I am using Jupyter notebook to write and running my qiskit codes (python 3.6) and every time I encounter the message: LookupError: backend "ibmqx4" is not found, right now the ibmqx4 computer is not in maintenance and it running well.

I tried to regenerate Apitoken in the advanced option and make correspondingly the file Qconfig.py but the message still appears.

look at the example for a code that I write:

import numpy as np
import matplotlib.pyplot as plt
import qiskit as qk
from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister, execute, register


import Qconfig



    batch = []
n=50
t=450
// we defining n similar quantum circuits
for j in range(1,n+1):
    q = QuantumRegister(1)
    c = ClassicalRegister(1)
    qc = QuantumCircuit(q, c)
    qc.x(q)
    qc.barrier()


// we decide the pausing time of the system be t*Gate Time
    for i in range(t):
        qc.iden(q)
    qc.measure(q,c)    
   //we append every circuit we create to batch so batch is like a list of n 
  circuits
    batch.append(qc)

// we executing all the n circuits and for each we run 1024 shots
shots = 1024
job = execute(qc,'ibmqx4',shots=shots)

and I get the output from the last row:

---------------------------------------------------------------------------
LookupError                               Traceback (most recent call last)
<ipython-input-38-a89f2171589f> in <module>()
      1 # we executing all the n circuits and for each we run 1024 shots
      2 shots = 1024
----> 3 job = execute(qc,'ibmqx4',shots=shots)

~\Anaconda3\envs\QISKit\lib\site-packages\qiskit\wrapper\_wrapper.py in execute(circuits, backend, config, basis_gates, coupling_map, initial_layout, shots, max_credits, seed, qobj_id, hpc, skip_transpiler)
    270     # pylint: disable=missing-param-doc, missing-type-doc
    271     if isinstance(backend, str):
--> 272         backend = _DEFAULT_PROVIDER.get_backend(backend)
    273     qobj = compile(circuits, backend,
    274                    config, basis_gates, coupling_map, initial_layout,

~\Anaconda3\envs\QISKit\lib\site-packages\qiskit\wrapper\defaultqiskitprovider.py in get_backend(self, name)
     29 
     30     def get_backend(self, name):
---> 31         name = self.resolve_backend_name(name)
     32         for provider in self.providers:
     33             try:

~\Anaconda3\envs\QISKit\lib\site-packages\qiskit\wrapper\defaultqiskitprovider.py in resolve_backend_name(self, name)
    218 
    219         if resolved_name not in available:
--> 220             raise LookupError('backend "{}" not found.'.format(name))
    221 
    222         return resolved_name

LookupError: backend "ibmqx4" not found.

please help me to solve this repetitive bug...

up vote 2 down vote accepted

I got it, there is need to add two codes :

the first one in the begining before starting programming:

import  sys,,  time,,  getpass
try:
    sys.path.append("../../") # go to parent dir
    import Qconfig
    qx_config = {
        "APItoken": Qconfig.APItoken,
        "url": Qconfig.config['url']}
    print('Qconfig loaded from %s.' % Qconfig.__file__)
except:
    APItoken = getpass.getpass('Please input your token and hit enter: ')
    qx_config = {
        "APItoken": APItoken,
        "url":"https://quantumexperience.ng.bluemix.net/api"}
    print('Qconfig.py not found in qiskit-tutorial directory; Qconfig loaded using user input.')

the second one is before execution:

register(qx_config['APItoken'], qx_config['url'])

hope I helped anyone with similar error!

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