What is the meaning of this ImportError when importing a Cython generated .so file?

Question

I am walking through the Cython documentation and building each of the example apps. I'm a little stuck at Using C Libraries. After successfully building the .so file and attempting to import it in a python file called test.py the following error is thrown.

$ python3.2 test.py 
Traceback (most recent call last):
  File "test.py", line 12, in <module>
    from queue import Queue
ImportError: dlopen(/Users/jeremy/Development/labs/python/cython_lib_wrapper/queue.so, 2): Symbol not found: _queue_free
  Referenced from: /Users/jeremy/Development/labs/python/cython_lib_wrapper/queue.so
  Expected in: flat namespace
 in /Users/jeremy/Development/labs/python/cython_lib_wrapper/queue.so

The .so file sits right next to the test.py file. So, it seems as though it should be found. This is running the latest version of Cython, with Python 3.2 on a OSX 10.6.

Any insights?

Edit - adding build command and output

$ python3.2 setup.py build_ext --inplace
running build_ext
cythoning queue.pyx to queue.c
building 'queue' extension
gcc-4.2 -fno-strict-aliasing -fno-common -dynamic -DNDEBUG -g -O3 -isysroot /Developer/SDKs/MacOSX10.6.sdk -arch i386 -arch x86_64 -isysroot /Developer/SDKs/MacOSX10.6.sdk -I/Library/Frameworks/Python.framework/Versions/3.2/include/python3.2m -c queue.c -o build/temp.macosx-10.6-intel-3.2/queue.o
    queue.c: In function ‘__pyx_f_5queue_5Queue_append’:
    queue.c:627: warning: cast to pointer from integer of different size
    queue.c: In function ‘__pyx_f_5queue_5Queue_extend’:
    queue.c:740: warning: cast to pointer from integer of different size
    queue.c: In function ‘__pyx_f_5queue_5Queue_peek’:
    queue.c:813: warning: cast from pointer to integer of different size
    queue.c: In function ‘__pyx_f_5queue_5Queue_pop’:
    queue.c:965: warning: cast from pointer to integer of different size
    gcc-4.2 -bundle -undefined dynamic_lookup -arch i386 -arch x86_64 -isysroot /Developer/SDKs/MacOSX10.6.sdk -isysroot /Developer/SDKs/MacOSX10.6.sdk -g build/temp.macosx-10.6-intel-3.2/queue.o -o 

Edit 2 - adding "otool" cmd requested in comment

queue.so:
    /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 125.2.0)

Edit 3 - adding "nm" output

U ___stack_chk_fail
U ___stack_chk_guard
U _queue_free
U _queue_is_empty
U _queue_new
U _queue_peek_head
U _queue_pop_head
U _queue_push_tail
U dyld_stub_binder

grep cmd outputs this:

(undefined) external _queue_free (dynamically looked up)

Answer 1

EDIT:

Ah, you didn't mention you had a dependency on the code in libcalg. That stuff needs to be compiled and included when you build the cextension.

Just modify setup.py:

# setup.py
# ...
ext_modules = [Extension("queue", ["queue.pyx", "libcalg/queue.c"])]
# ...

---

We could step back and see if you can build a really simple example:

I've tried the following (3 files, myext.pyx, test.py, setup.py) and it appears to work fine. Of course I'm on OS X 10.7 so it's not exactly the same as your environment. To rule out differences perhaps you can copy these and build them as a sanity check.

Contents of myext.pyx:

# myext.pyx
def square(x):
    return x * x

Contents of test.py

# test.py
from myext import square
print "%d squared is %d"%(4, square(4))

Contents of setup.py:

# setup.py
from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

ext_modules = [Extension("myext", ["myext.pyx"])]

setup(
  name = 'Hello world app',
  cmdclass = {'build_ext': build_ext},
  ext_modules = ext_modules
)

I build in the directory containing these 3 files:

cython_test$ /usr/bin/python setup.py build_ext --inplace 
running build_ext
cythoning myext.pyx to myext.c
building 'myext' extension
creating build
creating build/temp.macosx-10.7-intel-2.7
llvm-gcc-4.2 -fno-strict-aliasing -fno-common -dynamic -g -Os -pipe -fno-common -fno-strict-aliasing -fwrapv -mno-fused-madd -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE -arch i386 -arch x86_64 -pipe -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c myext.c -o build/temp.macosx-10.7-intel-2.7/myext.o
llvm-gcc-4.2 -Wl,-F. -bundle -undefined dynamic_lookup -Wl,-F. -arch i386 -arch x86_64 build/temp.macosx-10.7-intel-2.7/myext.o -o /Users/steder/SO/cython_test/myext.so

cython_test$ python test.py
4 squared is 16:

My environment has similar otool output and DYLD_LIBRARY_PATH is also unset but nm -m shows squared as defined.

Specifically:

00000000000011d0 (__DATA,__data) non-external ___pyx_k__square
00000000000011e0 (__DATA,__data) non-external ___pyx_mdef_5myext_square
0000000000001218 (__DATA,__bss) non-external ___pyx_n_s__square
0000000000000c80 (__TEXT,__text) non-external ___pyx_pf_5myext_square

Please give this a shot and see what it nm -m shows in your environment.