dag_drawer should check the existence of filename extension

[we-taper]

Information

• Qiskit Terra version: 0.10.0.dev0+831d942
• Python version: 3.7
• Operating system: Mac

What is the current behavior?

If a filename without extension is passed to the function dag_drawer, this line reports two errors:

nxpd.pydot.InvocationException: Program terminated with status: 1. stderr follows: Format: "XXXXX" not recognized. Use one of: ......
During handling of the above exception, another exception occurred:
qiskit.visualization.exceptions.VisualizationError: 'dag_drawer requires GraphViz installed in the system. Check https://www.graphviz.org/download/ for details on how to install GraphViz in your system.'

This is confusing because the second error thrown by Qiskit is not the cause of the problem.

Steps to reproduce the problem

Try dag_drawer(dag, filename='abc')

What is the expected behavior?

Make the error catching better.

Suggested solutions

We could either catch this error by reading and filtering the error message, or we could check the existence of the filename's extension, and provide a default one.

[kdk]

Hi @we-taper , thanks for reporting, and I agree the conflicting error messages are confusing. It looks like the error catching around https://github.com/Qiskit/qiskit-terra/blob/d090eca91dc1afdb68f563885c4ccf13b31de20e/qiskit/visualization/dag_visualization.py#L91 could be refined to either inspect the raised nxpd.pydot.InvocationException to try to determine the cause of the error, or at least include the generated stderr in the VisualizationError that's raised by qiskit.

[Rajan117]

Newcomer here, recently started learning QisKit. Would not mind taking a crack at this.

[javabster]

go for it @Rajan117! I've assigned to you, let us know if you have any questions

[Rajan117]

Thanks.
I am using this code to reproduce the error.
`from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
from qiskit.dagcircuit import DAGCircuit
from qiskit.converters import circuit_to_dag
from qiskit.visualization import dag_drawer

q = QuantumRegister(3, 'q')
c = ClassicalRegister(3, 'c')
circ = QuantumCircuit(q, c)
circ.h(q[0])
circ.cx(q[0], q[1])
circ.measure(q[0], c[0])
circ.rz(0.5, q[1]).c_if(c, 2)

dag = circuit_to_dag(circ)
dag_drawer(dag, filename='abc')`

But I instead get this error.

I assume I have done something wrong in setting up my test environment but unsure how, followed the official QisKit YouTube guide.

[Rajan117]

I will be honest, I have had a lot of issues in setting up test environments to attempt fixing issues. Is there an easy way to do it?

[javabster]

I will be honest, I have had a lot of issues in setting up test environments to attempt fixing issues. Is there an easy way to do it?

It's best to follow the steps outlined here for the best way to set up your environment (including pip install -e .)

[Rajan117]

Thanks! Will give this another go over the weekend then :)

[1ucian0]

unassigning due to lack of activity.

[iuliazidaru]

from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
from qiskit.dagcircuit import DAGCircuit
from qiskit.converters import circuit_to_dag
from qiskit.visualization import dag_drawer

q = QuantumRegister(3, 'q')
c = ClassicalRegister(3, 'c')
circ = QuantumCircuit(q, c)
circ.h(q[0])
circ.cx(q[0], q[1])
circ.measure(q[0], c[0])
circ.rz(0.5, q[1]).c_if(c, 2)

dag = circuit_to_dag(circ)
dag_drawer(dag, filename='abc')

"dot" with args ['-Tabc', '/tmp/tmp9ngv_7rl'] returned code: 1

stdout, stderr:
 b''
b'Format: "abc" not recognized. Use one of: bmp canon cmap cmapx cmapx_np dot dot_json eps fig gd gd2 gif gv ico imap imap_np ismap jpe jpeg jpg json json0 mp pdf pic plain plain-ext png pov ps ps2 svg svgz tif tiff tk vdx vml vmlz vrml wbmp webp xdot xdot1.2 xdot1.4 xdot_json\n'

Traceback (most recent call last):
  File "/tmp/ipykernel_78/1404195018.py", line 15, in <module>
    dag_drawer(dag, filename='abc')
  File "/opt/conda/lib/python3.8/site-packages/qiskit/visualization/dag_visualization.py", line 170, in dag_drawer
    dot.write(filename, format=extension)
  File "/opt/conda/lib/python3.8/site-packages/pydot.py", line 1828, in write
    s = self.create(prog, format, encoding=encoding)
  File "/opt/conda/lib/python3.8/site-packages/pydot.py", line 1956, in create
    assert process.returncode == 0, (
AssertionError: "dot" with args ['-Tabc', '/tmp/tmp9ngv_7rl'] returned code: 1

Use %tb to get the full traceback.

I think the message is pretty clear now.

[javabster]

thanks for checking if this is still valid @iuliazidaru! However I think the error is still not very user friendly and it would be good if we could catch it better in the dag_drawer

[iuliazidaru]

@javabster Please assign the issue to me.