From 9bb69d6403b2640ea72c77b16e9d088c3f0fc6c7 Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 08:36:29 -0400
Subject: [PATCH 01/15] use last segment of module name (path) when adding to
 parent

---
 src/types/module.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/types/module.rs b/src/types/module.rs
index 72ba2d341d9..cb3aca500f2 100644
--- a/src/types/module.rs
+++ b/src/types/module.rs
@@ -514,7 +514,7 @@ impl<'py> PyModuleMethods<'py> for Bound<'py, PyModule> {
     }
 
     fn add_submodule(&self, module: &Bound<'_, PyModule>) -> PyResult<()> {
-        let name = module.name()?;
+        let name = module.name()?.call_method1("rpartition", (".",))?.get_item(2)?.downcast_into::<PyString>()?;
         self.add(name, module)
     }
 

From f3a764519a24da0528dbdcac60e9f49cf4d0146e Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 08:51:15 -0400
Subject: [PATCH 02/15] add newsfragment

---
 newsfragments/5375.fixed.md | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 newsfragments/5375.fixed.md

diff --git a/newsfragments/5375.fixed.md b/newsfragments/5375.fixed.md
new file mode 100644
index 00000000000..ee6b3893e76
--- /dev/null
+++ b/newsfragments/5375.fixed.md
@@ -0,0 +1 @@
+Fixed `PyModuleMethods::add_submodul()` to use the last segment of the submodule name as the attribute name on the parent module instead of using the full name.
\ No newline at end of file

From 4455590c24c4a5aff64b69bce79894f6fec4caab Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 08:51:19 -0400
Subject: [PATCH 03/15] format

---
 src/types/module.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/types/module.rs b/src/types/module.rs
index cb3aca500f2..1c67ca90080 100644
--- a/src/types/module.rs
+++ b/src/types/module.rs
@@ -514,7 +514,11 @@ impl<'py> PyModuleMethods<'py> for Bound<'py, PyModule> {
     }
 
     fn add_submodule(&self, module: &Bound<'_, PyModule>) -> PyResult<()> {
-        let name = module.name()?.call_method1("rpartition", (".",))?.get_item(2)?.downcast_into::<PyString>()?;
+        let name = module
+            .name()?
+            .call_method1("rpartition", (".",))?
+            .get_item(2)?
+            .downcast_into::<PyString>()?;
         self.add(name, module)
     }
 

From 01ca587c15afd92f6542955e0ca624e126c53c2e Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 09:03:58 -0400
Subject: [PATCH 04/15] preliminary tests

---
 tests/test_module.rs | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/tests/test_module.rs b/tests/test_module.rs
index 5e6244420e0..d55781a08f6 100644
--- a/tests/test_module.rs
+++ b/tests/test_module.rs
@@ -285,10 +285,10 @@ fn superfunction() -> String {
 #[pymodule]
 fn supermodule(module: &Bound<'_, PyModule>) -> PyResult<()> {
     module.add_function(wrap_pyfunction!(superfunction, module)?)?;
-    let module_to_add = PyModule::new(module.py(), "submodule")?;
+    let module_to_add = PyModule::new(module.py(), "supermodule.submodule")?;
     submodule(&module_to_add)?;
     module.add_submodule(&module_to_add)?;
-    let module_to_add = PyModule::new(module.py(), "submodule_with_init_fn")?;
+    let module_to_add = PyModule::new(module.py(), "supermodule.submodule_with_init_fn")?;
     submodule_with_init_fn(&module_to_add)?;
     module.add_submodule(&module_to_add)?;
     Ok(())
@@ -316,6 +316,21 @@ fn test_module_nesting() {
             supermodule,
             "supermodule.submodule_with_init_fn.subfunction() == 'Subfunction'"
         );
+        py_assert!(
+            py,
+            supermodule,
+            "supermodule.submodule.__name__ == 'supermodule.submodule'"
+        );
+        py_assert!(
+            py,
+            supermodule,
+            "'submodule' in supermodule.__dict__"
+        );
+        py_assert!(
+            py,
+            supermodule,
+            "'supermodule.submodule' not in supermodule.__dict__"
+        );
     });
 }
 

From 0d6fdff677144e85361ce9cbf483d7c752e54534 Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 09:21:12 -0400
Subject: [PATCH 05/15] fmt

---
 tests/test_module.rs | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/tests/test_module.rs b/tests/test_module.rs
index d55781a08f6..ca154996839 100644
--- a/tests/test_module.rs
+++ b/tests/test_module.rs
@@ -321,11 +321,7 @@ fn test_module_nesting() {
             supermodule,
             "supermodule.submodule.__name__ == 'supermodule.submodule'"
         );
-        py_assert!(
-            py,
-            supermodule,
-            "'submodule' in supermodule.__dict__"
-        );
+        py_assert!(py, supermodule, "'submodule' in supermodule.__dict__");
         py_assert!(
             py,
             supermodule,

From fb5725e72d167bf97689a200a7de9edf24bd61ea Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 15:34:30 -0400
Subject: [PATCH 06/15] doc comment and more test (that fails right now)

---
 src/types/module.rs  |  3 ++-
 tests/test_module.rs | 11 +++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/types/module.rs b/src/types/module.rs
index 1c67ca90080..57e9177299c 100644
--- a/src/types/module.rs
+++ b/src/types/module.rs
@@ -35,7 +35,8 @@ pub struct PyModule(PyAny);
 pyobject_native_type_core!(PyModule, pyobject_native_static_type_object!(ffi::PyModule_Type), #checkfunction=ffi::PyModule_Check);
 
 impl PyModule {
-    /// Creates a new module object with the `__name__` attribute set to `name`.
+    /// Creates a new module object with the `__name__` attribute set to `name`.  When creating
+    /// a submodule pass the full path as the name such as `top_level.name`.
     ///
     /// # Examples
     ///
diff --git a/tests/test_module.rs b/tests/test_module.rs
index ca154996839..09529eb5371 100644
--- a/tests/test_module.rs
+++ b/tests/test_module.rs
@@ -327,6 +327,17 @@ fn test_module_nesting() {
             supermodule,
             "'supermodule.submodule' not in supermodule.__dict__"
         );
+        py_assert!(
+            py,
+            supermodule,
+            "supermodule.submodule.__name__ == 'supermodule.submodule_with_init_fn'"
+        );
+        py_assert!(py, supermodule, "'submodule_with_init_fn' in supermodule.__dict__");
+        py_assert!(
+            py,
+            supermodule,
+            "'supermodule.submodule_with_init_fn' not in supermodule.__dict__"
+        );
     });
 }
 

From 7c4b890dd08ce307c8464a2626e8dbe6c3811f6f Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 15:57:43 -0400
Subject: [PATCH 07/15] fmt

---
 tests/test_module.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tests/test_module.rs b/tests/test_module.rs
index 09529eb5371..1b05eddff90 100644
--- a/tests/test_module.rs
+++ b/tests/test_module.rs
@@ -332,7 +332,11 @@ fn test_module_nesting() {
             supermodule,
             "supermodule.submodule.__name__ == 'supermodule.submodule_with_init_fn'"
         );
-        py_assert!(py, supermodule, "'submodule_with_init_fn' in supermodule.__dict__");
+        py_assert!(
+            py,
+            supermodule,
+            "'submodule_with_init_fn' in supermodule.__dict__"
+        );
         py_assert!(
             py,
             supermodule,

From c05362287d96b485ea2c5ba53d7bef4524105193 Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 16:17:41 -0400
Subject: [PATCH 08/15] separate

---
 tests/test_module.rs | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tests/test_module.rs b/tests/test_module.rs
index 1b05eddff90..250833b5f5c 100644
--- a/tests/test_module.rs
+++ b/tests/test_module.rs
@@ -316,6 +316,16 @@ fn test_module_nesting() {
             supermodule,
             "supermodule.submodule_with_init_fn.subfunction() == 'Subfunction'"
         );
+    });
+}
+
+#[test]
+fn test_submodule_attribute_and_dunder_names() {
+    use pyo3::wrap_pymodule;
+
+    Python::attach(|py| {
+        let supermodule = wrap_pymodule!(supermodule)(py);
+
         py_assert!(
             py,
             supermodule,

From ec749321cead6f3e9dbc08af6a88bfcc1cd2437b Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 16:20:38 -0400
Subject: [PATCH 09/15] tweak test layout

---
 tests/test_module.rs | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/tests/test_module.rs b/tests/test_module.rs
index 250833b5f5c..91111898024 100644
--- a/tests/test_module.rs
+++ b/tests/test_module.rs
@@ -320,7 +320,7 @@ fn test_module_nesting() {
 }
 
 #[test]
-fn test_submodule_attribute_and_dunder_names() {
+fn test_submodule_attribute_and_dunder_names_submodule() {
     use pyo3::wrap_pymodule;
 
     Python::attach(|py| {
@@ -337,10 +337,20 @@ fn test_submodule_attribute_and_dunder_names() {
             supermodule,
             "'supermodule.submodule' not in supermodule.__dict__"
         );
+    });
+}
+
+#[test]
+fn test_submodule_attribute_and_dunder_names_submodule_with_init_fn() {
+    use pyo3::wrap_pymodule;
+
+    Python::attach(|py| {
+        let supermodule = wrap_pymodule!(supermodule)(py);
+
         py_assert!(
             py,
             supermodule,
-            "supermodule.submodule.__name__ == 'supermodule.submodule_with_init_fn'"
+            "supermodule.submodule_with_init_fn.__name__ == 'supermodule.submodule_with_init_fn'"
         );
         py_assert!(
             py,

From 9c9f0026ada83a78e42f8f3005dae3c20a9ee657 Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 16:34:10 -0400
Subject: [PATCH 10/15] recombine for PyO3 modules compiled for CPython 3.8 or
 older may only be initialized once per interpreter process

---
 tests/test_module.rs | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/tests/test_module.rs b/tests/test_module.rs
index 91111898024..80082921540 100644
--- a/tests/test_module.rs
+++ b/tests/test_module.rs
@@ -320,12 +320,13 @@ fn test_module_nesting() {
 }
 
 #[test]
-fn test_submodule_attribute_and_dunder_names_submodule() {
+fn test_submodule_attribute_and_dunder_names() {
     use pyo3::wrap_pymodule;
 
     Python::attach(|py| {
         let supermodule = wrap_pymodule!(supermodule)(py);
 
+        // submodule
         py_assert!(
             py,
             supermodule,
@@ -337,16 +338,8 @@ fn test_submodule_attribute_and_dunder_names_submodule() {
             supermodule,
             "'supermodule.submodule' not in supermodule.__dict__"
         );
-    });
-}
-
-#[test]
-fn test_submodule_attribute_and_dunder_names_submodule_with_init_fn() {
-    use pyo3::wrap_pymodule;
-
-    Python::attach(|py| {
-        let supermodule = wrap_pymodule!(supermodule)(py);
 
+        // submodule_with_init_fn
         py_assert!(
             py,
             supermodule,

From 4cb3eb6263e865f989296239c17cb3a108c6f128 Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 16:49:03 -0400
Subject: [PATCH 11/15] again

---
 tests/test_module.rs | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/tests/test_module.rs b/tests/test_module.rs
index 80082921540..4e998fc464b 100644
--- a/tests/test_module.rs
+++ b/tests/test_module.rs
@@ -316,17 +316,9 @@ fn test_module_nesting() {
             supermodule,
             "supermodule.submodule_with_init_fn.subfunction() == 'Subfunction'"
         );
-    });
-}
 
-#[test]
-fn test_submodule_attribute_and_dunder_names() {
-    use pyo3::wrap_pymodule;
-
-    Python::attach(|py| {
-        let supermodule = wrap_pymodule!(supermodule)(py);
 
-        // submodule
+        // submodule dunder name and attribute name
         py_assert!(
             py,
             supermodule,
@@ -339,7 +331,7 @@ fn test_submodule_attribute_and_dunder_names() {
             "'supermodule.submodule' not in supermodule.__dict__"
         );
 
-        // submodule_with_init_fn
+        // submodule_with_init_fn dunder name and attribute name
         py_assert!(
             py,
             supermodule,

From b69ef460dd1ece1ea4f5b47ec05d9e21b910a177 Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Fri, 29 Aug 2025 16:59:30 -0400
Subject: [PATCH 12/15] fmt

---
 tests/test_module.rs | 1 -
 1 file changed, 1 deletion(-)

diff --git a/tests/test_module.rs b/tests/test_module.rs
index 4e998fc464b..b310c113c76 100644
--- a/tests/test_module.rs
+++ b/tests/test_module.rs
@@ -317,7 +317,6 @@ fn test_module_nesting() {
             "supermodule.submodule_with_init_fn.subfunction() == 'Subfunction'"
         );
 
-
         // submodule dunder name and attribute name
         py_assert!(
             py,

From e1909d5a2989da9311bee1d330e11f72d07b94b0 Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Mon, 3 Nov 2025 11:28:05 -0500
Subject: [PATCH 13/15] catchup

---
 src/types/module.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/types/module.rs b/src/types/module.rs
index 389bf73680c..08ec262523f 100644
--- a/src/types/module.rs
+++ b/src/types/module.rs
@@ -518,7 +518,7 @@ impl<'py> PyModuleMethods<'py> for Bound<'py, PyModule> {
             .name()?
             .call_method1("rpartition", (".",))?
             .get_item(2)?
-            .downcast_into::<PyString>()?;
+            .cast_into::<PyString>()?;
         self.add(name, module)
     }
 

From e89192301924fbe456d91af2b8983d3458404384 Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Mon, 3 Nov 2025 12:38:55 -0500
Subject: [PATCH 14/15] declarative

---
 pyo3-macros-backend/src/module.rs |  6 ++++--
 tests/test_declarative_module.rs  | 29 ++++++++++++++++++++++++++++-
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/pyo3-macros-backend/src/module.rs b/pyo3-macros-backend/src/module.rs
index df00f319ac0..a876a724b9d 100644
--- a/pyo3-macros-backend/src/module.rs
+++ b/pyo3-macros-backend/src/module.rs
@@ -392,6 +392,7 @@ pub fn pymodule_module_impl(
     let gil_used = options.gil_used.is_some_and(|op| op.value.value);
 
     let initialization = module_initialization(
+        &full_name,
         &name,
         ctx,
         quote! { __pyo3_pymodule },
@@ -451,6 +452,7 @@ pub fn pymodule_function_impl(
     let gil_used = options.gil_used.is_some_and(|op| op.value.value);
 
     let initialization = module_initialization(
+        &name.to_string(),
         &name,
         ctx,
         quote! { ModuleExec::__pyo3_module_exec },
@@ -499,6 +501,7 @@ pub fn pymodule_function_impl(
 }
 
 fn module_initialization(
+    full_name: &str,
     name: &syn::Ident,
     ctx: &Ctx,
     module_exec: TokenStream,
@@ -508,8 +511,7 @@ fn module_initialization(
 ) -> TokenStream {
     let Ctx { pyo3_path, .. } = ctx;
     let pyinit_symbol = format!("PyInit_{name}");
-    let name = name.to_string();
-    let pyo3_name = LitCStr::new(&CString::new(name).unwrap(), Span::call_site());
+    let pyo3_name = LitCStr::new(&CString::new(full_name).unwrap(), Span::call_site());
 
     let mut result = quote! {
         #[doc(hidden)]
diff --git a/tests/test_declarative_module.rs b/tests/test_declarative_module.rs
index c01933c35e4..80eb8b87299 100644
--- a/tests/test_declarative_module.rs
+++ b/tests/test_declarative_module.rs
@@ -150,7 +150,7 @@ fn double_value(v: &ValueClass) -> usize {
     v.value * 2
 }
 
-#[pymodule]
+#[pymodule(module="declarative_module")]
 mod declarative_submodule {
     #[pymodule_export]
     use super::{double, double_value};
@@ -204,6 +204,33 @@ fn test_declarative_module() {
         py_assert!(py, m, "not hasattr(m, 'BAR')");
         py_assert!(py, m, "m.type == '!'");
         py_assert!(py, m, "not hasattr(m, 'NOT_EXPORTED')");
+
+        // submodule dunder name and attribute name
+        // declarative_module.inner is declared inside
+        py_assert!(
+            py,
+            m,
+            "m.inner.__name__ == 'declarative_module.inner'"
+        );
+        py_assert!(py, m, "'inner' in m.__dict__");
+        py_assert!(
+            py,
+            m,
+            "'declarative_module.inner' not in m.__dict__"
+        );
+
+        // since declarative_submodule is declared outside, but the parent module name is passed
+        py_assert!(
+            py,
+            m,
+            "m.declarative_submodule.__name__ == 'declarative_module.declarative_submodule'"
+        );
+        py_assert!(py, m, "'declarative_submodule' in m.__dict__");
+        py_assert!(
+            py,
+            m,
+            "'declarative_module.declarative_submodule' not in m.__dict__"
+        );
     })
 }
 

From cd69d883b75b993890274d91a6de27684135fbca Mon Sep 17 00:00:00 2001
From: Kyle Altendorf <sda@fstab.net>
Date: Mon, 3 Nov 2025 13:43:01 -0500
Subject: [PATCH 15/15] fmt

---
 tests/test_declarative_module.rs | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/tests/test_declarative_module.rs b/tests/test_declarative_module.rs
index 80eb8b87299..e0d77f69e97 100644
--- a/tests/test_declarative_module.rs
+++ b/tests/test_declarative_module.rs
@@ -150,7 +150,7 @@ fn double_value(v: &ValueClass) -> usize {
     v.value * 2
 }
 
-#[pymodule(module="declarative_module")]
+#[pymodule(module = "declarative_module")]
 mod declarative_submodule {
     #[pymodule_export]
     use super::{double, double_value};
@@ -207,17 +207,9 @@ fn test_declarative_module() {
 
         // submodule dunder name and attribute name
         // declarative_module.inner is declared inside
-        py_assert!(
-            py,
-            m,
-            "m.inner.__name__ == 'declarative_module.inner'"
-        );
+        py_assert!(py, m, "m.inner.__name__ == 'declarative_module.inner'");
         py_assert!(py, m, "'inner' in m.__dict__");
-        py_assert!(
-            py,
-            m,
-            "'declarative_module.inner' not in m.__dict__"
-        );
+        py_assert!(py, m, "'declarative_module.inner' not in m.__dict__");
 
         // since declarative_submodule is declared outside, but the parent module name is passed
         py_assert!(