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webassembly/objjsproxy: Lookup attributes without testing they exist.

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In JavaScript when accessing an attribute such as `obj.attr` a value of
`undefined` is returned if the attribute does not exist.  This is unlike
Python semantics where an `AttributeError` is raised.  Furthermore, in some
cases in JavaScript (eg a Proxy instance) `attr in obj` can return false
yet `obj.attr` is still valid and returns something other than `undefined`.
So the source of truth for whether a JavaScript attribute exists is to just
right away attempt `obj.attr`.

To more closely match these JavaScript semantics when proxying a JavaScript
object through to Python, change the attribute lookup logic on a `JsProxy`
so that it immediately attempts `obj.attr` instead of first testing if the
attribute exists via `attr in obj`.

This allows JavaScript objects which dynamically create attributes to work
correctly on the Python side, with both `obj.attr` and `obj["attr"]`.  Note
that `obj["attr"]` already works in all cases because it immediately does
the subscript access without first testing if the attribute exists.

As a benefit, this new behaviour matches the Pyodide behaviour.

Signed-off-by: Damien George <damien@micropython.org>
This commit is contained in:
Damien George
2024-06-24 13:03:44 +10:00
parent 5dff78f38e
commit 95c19e05ff
3 changed files with 51 additions and 2 deletions
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10
ports/webassembly/objjsproxy.c
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@@ -46,8 +46,14 @@ EM_JS(bool, has_attr, (int jsref, const char *str), {
EM_JS(bool, lookup_attr, (int jsref, const char *str, uint32_t * out), {
const base = proxy_js_ref[jsref];
const attr = UTF8ToString(str);
if (attr in base) {
let value = base[attr];
// Attempt to lookup the requested attribute from the base object:
// - If the value is not `undefined` then the attribute exists with that value.
// - Otherwise if the value is `undefined` and the `in` operator returns true, then
// that attribute does exist and is intended to have a value of `undefined`.
// - Otherwise, the attribute does not exist.
let value = base[attr];
if (value !== undefined || attr in base) {
if (typeof value === "function") {
if (base !== globalThis) {
if ("_ref" in value) {