Use of matplotlib.colors.Colormap objects as cmap argument results in TypeError: argument of type 'LinearSegmentedColormap' is not iterable

[vadmbertr]

Hi,

ALL software version info

(this library, plus any other relevant software, e.g. bokeh, python, notebook, OS, browser, etc should be added within the dropdown below.)

Software Version Info

- macOS 14.6.1
- Python 3.11.10
- holoviews 1.20.0
- matplotlib 3.9.2
- bokeh 3.5.2
- jupyter_bokeh 4.0.5
- cmocean 4.0.3
- jupyterlab 4.3.0

Description of expected behavior and the observed behavior

I'm plotting a 2d field using hvplot.image (same with hvplot.quadmesh) using a colormap from cmocean (same with any matplotlib.colors.Colormap object, including matplotlib.colormaps ones).
I expect it to just do the plot using the provided colormap, but instead I get the following error: TypeError: argument of type 'LinearSegmentedColormap' is not iterable.
I believe it is because the argument passed through cmap is at some point assumed to be a string (not the case here). Adding the dummy argument color="black" and it works as expected (because a test evaluates to False in the implementation).

Complete, minimal, self-contained example code that reproduces the issue

# using cmocean
import cmocean.cm as cmo
import hvplot.xarray  # noqa
import xarray as xr


ds = xr.tutorial.open_dataset("air_temperature")
da = ds.air.isel(time=0)

plot = da.hvplot.image(cmap=cmo.thermal)  # this works: plot = da.hvplot.image(cmap=cmo.thermal, color="black")

# using matplotlib
import hvplot.xarray  # noqa
from matplotlib import colormaps
import xarray as xr


ds = xr.tutorial.open_dataset("air_temperature")
da = ds.air.isel(time=0)

plot = da.hvplot.image(cmap=colormaps["viridis"])  # this works: plot = da.hvplot.image(cmap=colormaps["viridis"] color="black")

Stack traceback and/or browser JavaScript console output

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[5], [line 9](vscode-notebook-cell:?execution_count=5&line=9)
      [6](vscode-notebook-cell:?execution_count=5&line=6) ds = xr.tutorial.open_dataset("air_temperature")
      [7](vscode-notebook-cell:?execution_count=5&line=7) da = ds.air.isel(time=0)
----> [9](vscode-notebook-cell:?execution_count=5&line=9) plot = da.hvplot.image(cmap=cmo.thermal)

File ~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:2155, in hvPlot.image(self, x, y, z, colorbar, **kwds)
   [2103](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:2103) def image(self, x=None, y=None, z=None, colorbar=True, **kwds):
   [2104](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:2104)     """
   [2105](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:2105)     Image plot
   [2106](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:2106) 
   (...)
   [2153](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:2153)     - Plotly: https://plotly.com/python/imshow/
   [2154](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:2154)     """
-> [2155](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:2155)     return self(x, y, z=z, kind='image', colorbar=colorbar, **kwds)

File ~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:95, in hvPlotBase.__call__(self, x, y, kind, **kwds)
     [92](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:92)         plot = self._get_converter(x, y, kind, **kwds)(kind, x, y)
     [93](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:93)         return pn.panel(plot, **panel_dict)
---> [95](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:95) return self._get_converter(x, y, kind, **kwds)(kind, x, y)

File ~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:102, in hvPlotBase._get_converter(self, x, y, kind, **kwds)
    [100](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:100) y = y or params.pop('y', None)
    [101](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:101) kind = kind or params.pop('kind', None)
--> [102](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/plotting/core.py:102) return HoloViewsConverter(self._data, x, y, kind=kind, **params)

File ~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:796, in HoloViewsConverter.__init__(self, data, x, y, kind, by, use_index, group_label, value_label, backlog, persist, use_dask, crs, fields, groupby, dynamic, grid, legend, rot, title, xlim, ylim, clim, robust, symmetric, logx, logy, loglog, hover, hover_cols, hover_formatters, hover_tooltips, subplots, label, invert, stacked, colorbar, cticks, datashade, rasterize, downsample, resample_when, row, col, debug, framewise, aggregator, projection, global_extent, geo, precompute, flip_xaxis, flip_yaxis, dynspread, x_sampling, y_sampling, pixel_ratio, project, tools, attr_labels, coastline, tiles, tiles_opts, sort_date, check_symmetric_max, transforms, stream, cnorm, features, rescale_discrete_levels, autorange, subcoordinate_y, **kwds)
    [793](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:793)     if plotwd in kwds:
    [794](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:794)         plot_opts[plotwd] = kwds.pop(plotwd)
--> [796](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:796) self._style_opts, plot_opts, kwds = self._process_style(kwds, plot_opts)
    [798](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:798) for axis_name in ['xaxis', 'yaxis']:
    [799](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:799)     if axis_name in kwds:

File ~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1488, in HoloViewsConverter._process_style(self, kwds, plot_opts)
   [1486](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1486)     color = style_opts['color']
   [1487](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1487) elif not isinstance(cmap, dict):
-> [1488](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1488)     if cmap and any(c in cmap for c in categories):
   [1489](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1489)         color = process_cmap(cmap or self._default_cmaps['categorical'], categorical=True)
   [1490](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1490)     else:

File ~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1488, in <genexpr>(.0)
   [1486](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1486)     color = style_opts['color']
   [1487](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1487) elif not isinstance(cmap, dict):
-> [1488](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1488)     if cmap and any(c in cmap for c in categories):
   [1489](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1489)         color = process_cmap(cmap or self._default_cmaps['categorical'], categorical=True)
   [1490](https://file+.vscode-resource.vscode-cdn.net/Users/bertrava/projects/vb-phd/notebooks/daily/~/miniconda3/envs/vb-phd/lib/python3.11/site-packages/hvplot/converter.py:1490)     else:

TypeError: argument of type 'LinearSegmentedColormap' is not iterable

• I may be interested in making a pull request to address this

[maximlt]

Duplicate of #576