SWIG, Python and interface file with inline directive

Question

I have Ubuntu-20.04, Anaconda-3 (installed into the user dir) with Python-3.7.9 and SWIG-4.0.

Here are my files.

a.h:

void foo(void);

a.c:

#include <stdio.h>
#include "a.h"
void foo(void) { printf("Foo!\n"); }

a.i:

%module a
%inline %{
#include "a.h"
%}

test.py:

import a
a.foo()

Compile script compile.sh:

A=$HOME/opt/anaconda3
I=$A/include/python3.7m
gcc -c -fpic -DHAVE_CONFIG_H -I$I a.c
$A/bin/swig -python -py3 a.i
gcc -c -fpic -DHAVE_CONFIG_H -I$I a_wrap.c
gcc -shared a.o a_wrap.o -o _a.so

After compilation the test script produces

Traceback (most recent call last):
  File "test.py", line 2, in <module>
    a.foo()
AttributeError: module 'a' has no attribute 'foo'

However, if I write a longer interface file everything is OK:

%module a
%{
#include "a.h"
%}
%include "a.h"

UPD @Jens suggested to replace # with % in the first (short) interface file. In this case I got

a_wrap.c:2701:1: error: expected identifier or '(' before '%' token
 2701 | %include "a.h"
      | ^
a_wrap.c:2720:12: error: '_wrap_foo' undeclared here (not in a function)
 2720 |   { "foo", _wrap_foo, METH_NOARGS, NULL},
      |            ^~~~~~~~~
gcc: error: a_wrap.o: No such file or directory

Answer 1

%inline both includes the braced code directly in the SWIG-generated wrapper code, but also processes it to generated target language interfaces. So this:

%module a
%inline %{
void foo(void);
%}

is equivalent to:

%module a
%{
void foo(void) {}
%}
void foo(void) {}

But this:

%module a
%inline %{
#include "a.h"
%}

is equivalent to:

%module a
%{
#include "a.h"
%}
#include "a.h"  // NOT the same as %include "a.h" and isn't processed for interfaces

To show that %inline is both included and processed, I made:

%module a
%inline %{
#include "a.h"   // ignored in "include-in-wrapper" pass
#ifdef SWIG
%include "a.h"   // processed in "wrap-with-SWIG" pass
#endif
%}

The above did the right thing and exposed the interface, but it's worse than just using:

%module a
%{
#include "a.h"
%}
%include "a.h"

%inline is really for inserting new functions into the wrapper and exposing the interface, such as this:

%module a
%inline %{
class myClass
{
private: 
  int a;
public: 
  myClass(){} 
  void foo(){}
};
%}

where otherwise you'd have to write, at a minimum:

%module a

%{ // new functionality added to wrapper
class myClass
{
private: 
  int a;
public: 
  myClass(){} 
  void foo(){}
};
%}

class myClass   // process the interface
{
public:         // note don't need to repeat private part or
  myClass();    // implementation, just public declarations to be exposed.
  void foo();
};