Use a more robust method to improve our ModuleNotFound errors

[hoodmane]

@ryanking13 added these very nice error messages to the ModuleNotFound errors. However they introduce a few problems:

1. find_spec is supposed to return None or a spec and not to raise an error. If it raises errors, it can cause trouble in code that tries to check if a module is installed or not.
2. Other code that tries to add new import hooks has to know to insert them before these error-raising import hooks.

See the discussion in #3262.

This instead patches importlib._bootstrap to create a function called _get_module_not_found_error. We then can monkey patch this to modify the error messages that importlib raises.

Checklists

• Add a CHANGELOG entry
• Add / update tests

[hoodmane]

I also made a thread about this on discuss.python.org:
https://discuss.python.org/t/a-hook-to-allow-adding-notes-to-modulenotfounderror/21151/2

[ryanking13]

Thanks for the fix @hoodmane!

What do think of modifying sys.excepthook, something like this? (I have never tried, just an idea).

def pyodide_except_hook(type, value, traceback):
    orig_excepthook = sys.excepthook
    if isinstance(type, ModuleNotFoundError):
        ...
    else:
        return orig_excepthook(...)

sys.excepthook = pyodide_except_hook

I think this won't require patching CPython.

[hoodmane]

We currently never call sys.excepthook. Instead of that, we turn errors into JavaScript PythonError exceptions when they reach the bottom of the stack.

[ryanking13]

Oh, okay. You are right. I was able to check that sys.excepthook is never called.

[rth]

No objection to this change, but it would be worth opening a feature request at Cpython to ask to make it possible change this message without patching. Or if they don't want to update the public API of importlib for this, maybe at least they would be OK with adding this patch upstream so that it can be modified via monkeypatching (instead of real patching). As the patch to Python/importlib.h in particular looks a bit annoying to maintain.

[hoodmane]

the patch to Python/importlib.h in particular looks a bit annoying to maintain.

Well it's autogenerated, you just have to run make regen-importlib.

[hoodmane]

at least they would be OK with adding this patch upstream so that it can be modified via monkeypatching (instead of real patching).

I bet they would accept that, I'll try.

[ryanking13]

I am okay with merging this, and would prefer to include this in 0.22.