SWIG Symbol not found, leading underscore in symbol name - python-3.x

Question

I'm following the tutorial for SWIG and Python, but I'm getting an error when I try to import the compiled extension in Python.
Python 3.5.2 (default, Oct 11 2016, 15:01:25)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.38)] on darwin
>>> import _example
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: dlopen(/pat/to/_example.cpython-35m-darwin.so, 2): Symbol not found: _fact
Referenced from: /path/to/_example.cpython-35m-darwin.so
Expected in: flat namespace
in /path/to/_example.cpython-35m-darwin.so
Note that the error says it can't find the symbol _fact. I'm not sure where the leading underscore is coming from, and I can't find any information about how to resolve this problem.
Here is example.i (matches what's in the tutorial):
%module example
%{
#define SWIG_FILE_WITH_INIT
#include "example.h"
%}
int fact(int n);
example.h and example.c also match what's in the tutorial; I can post here if necessary.
Here's setup.py:
# coding=utf-8
from setuptools import Extension, setup
setup(
name = 'SWIG Test',
ext_modules = [Extension('_example', ['example.i'])],
py_modules = ['example'],
)
This is the relevant output of pip install -e . -v:
running build_ext
building '_example' extension
swigging example.i to example_wrap.c
swig -python -o example_wrap.c example.i
creating build
creating build/temp.macosx-10.12-x86_64-3.5
/usr/bin/clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -pipe -Os -I/opt/local/Library/Frameworks/Python.framework/Versions/3.5/include/python3.5m -c example_wrap.c -o build/temp.macosx-10.12-x86_64-3.5/example_wrap.o
creating build/lib.macosx-10.12-x86_64-3.5
/usr/bin/clang -bundle -undefined dynamic_lookup -L/opt/local/lib -Wl,-headerpad_max_install_names build/temp.macosx-10.12-x86_64-3.5/example_wrap.o -o build/lib.macosx-10.12-x86_64-3.5/_example.cpython-35m-darwin.so
copying build/lib.macosx-10.12-x86_64-3.5/_example.cpython-35m-darwin.so ->
Some things that I've tried that didn't work:
Declaring fact as int fact(int n) asm("fact") (after building, I get Symbol not found: fact when importing).
Adding swig_opts=['-py3'] to the extension in setup.py.

Answer 1

The leading underscore was a red herring. Turns out, I simply forgot to include example.c in my extension sources.
Here's what setup.py is supposed to look like:
# coding=utf-8
from setuptools import Extension, setup
setup(
name = 'SWIG Test',
ext_modules = [Extension('_example', ['example.i', 'example.c'])],
py_modules = ['example'],
)
Note that the extension has both example.i and example.c in its sources.
Now everything works as expected (:
Python 3.5.2 (default, Oct 11 2016, 15:01:25)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.38)] on darwin
>>> import example
>>> example.fact(42)
0
>>> example.fact(0)
1