From 99773fc5f6b148900a8b5e61a4a683a17052261d Mon Sep 17 00:00:00 2001
From: Yannick Jadoul <yannick.jadoul@belgacom.net>
Date: Tue, 13 Oct 2020 05:00:54 +0200
Subject: [PATCH] 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 | 6 +++++-
 tests/test_pytypes.cpp     | 3 +++
 tests/test_pytypes.py      | 8 ++++++++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/include/pybind11/pytypes.h b/include/pybind11/pytypes.h
index 30d7f9ae..1010ad71 100644
--- a/include/pybind11/pytypes.h
+++ b/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);
diff --git a/tests/test_pytypes.cpp b/tests/test_pytypes.cpp
index 45fd8718..ceaac365 100644
--- a/tests/test_pytypes.cpp
+++ b/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); });
 }
diff --git a/tests/test_pytypes.py b/tests/test_pytypes.py
index ab51b551..403c4da4 100644
--- a/tests/test_pytypes.py
+++ b/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