure out how to implement them to solve the UnboundLocalError. Sorry for the long program I put here:

 # initialization

import numpy as np

# import Qiskit

from qiskit import IBMQ, BasicAer
from qiskit.providers.ibmq import least_busy
from qiskit import QuantumCircuit, execute
from qiskit.tools.jupyter import *
#provider= IBMQ.load.account()
 provider= IBMQ.load_account()

#import basic plot tools
from qiskit.visualization import plot_histogram

Here is the Dj function

 def dj_oracle(case, n):
#We need to make a QuantumCircuit object to return
#This circuit has n+1 qubits: the size of the input,
#plus one output qubit
oracle_qc=QuantumCircuit(n+1)

#First, let's deal with the case in which oracle is balanced

if case == "balanced":
    # We apply controlled-NOT gates for each qubit, using the
    #output qubit as target
    
    for qubit in range(n):
        oracle_qc.cx(qubit, n)
        
 # case in oracle is constant
if case == "constant":

# First decide what the fixed output of the oracle will be
# (either always 0 or always 1)
 
    output = np.random.randint(2)
    
if output == 1:
    
    oracle_qc.x(n)
    
oracle_gate = oracle_qc.to_gate()
oracle_gate.name ="Oracle"
return oracle_gate

and Here is the dj_algorithm

  def dj_algorithm(n, case='random'):
dj_circuit=QuantumCircuit(n+1, n)
# Set up the input register:
for qubit in range(n):
    dj_circuit.h(qubit)
#And set up the output qubit:
dj_circuit.x(n)
dj_circuit.h(n)
#Let's append the oracle gate to our circuit:

if case == 'random':
    random = np.random.randint(2)
    if random == 0:
        case = 'constant'
else: 
    case = 'balanced'
    oracle = dj_oracle(case, n)
    dj_circuit.append(oracle, range(n+1))
 # Finally, perform the H-gates again and measure:
for i in range(n):
        dj_circuit.h(i)
        dj_circuit.measure(i,i)
return dj_circuit

Now when I want to use the local simulator to get the results, I face the UnboundLocalError, Can anybody help me to solve it?

This is the error I get:

---

  UnboundLocalError                         Traceback (most recent call last)
  <ipython-input-112-f0c5e2ae7e9a> in <module>
  2 backend = BasicAer.get_backend('qasm_simulator')
  3 shots = 1024
  ----> 4 dj_circuit = dj_algorithm(n,'constant')
  5 results = execute(dj_circuit, backend=backend, shots=shots).result()
  6 answer = results.get_counts()

 <ipython-input-89-6104e1173619> in dj_algorithm(n, case)
 15     else:
 16         case = 'balanced'
 ---> 17         oracle = dj_oracle(case, n)
 18         dj_circuit.append(oracle, range(n+1))
 19      # Finally, perform the H-gates again and measure:

 <ipython-input-110-75bb51aa8b15> in dj_oracle(case, n)
 22         output = np.random.randint(2)
 23 
 ---> 24     if output == 1:
 25 
 26         oracle_qc.x(n)

 UnboundLocalError: local variable 'output' referenced before assignment

Answer 1

This is a Python related question. I think it should be asked in stackoverflow

This error happens because you try to check the value of the variable output before assigning a value to it.
As you may know, Python is space sensitive. Hence, the line where you check if output equals 1 will be executed regardless of case value. On the other hand, the variable output will be initialized only if case equals "constant".
This means this error will happen whenever case equals "balanced"