[BUG]: cannot register submodules for embedded modules

[rkaminsk]

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What version (or hash if on master) of pybind11 are you using?

c125cc7

Problem description

Pybind's module_::def_submodule function requires the module to have a __file__ attribute. However, when creating an embedded module this attribute is not set.

I would think that adding submodules is a valid use-case that should not need any workarounds.

Reproducible example code

As a workaround, I am currently setting the attribute manually:

PYBIND11_EMBEDDED_MODULE(x, m) {
    m.attr("__file__") = "<embedded>";
    m.def_submodule("y");
}

Is this a regression? Put the last known working version here if it is.

The issue was introduced by 8f00d1e. @henryiii maybe you can have a look.

[henryiii]

I think we should just not set the attribute if it's not available, then?

[rkaminsk]

I think that's a good idea:

• In the first case, one can simply check with hasattr whether the parent has a __file__ attribute.
• In the second case, one can handle the system error if the parent does not have a __file__ attribute.

[rkaminsk]

What about the following:

diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index e0538094..38338539 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1342,13 +1342,20 @@ public:
         // GraalPy doesn't support PyModule_GetFilenameObject,
         // so getting by attribute (see PR #5584)
         handle this_module = m_ptr;
-        result.attr("__file__") = this_module.attr("__file__");
+        if (hasattr(this_module, "__file__")) {
+            result.attr("__file__") = this_module.attr("__file__");
+        }
 #else
         handle this_file = PyModule_GetFilenameObject(m_ptr);
-        if (!this_file) {
+        if (this_file) {
+            result.attr("__file__") = this_file;
+        }
+        else if (PyErr_ExceptionMatches(PyExc_SystemError) != 0) {
+            PyErr_Clear();
+        }
+        else {
             throw error_already_set();
         }
-        result.attr("__file__") = this_file;
 #endif
         attr(name) = result;
         return result;