The first thing that importlib.reload() does is to verify that the passed module is an instance of types.ModuleType (Lib/importlib/__init__.py:107).  This check seems unnecessary to me.  We really don't have a functional need for the check (that I know of).  Furthermore, there has been at least one serious proposal recently that suggested using custom module types.

The only benefit that I can think of to the type check is it makes the failure more clear when someone tries to "reload" an attribute in a module (thinking just the attribute will get reloaded!).  However, does that matter all that much now that reload() is not a builtin (ergo less likely to get misused very often)?

I'm not invested in removing these 2 lines (or at least loosening the restriction).  I've brought it up simply because it keeps staring me in the face lately. :-)  If anyone has any objections, I'll drop it (at least it will be recorded here in the tracker).  That said, I'm glad to remove the restriction otherwise.

Can we or can't we remove the check as Eric has proposed in msg201874?

I think the check can be removed as long as an AttributeError is caught when trying to access module.__name__ and a message mentioning that the user probably meant to pass in a module is used, e.g.:

try:
    name = module.__spec__.name
except AttributeError:
    try:
        name = module.__name__
    except AttributeError:
        raise TypeError("argument should be a module")

PR has been sent.

New changeset fef1fae9df3b03510f9defb25bd0388135b4c591 by Furkan Önder in branch 'master':
bpo-19468: delete unnecessary instance check in importlib.reload() (GH-19424)
https://github.com/python/cpython/commit/fef1fae9df3b03510f9defb25bd0388135b4c591

Thanks for the PR, Furkan !