Fix a bug in QuantumCircuit.draw related to vertical_compression

[rainij]

Summary

When using QuantumCircuit.draw('text') there was a wrong character used for drawing boxes in case of vertical_compression="low". I fixed this bug by a oneliner and updated the tests. I did not find a corresponding open issue.

Details and comments

The bug is appears in this example:

from qiskit import QuantumCircuit

qc = QuantumCircuit(2)
qc.h(0)
qc.h(1)

drawing_high = qc.draw("text", vertical_compression="high")
drawing_normal = qc.draw("text")
drawing_low = qc.draw("text", vertical_compression="low")

# Skip drawing_normal since it leads to the same result as drawing_high
print(drawing_high)
print(drawing_low)

The "high"-circuit is drawn correctly, but the second Hadamard of the seconed circuit is drawn incorrectly. It incorrectly uses the character ┤ for the upper right corner of the Box of the Gate.

     ┌───┐
q_0: ┤ H ├
     ├───┤
q_1: ┤ H ├
     └───┘
     ┌───┐
q_0: ┤ H ├
     └───┘
     ┌───┤
q_1: ┤ H ├
     └───┘

After the fix the output looks like this:

     ┌───┐
q_0: ┤ H ├
     ├───┤
q_1: ┤ H ├
     └───┘
     ┌───┐
q_0: ┤ H ├
     └───┘
     ┌───┐
q_1: ┤ H ├
     └───┘

Btw: I observed that the argument icod of the method merge_lines is somewhat "abused". Its docstring says icod (top or bot): in case of doubt, which line should have priority? Default: "top".. I am aware that I did not adhere to this statement. But My code change is consistent with other similar cases. See for example the case for the upper left corner which is treated in the same way. If somebody feels that this inconsistency is an issue I would propose to open an issue, but right now my change just fixes the bug in the simplest way possible without introducing more inconsistencies.

Tests: I actually found another kind of bug in the tests. The test class TestTextDrawerVerticalCompressionLow certainly wants to test the vertical_compression="low" feature. But two of those tests actually did not use low compression. I changed those to use low compression hence establishing two tests for my fix. I carefully checked that otherwise the output looks as before.

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[rainij]

As far as I can see the two failing tests already fail on the main branch (and seem to be completely unrelated to what I did). I observed also a linter error in the file I updated but in line 647, far away from my change. I assume that the linter is run "lazily" in some way and some update on the linter side (in the past) triggered this now.

[jakelishman]

Thanks for spotting this, and for the fix! Please could you add a one-line bugfix release note (reno new --edit fix-circuit-drawing-low-compression, then delete all but the "fixes" bit and edit) saying that you've fixed the top-right corner of low-compression circuit visualisations?

I didn't happen to see what the test failures were, but given the time you opened the PR, it probably coincided with the time that symengine version 0.10 had just released, which broke our CI temporarily.

[coveralls]

Pull Request Test Coverage Report for Build 4597914946

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 313 unchanged lines in 16 files lost coverage.
• Overall coverage increased (+0.03%) to 85.411%

---

Files with Coverage Reduction	New Missed Lines	%
qiskit/primitives/backend_estimator.py	1	95.37%
qiskit/primitives/backend_sampler.py	1	97.73%
qiskit/transpiler/passes/calibration/pulse_gate.py	1	95.45%
qiskit/transpiler/preset_passmanagers/level3.py	5	92.5%
qiskit/pulse/instructions/instruction.py	6	86.09%
qiskit/pulse/transforms/alignments.py	8	94.08%
qiskit/circuit/library/standard_gates/multi_control_rotation_gates.py	10	93.38%
qiskit/transpiler/passmanager.py	12	93.68%
qiskit/circuit/quantumcircuitdata.py	14	87.04%
qiskit/pulse/library/symbolic_pulses.py	16	94.79%

Totals
Change from base Build 4564463001:	0.03%
Covered Lines:	67422
Relevant Lines:	78938

---

💛 - Coveralls

[rainij]

@jakelishman I added the release note. Thanks for the info on symengine.

[jakelishman]

Super, thanks for this!