Convert long Python ints to Javascript floats

src/python2js.c

@@ -131,6 +131,21 @@ _python2js(PyObject* x, PyObject* map)
     if (x_long == -1 && PyErr_Occurred()) {
       return HW_ERROR;
     }
+    // Since Javascript doesn't support > 32-bit ints, use floats
+    // when the Python int gets too large.  This will lose precision,
+    // but is less problematic than truncation.
+    if ((unsigned long)x_long > 0x7fffffff) {
+      PyObject *py_float = PyNumber_Float(x);
+      if (py_float == NULL) {
+        return HW_ERROR;
+      }
+      double x_double = PyFloat_AsDouble(py_float);
+      Py_DECREF(py_float);
+      if (x_double == -1.0 && PyErr_Occurred()) {
+        return HW_ERROR;
+      }
+      return hiwire_double(x_double);
+    }
     return hiwire_int(x_long);
   } else if (PyFloat_Check(x)) {
     double x_double = PyFloat_AsDouble(x);

test/test_python.py

@@ -58,6 +58,13 @@ def test_python2js(selenium):
         """)
 
 
+def test_python2js_long_ints(selenium):
+    assert selenium.run('2**30') == 2**30
+    assert selenium.run('2**31') == 2**31
+    assert selenium.run('2**30 - 1 + 2**30') == (2**30 - 1 + 2**30)
+    assert selenium.run('2**32 / 2**4') == (2**32 / 2**4)
+
+
 def test_pythonexc2js(selenium):
     try:
         selenium.run_js('return pyodide.runPython("5 / 0")')