refs PyO3#4286 -- allow setting submodule on declarative pymodules

guide/src/module.md

@@ -154,6 +154,8 @@ The `#[pymodule]` macro automatically sets the `module` attribute of the `#[pycl
 For nested modules, the name of the parent module is automatically added.
 In the following example, the `Unit` class will have for `module` `my_extension.submodule` because it is properly nested
 but the `Ext` class will have for `module` the default `builtins` because it not nested.
+
+You can provide the `submodule` argument to `pymodule()` for modules that are not top-level modules.
 ```rust
 # mod declarative_module_module_attr_test {
 use pyo3::prelude::*;
@@ -168,7 +170,7 @@ mod my_extension {
     #[pymodule_export]
     use super::Ext;
 
-    #[pymodule]
+    #[pymodule(submodule)]
     mod submodule {
         use super::*;
         // This is a submodule

newsfragments/4301.added.md

@@ -0,0 +1 @@
+allow setting `submodule` on declarative `#[pymodule]`s

pyo3-macros-backend/src/module.rs

@@ -75,7 +75,7 @@ impl PyModuleOptions {
     }
 }
 
-pub fn pymodule_module_impl(mut module: syn::ItemMod) -> Result<TokenStream> {
+pub fn pymodule_module_impl(mut module: syn::ItemMod, is_submodule: bool) -> Result<TokenStream> {
     let syn::ItemMod {
         attrs,
         vis,
@@ -286,7 +286,7 @@ pub fn pymodule_module_impl(mut module: syn::ItemMod) -> Result<TokenStream> {
         }
     }
 
-    let initialization = module_initialization(&name, ctx);
+    let initialization = module_initialization(&name, ctx, is_submodule);
     Ok(quote!(
         #(#attrs)*
         #vis mod #ident {
@@ -335,7 +335,7 @@ pub fn pymodule_function_impl(mut function: syn::ItemFn) -> Result<TokenStream>
     let vis = &function.vis;
     let doc = get_doc(&function.attrs, None, ctx);
 
-    let initialization = module_initialization(&name, ctx);
+    let initialization = module_initialization(&name, ctx, false);
 
     // Module function called with optional Python<'_> marker as first arg, followed by the module.
     let mut module_args = Vec::new();
@@ -400,28 +400,34 @@ pub fn pymodule_function_impl(mut function: syn::ItemFn) -> Result<TokenStream>
     })
 }
 
-fn module_initialization(name: &syn::Ident, ctx: &Ctx) -> TokenStream {
+fn module_initialization(name: &syn::Ident, ctx: &Ctx, is_submodule: bool) -> 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(), ctx);
 
-    quote! {
+    let mut base = quote! {
         #[doc(hidden)]
         pub const __PYO3_NAME: &'static ::std::ffi::CStr = #pyo3_name;
 
         pub(super) struct MakeDef;
         #[doc(hidden)]
         pub static _PYO3_DEF: #pyo3_path::impl_::pymodule::ModuleDef = MakeDef::make_def();
-
-        /// This autogenerated function is called by the python interpreter when importing
-        /// the module.
-        #[doc(hidden)]
-        #[export_name = #pyinit_symbol]
-        pub unsafe extern "C" fn __pyo3_init() -> *mut #pyo3_path::ffi::PyObject {
-            #pyo3_path::impl_::trampoline::module_init(|py| _PYO3_DEF.make_module(py))
-        }
+    };
+    if !is_submodule {
+        base = quote! {
+            #base
+
+            /// This autogenerated function is called by the python interpreter when importing
+            /// the module.
+            #[doc(hidden)]
+            #[export_name = #pyinit_symbol]
+            pub unsafe extern "C" fn __pyo3_init() -> *mut #pyo3_path::ffi::PyObject {
+                #pyo3_path::impl_::trampoline::module_init(|py| _PYO3_DEF.make_module(py))
+            }
+        };
     }
+    base
 }
 
 /// Finds and takes care of the #[pyfn(...)] in `#[pymodule]`

pyo3-macros/src/lib.rs

@@ -3,7 +3,7 @@
 
 #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
 use proc_macro::TokenStream;
-use proc_macro2::TokenStream as TokenStream2;
+use proc_macro2::{Span, TokenStream as TokenStream2};
 use pyo3_macros_backend::{
     build_derive_from_pyobject, build_py_class, build_py_enum, build_py_function, build_py_methods,
     pymodule_function_impl, pymodule_module_impl, PyClassArgs, PyClassMethodsType,
@@ -35,10 +35,26 @@ use syn::{parse::Nothing, parse_macro_input, Item};
 /// [1]: https://pyo3.rs/latest/module.html
 #[proc_macro_attribute]
 pub fn pymodule(args: TokenStream, input: TokenStream) -> TokenStream {
-    parse_macro_input!(args as Nothing);
     match parse_macro_input!(input as Item) {
-        Item::Mod(module) => pymodule_module_impl(module),
-        Item::Fn(function) => pymodule_function_impl(function),
+        Item::Mod(module) => {
+            let is_submodule = match parse_macro_input!(args as Option<syn::Ident>) {
+                Some(i) if i == "submodule" => true,
+                Some(_) => {
+                    return syn::Error::new(
+                        Span::call_site(),
+                        "#[pymodule] only accepts submodule as an argument",
+                    )
+                    .into_compile_error()
+                    .into();
+                }
+                None => false,
+            };
+            pymodule_module_impl(module, is_submodule)
+        }
+        Item::Fn(function) => {
+            parse_macro_input!(args as Nothing);
+            pymodule_function_impl(function)
+        }
         unsupported => Err(syn::Error::new_spanned(
             unsupported,
             "#[pymodule] only supports modules and functions.",

tests/test_declarative_module.rs

@@ -108,7 +108,7 @@ mod declarative_module {
         }
     }
 
-    #[pymodule]
+    #[pymodule(submodule)]
     #[pyo3(module = "custom_root")]
     mod inner_custom_root {
         use super::*;