Add pytest for visualization.py

[tamakoshi2001]

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Description
pytest for #2170

[tamakoshi2001]

Here is a sample pytest for functions in visualization.py

[coveralls]

coverage: 83.199% (+5.0%) from 78.186% when pulling 385746c on tamakoshi2001:pytest-visualization into 7cf468c on qutip:master.

[Ericgig]

• It's better not to call qutip function in the parametrization step, if there is a bug in that function, it stop all tests from running.
• It's better to just parametrize on the variable we actually are interested in. All the None make it unclear what it tested in that particular test. It also makes test hard to read in the log:

qutip/tests/test_visualization.py::test_hinton[rho0-None-None-scaled-True-None-None-None-None] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton[rho1-None-None-scaled-True-None-None-None-None] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton[rho2-None-None-scaled-True-None-None-None-None] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton[rho3-None-None-scaled-True-None-None-None-None] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton[rho4-x_basis4-None-scaled-True-None-None-None-None] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton[rho5-None-y_basis5-scaled-True-None-None-None-None] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton[rho6-None-None-scaled-True-None-None-None-None] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton[rho7-None-None-threshold-True-None-None-None-None] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton[rho8-None-None-phase-True-None-None-None-None] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton_ValueError[rho0-None-None-scaled-True-None-None-None-None-Input quantum object must be an operator or superoperator.] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton_ValueError[rho1-None-None-color_style-True-None-None-None-None-Unknown color style color_style for Hinton diagrams.] PASSED [ 16%]
qutip/tests/test_visualization.py::test_hinton_ValueError[rho2-None-None-scaled-True-None-None-None-None-Hinton plots of superoperators are currently only supported for qubits.] PASSED [ 16%]

Suggested change

	@pytest.mark.parametrize('rho, x_basis, y_basis, color_style,\
	label_top, cmap, colorbar, fig, ax', [
	(qutip.rand_dm(5), None, None, "scaled", True, None, None, None, None),
	(qutip.operator_to_vector(qutip.rand_dm(5)),
	None, None, "scaled", True, None, None, None, None),
	(qutip.operator_to_vector(qutip.rand_dm(5)).dag(),
	None, None, "scaled", True, None, None, None, None),
	(qutip.spre(qutip.rand_dm(4)),
	None, None, "scaled", True, None, None, None, None),
	(qutip.rand_dm(5),
	[1, 2, 3, 4, 5], None, "scaled", True, None, None, None, None),
	(qutip.rand_dm(5),
	None, [1, 2, 3, 4, 5], "scaled", True, None, None, None, None),
	(qutip.rand_dm(5), None, None, "scaled", True, None, None, None, None),
	(qutip.rand_dm(5), None, None, "threshold", True, None, None, None, None),
	(qutip.rand_dm(5), None, None, "phase", True, None, None, None, None),
	])
	def test_hinton(rho, x_basis, y_basis, color_style,
	label_top, cmap, colorbar, fig, ax):
	fig, ax = qutip.hinton(rho, x_basis, y_basis, color_style,
	label_top, cmap=cmap, colorbar=colorbar,
	fig=fig, ax=ax)
	def to_oper_bra(oper):
	return qutip.operator_to_vector(oper).dag()
	def to_oper(oper):
	return oper
	@pytest.mark.parametrize('transform, args', [
	(to_oper, {}),
	(qutip.operator_to_vector(), {}),
	(to_oper_bra, {}),
	(qutip.to_super(), {}),
	(to_oper, {'x_basis': [1, 2, 3, 4, 5]),
	(to_oper, {'y_basis': [1, 2, 3, 4, 5]),
	(to_oper, {'color_style': 'threshold'),
	(to_oper, {'color_style': 'phase'),
	])
	def test_hinton(transform, args):
	rho = transform(qutip.rand_dm(5))
	fig, ax = qutip.hinton(rho, **args)

[tamakoshi2001]

@Ericgig Hi. The Github test fails because of plot_spin_distribution. plot_spin_distribution plots color at (x, y), but the correct way to use pcolor is plotting color in rectangles made by four matrix elements (the link). It may be difficult to fix this warning and hence we can not add pytests for it.
https://matplotlib.org/3.1.1/api/_as_gen/matplotlib.pyplot.pcolor.html

[Ericgig]

You can ignore warnings with:
@pytest.mark.filterwarnings("ignore:The input coordinates to pcolor:UserWarning")
The The input coordinates to pcolor is the start of the warning message.
UserWarning is the kind of warnings.

[tamakoshi2001]

@Ericgig Hi. I added pytests for functions except for plot_wigner_sphere, matrix_histogram, and matrix_histogram_complex. Do you have any advice on them? I will cover inner functions this weekends

[Ericgig]

There are a few options:

• Wait for Update plot_wigner_sphere and delete matrix_histogram_complex #2193 to be merged and add the missing tests after.
• Merge this and add the tests for matrix_histogram, etc. in Update plot_wigner_sphere and delete matrix_histogram_complex #2193.

Any is good, just tell me when you are ready for a review.

[tamakoshi2001]

@Ericgig Hi. I prefer the latter. I am adding tests in #2193 now.

[tamakoshi2001]

@Ericgig This PR is ready for review.

[Ericgig]

There are a few unneeded parametrization, but overall look great.