fix: throw error_already_set in py::len on failing PyObject_Length (#2575)

* Throw error_already_set in py::len on failing PyObject_Length

* Fix tests to mach error message on PyPy

include/pybind11/pytypes.h

@@ -1530,13 +1530,17 @@ inline memoryview memoryview::from_buffer(
 
 /// \addtogroup python_builtins
 /// @{
+
+/// Get the length of a Python object.
 inline size_t len(handle h) {
     ssize_t result = PyObject_Length(h.ptr());
     if (result < 0)
-        pybind11_fail("Unable to compute length of object");
+        throw error_already_set();
     return (size_t) result;
 }
 
+/// Get the length hint of a Python object.
+/// Returns 0 when this cannot be determined.
 inline size_t len_hint(handle h) {
 #if PY_VERSION_HEX >= 0x03040000
     ssize_t result = PyObject_LengthHint(h.ptr(), 0);

tests/test_pytypes.cpp

@@ -407,4 +407,7 @@ TEST_SUBMODULE(pytypes, m) {
             buf, static_cast<ssize_t>(strlen(buf)));
     });
 #endif
+
+    // test_builtin_functions
+    m.def("get_len", [](py::handle h) { return py::len(h); });
 }

tests/test_pytypes.py

@@ -424,3 +424,11 @@ def test_memoryview_from_memory():
     assert isinstance(view, memoryview)
     assert view.format == 'B'
     assert bytes(view) == b'\xff\xe1\xab\x37'
+
+
+def test_builtin_functions():
+    assert m.get_len([i for i in range(42)]) == 42
+    with pytest.raises(TypeError) as exc_info:
+        m.get_len(i for i in range(42))
+    assert str(exc_info.value) in ["object of type 'generator' has no len()",
+                                   "'generator' has no length"]  # PyPy