Create a duplicate of the dictionary for the default colorbar parameters of plot2D

[oskooi]

The colorbar feature for plot2D added in #2289 produces an error when trying to add colorbars to more than one subplots of a single matplotlib.figure object. This is because the dictionary of default parameters for the colorbar (default_colorbar_parametrs of visualization.py) is passed by reference and the use of pop to retrieve its parameters when adding the colorbar to the first subplot removes them from the dictionary. Subsequent calls to retrieve the same parameters in turn triggers an error. The fix involves simply creating a duplicate of the dictionary.

Here is an example which demonstrates this bug:

""                                                                                                               
Plots the fields for an Er point source at r=z=0 in vacuum.                                                       
"""

import meep as mp
import numpy as np
import matplotlib
matplotlib.use('agg')
import matplotlib.pyplot as plt


res = 50
dpml = 1.0
s = 5.0
m = -1
fcen = 1.

cell_size = mp.Vector3(s+dpml,0,s+2*dpml)

pml_layers = [
    mp.PML(dpml, direction=mp.R),
    mp.PML(dpml, direction=mp.Z),
]

sources = [
    mp.Source(
	src=mp.ContinuousSource(fcen),
        center=mp.Vector3(2.3),
        component=mp.Er,
    ),
]

sim = mp.Simulation(
    resolution=res,
    cell_size=cell_size,
    dimensions=mp.CYLINDRICAL,
    m=m,
    sources=sources,
    boundary_layers=pml_layers,
)

sim.run(until=26.5)

fig, ax = plt.subplots(ncols=2)
fig.subplots_adjust(wspace=0.7)

sim.plot2D(
    ax=ax[0],
    fields=mp.Er,
    field_parameters={
	'colorbar':True,
        'cmap':'inferno',
        'post_process':lambda x: np.log10(np.abs(np.real(x))),
    },
)
ax[0].set_title("$|\Re(E_r)|$")

sim.plot2D(
    ax=ax[1],
    fields=mp.Er,
    field_parameters={
        'colorbar':True,
        'cmap':'inferno',
	'post_process':lambda x: np.log10(np.abs(np.imag(x))),
    },
)
ax[1].set_title("$|\Im(E_r)|$")

if mp.am_master():
    plt.savefig(
        f'cyl_er_real_imag.png',
        dpi=150,
        bbox_inches='tight'
    )

Executing this script using the current master branch triggers an error related to a missing pad key entry in the dictionary when trying to add a colorbar to the second subplot:

Traceback (most recent call last):
  File "bug_colorbar.py", line 58, in <module>
    sim.plot2D(
  File "/meep/python/meep/simulation.py", line 4680, in plot2D
    return vis.plot2D(
  File "/meep/python/meep/visualization.py", line 977, in plot2D
    ax = plot_fields(
  File "/meep/python/meep/visualization.py", line 885, in plot_fields
    _add_colorbar(
  File "/meep/python/meep/visualization.py", line 491, in _add_colorbar
    pad=colorbar_parameters.pop("pad"),
KeyError: 'pad'

With the fix in this PR, colorbars are correctly added to both subplots as shown in the following image:

cc @thomasdorch

[codecov-commenter]

Codecov Report

Merging #2380 (a03a053) into master (f448ebd) will decrease coverage by 0.03%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master    #2380      +/-   ##
==========================================
- Coverage   72.59%   72.57%   -0.03%     
==========================================
  Files          17       17              
  Lines        5163     5166       +3     
==========================================
+ Hits         3748     3749       +1     
- Misses       1415     1417       +2     

Impacted Files	Coverage Δ
python/visualization.py	36.66% <16.66%> (-0.02%)	⬇️