If folks agree it's a regression error (& I think it kinda is) the fix is I think add something like

 if item is None: 
     item = mpl.rcParams['image.cmap'] 

which is a direct port of the get_cmap code

before the try in:

@mroeschke What is your use case? Are you passing in None as a literal or is it the value of some variable you have? Is this in library code or do you expect this to be needed in an interactive console.

We have discussed this on the last dev call and are not yet clear how to handle the default colormap case in the new API. Logically, colormaps itself was designed as a mapping of all explicit colormaps. The default colormap notion does not fit in there very well. It depends on the anticipated usage how we would handle that API-wise.

The use case would be for library code. In pandas, there are some APIs with matplotlib functionality that accept cmap that default to None and were passed into matplotlib.cm.get_cmap: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.io.formats.style.Styler.bar.html

We're happy to start using matplotlib.colormaps instead just noting in #23981 (comment) that I had to dig into what get_cmap(None) did before to maintain the prior behavior.

@mroeschke I was going to suggest that you do something like

cmap = cmap in cmap is not None else mpl.rcparams['image.cmap']

which should work on all versions of Matplotlib and leaves you an escape hatch if you want to make the default colormap controllable indepently. However, that probably is not enough for you as if you take str or ColorMap and actually want the color map (rather than passing through to some mpl function) you'll also have to have the logic to decide if you need to look up the color map or if the user directly gave you one.

When we decided to move to the registry model we mostly had end-users in mind, however this issues highlights that we did not take into account the library case enough. We have

if isinstance(inp, mcolors.Colormap):
    cmap = inp
else:
    cmap = mpl.colormaps[inp if inp is not None else mpl.rcParams["image.cmap"]]

There is just enough complexity in there it probably be a function we provide. I think this has to be a function/method rather than shoehorned into __getitem__ or get on the registry because that would be getting too far away from the Mapping API

If we have a registry method for this I propose

def ensure_colormap(self, cmap: str|Colormap, * default=None):
    if isinstance(cmap, mcolors.Colormap):
       return cmap

    default = default if default is not None else mpl.rcParams["image.cmap"]
    cmap = cmap if cmap is not None else default
    return self[cmap]

which could also live as a free method and use the global registry.

Yeah in pandas we used a similar workaround for translating cm.cmap(None) to remove the deprecation warning.

I don't have any strong opinions personally on colormaps[None] being valid. I mainly wanted to highlighting that the deprecation message wasn't entirely clear for the name=None case.

Discussed on the call, we are going to put the method on the registery class for 3.6.1 on the reasoning that this is not a new feature, but completing something that should have been in 3.6.1 as part of the deplication.