Support dict_converter and iterating over python objects in js (#138)

environment-wasm.yml

@@ -1,6 +1,6 @@
 name: pyjs-wasm-run
 channels:
-  - /Users/thorstenbeier/src/recipes/output 
+  #- /Users/thorstenbeier/src/recipes/output
   - conda-forge
   - https://repo.prefix.dev/emscripten-forge-bot/emscripten-forge-6x
 
@@ -12,4 +12,5 @@ dependencies:
   - numpy
   - zlib-static
   - libsqlite-static
-  - libbzip2-static
+  - libbzip2-static
+  - libexpat-static

include/pyjs/pre_js/init.js

@@ -225,6 +225,25 @@ def _add_resolve_done_callback(future, resolve, reject):
     Module._py_to_js = Module.eval("pyjs.to_js")
     Module._py_objects.push(Module._py_to_js);
 
+    // helpers to iterate over python objects from JavaScript (see make_proxy.js)
+    Module.exec(`
+def _pyjs_is_iterable(obj):
+    return hasattr(type(obj), "__iter__")
+
+def _pyjs_next(it, done):
+    try:
+        return next(it)
+    except StopIteration:
+        return done
+    `)
+
+    Module._py_is_iterable = Module.eval("_pyjs_is_iterable")
+    Module._py_objects.push(Module._py_is_iterable);
+    Module._py_iter = Module.eval("iter")
+    Module._py_objects.push(Module._py_iter);
+    Module._py_next = Module.eval("_pyjs_next")
+    Module._py_objects.push(Module._py_next);
+
     Module["to_js"] = function(obj){
         return Module._py_to_js.py_call(obj)
     }

include/pyjs/pre_js/make_proxy.js

@@ -5,9 +5,42 @@ Module['make_proxy'] = function(py_object) {
             if (ret !== undefined) {
                 return ret
             }
+            // python attribute names are strings, so symbols cannot be passed to _getattr
+            if (typeof property === 'symbol') {
+                if (property === Symbol.iterator && Module._py_is_iterable.py_call(target)) {
+                    return function() {
+                        return Module._py_object_iterator(target)
+                    }
+                }
+                return undefined
+            }
             return target._getattr(property);
         }
     };
 
     return new Proxy(py_object, handler);
 }
+
+// JavaScript iterator over an iterable python object
+Module['_py_object_iterator'] = function(py_object) {
+    // returned by _py_next when the python iterator is exhausted
+    const done = {}
+    let py_iter = Module._py_iter.py_call(py_object)
+    return {
+        next() {
+            if (py_iter === null) {
+                return { done: true, value: undefined }
+            }
+            const value = Module._py_next.py_call(py_iter, done)
+            if (value === done) {
+                py_iter.delete()
+                py_iter = null
+                return { done: true, value: undefined }
+            }
+            return { done: false, value: value }
+        },
+        [Symbol.iterator]() {
+            return this
+        }
+    }
+}

module/pyjs/convert_py_to_js.py

@@ -2,44 +2,58 @@
 from .core import new
 from pyjs_core import JsValue, js_array, js, js_undefined,internal
 
-def _py_list_like_to_js(value, cache, depth, max_depth):
+def _py_list_like_to_js(value, cache, depth, max_depth, dict_converter):
     vid = id(value)
     if vid in cache:
         return cache[vid]
     j_value = js_array()
     cache[vid] = j_value
     for v in value:
-        js_v = to_js(v, cache=cache, depth=depth + 1, max_depth=max_depth)
+        js_v = to_js(v, cache=cache, depth=depth + 1, max_depth=max_depth, dict_converter=dict_converter)
         j_value.push(js_v)
     return j_value
 
 
-def _py_dict_like_to_js(value, cache, depth, max_depth):
+def _py_dict_like_to_js(value, cache, depth, max_depth, dict_converter):
     vid = id(value)
     if vid in cache:
         return cache[vid]
+    if dict_converter is not None:
+        # The converted value is only cached after all entries are converted,
+        # so recursive dicts are not supported.
+        entries = js_array()
+        for k, v in value.items():
+            js_k = to_js(k, cache=cache, depth=depth + 1, max_depth=max_depth, dict_converter=dict_converter)
+            js_v = to_js(v, cache=cache, depth=depth + 1, max_depth=max_depth, dict_converter=dict_converter)
+            pair = js_array()
+            pair.push(js_k)
+            pair.push(js_v)
+            entries.push(pair)
+        j_value = dict_converter(entries)
+        cache[vid] = j_value
+        return j_value
     j_value = new(js.LiteralMap)
     cache[vid] = j_value
     for k, v in value.items():
-        js_k = to_js(k, cache=cache, depth=depth + 1, max_depth=max_depth)
-        js_v = to_js(v, cache=cache, depth=depth + 1, max_depth=max_depth)
+        js_k = to_js(k, cache=cache, depth=depth + 1, max_depth=max_depth, dict_converter=dict_converter)
+        js_v = to_js(v, cache=cache, depth=depth + 1, max_depth=max_depth, dict_converter=dict_converter)
         j_value.set(js_k, js_v)
     return j_value
 
 
-def _py_set_like_to_js(value, cache, depth, max_depth):
+def _py_set_like_to_js(value, cache, depth, max_depth, dict_converter):
     vid = id(value)
     if vid in cache:
         return cache[vid]
     j_value = new(js.Set)
     cache[vid] = j_value
     for v in value:
-        js_v = to_js(v, cache=cache, depth=depth + 1, max_depth=max_depth)
+        js_v = to_js(v, cache=cache, depth=depth + 1, max_depth=max_depth, dict_converter=dict_converter)
         j_value.add(js_v)
     return j_value
 
 
-def to_js(value, cache=None, depth=0, max_depth=None):
+def to_js(value, cache=None, depth=0, max_depth=None, dict_converter=None):
     """
     Convert a Python object to a JavaScript object.
 
@@ -48,7 +62,11 @@ def to_js(value, cache=None, depth=0, max_depth=None):
         cache: A dictionary to use as a cache for already converted objects.
         depth: The current depth of of nested object conversion.
         max_depth: The maximum depth of nested object conversion.
-
+        dict_converter: Optional function that receives a JavaScript array of
+            [key, value] pairs and returns the converted dict, e.g.
+            js.Object.fromEntries. If None, dicts are converted to a LiteralMap.
+            Recursive dicts are not supported when this is used.
+
     Returns:
 
         a JavaScript stored in a pyjs_core.JsValue
@@ -63,15 +81,15 @@ def to_js(value, cache=None, depth=0, max_depth=None):
         return value
     elif isinstance(value, (list, tuple)):
         return _py_list_like_to_js(
-            value=value, cache=cache, depth=depth, max_depth=max_depth
+            value=value, cache=cache, depth=depth, max_depth=max_depth, dict_converter=dict_converter
         )
     elif isinstance(value, dict):
         return _py_dict_like_to_js(
-            value=value, cache=cache, depth=depth, max_depth=max_depth
+            value=value, cache=cache, depth=depth, max_depth=max_depth, dict_converter=dict_converter
         )
     elif isinstance(value, set):
         return _py_set_like_to_js(
-            value=value, cache=cache, depth=depth, max_depth=max_depth
+            value=value, cache=cache, depth=depth, max_depth=max_depth, dict_converter=dict_converter
         )
     elif value is None:
         return js_undefined()
@@ -85,7 +103,7 @@ def to_js(value, cache=None, depth=0, max_depth=None):
 
     elif hasattr(value, "explicit_to_js"):
         return value.explicit_to_js()
-        
+
     elif hasattr(value, "implicit_to_js"):
         return value.implicit_to_js()
 

src/js_timestamp.cpp

@@ -1 +1 @@
-#define PYJS_JS_UTC_TIMESTAMP "2026-09-07 08:23:38.609625"
+#define PYJS_JS_UTC_TIMESTAMP "2026-10-27 15:17:57.728418"

tests/tests/test_pyjs.py

@@ -258,6 +258,24 @@ def test_to_js_dict():
     assert jsmap.get(1) == "a"
 
 
+def test_to_js_dict_converter():
+    pydict = {"a": 1, "b": [2, {"c": "three"}]}
+    jsobj = pyjs.to_js(pydict, dict_converter=pyjs.js.Object.fromEntries)
+    assert pyjs.js.JSON.stringify(jsobj) == '{"a":1,"b":[2,{"c":"three"}]}'
+
+
+def test_iterate_py_object_in_js():
+    # python objects passed to js are proxies that implement Symbol.iterator
+    blob = pyjs.js.Blob.new(["hello", "world"])
+    assert blob.size == 10
+
+    jsarray = getattr(pyjs.js.Array, "from")([1, "two", 3.5])
+    assert pyjs.js.JSON.stringify(jsarray) == '[1,"two",3.5]'
+
+    jsarray = getattr(pyjs.js.Array, "from")({"a": 1, "b": 2})
+    assert pyjs.js.JSON.stringify(jsarray) == '["a","b"]'
+
+
 def test_bytes_to_js():
     pyjs.to_js(b"\x00").byteLength == 1