Add JsProxy.to_py method

[hoodmane]

Currently converts Map to dict, Set to set and array-like objects to list. If the object is a base Object (not a subclass of Object, I check that Object.constructor.name === "object" and that it doesn't have a custom Symbol.toStringTag) then I convert it to a dict as well. Otherwise I leave it alone.

Takes a depth parameter which works in the same way as PyProxy.toJs.

[hoodmane]

Here's one limitation of the current implementation:

z = new Map();
z.set([1,1], 2);
pyodide.runPython(`
    from js import z
    z.to_py() # ==> TypeError: unhashable type: 'list'
`);

We could try to convert lists in keys to tuples, but then there's also:

z = new Map();
z.set({"a" : 2}, 2);
pyodide.runPython(`
    from js import z
    z.to_py() # ==> TypeError: unhashable type: 'dict'
`);

and:

z = new Map();
class Test {};
z.set(new Test(), 2);
pyodide.runPython(`
    from js import z
    z.to_py() # ==> TypeError: unhashable type: 'JsProxy'
`);

Of course the same problems happen when converting Set to set.

[rth]

Here's one limitation of the current implementation:
TypeError: unhashable type

I think that's an expected error from the Python perspective on unhashable keys, so better to raise it than try to find some non obvious workaround (such as converting keys to str).

[phorward]

Hello @hoodmane,
thanks for your work! I'm mostly well with this pull request, but what do you think about adding the failing tests as well to the test suite? See my last comment above.

[hoodmane]

@phorward Thanks for the review!

[phorward]

OK, I'm fine with this, thanks for updating the tests!

[rth]

Thanks! I have not reviewed the JS code in detail but the tests look good. Merging with a +1 above.