50

I have just compiled part of my C library as an extension using Cython, as a "proof of concept". I managed to hack the code (const correctnes problems etc aside), to finally get an extension built.

However, when I attempted to import the newly created extension, I got the following error:

ImportError: dynamic module does not define init function

What am I doing wrong and how do I fix this?

I am using Cythn 0.11.2 and Python 2.6.5 on Ubuntu 10.0.4

Homunculus Reticulli
  • 65,167
  • 81
  • 216
  • 341

9 Answers9

76

I've found that a frequent cause of this problem is, when using a distutils setup file to compile the code, that the .pyx base name does not match the extension name, e.g:

ext = Extension(name='different', sources=['cython_ext.pyx']) # Won't work

To avoid the problem the extension name should be exactly the same, in this case, cython_ext.

Dologan
  • 4,554
  • 2
  • 31
  • 33
  • Nice call. Does anyone know why this works, and why a different name does not? – dinkelk Jun 24 '14 at 19:42
  • 2
    @Dologan, thanks, it helped! I want to make your answer more complete: the `name` can be nested like `make.something.useful.cython_ext`. The point is that the module name (word after the last point) should be the same as the name of the source file. I guess, it would be helpful to put such positive example in your answer. – Charlie Aug 23 '18 at 13:21
  • thank you thank you. Amazing that this many years later there still isn't a * error message that explains this. – Neil May 03 '20 at 17:43
36

It appears that it's a bug/feature in Cython. I had the same thing, but simply added:

STUFF = "Hi"

to the top of my .pyx file and the issue went away. It appears if there is no global initialization (a cinit or setting a global variable), that the required initialization code isn't generated.

Tim
  • 361
  • 3
  • 4
2

This is a very late answer - but I just had the same error, and mine went away when I used __cinit__ instead of __init__. I'm still learing about extension types so currently I do not know why this happens. :) (You can take a look at http://docs.cython.org/src/reference/extension_types.html#initialization-cinit-and-init) Hope this is useful to someone.

joon
  • 3,899
  • 1
  • 40
  • 53
  • This is a _very_ late comment, but I think this answer would relate to a `cdef class` rather than a module, so probably not the problem reported. However, I realise it's probably difficult to know what issue you were fixing at this stage... – DavidW Nov 19 '19 at 08:56
1

Another really late answer in my case I had accidentally called cython in a terminal that was running python2, while trying to use the generated library from a terminal that was on another python environment, using python3.

Using the same python version everywhere fixed it.

Cola_Colin
  • 415
  • 5
  • 16
0

Likewise a late answer... but I kept finding my way back to this question in particular. It probably is related to the mismatched names issue that Dologan addresses.

What happened in my case was that I was adapting an example I'd gotten to work, and got the module does not define init function error. This was verified by using (e.g.)

nm -m build/lib.macosx-10.9-x86_64-2.7/myproj.so

In this command's output I searched for 'init' and found

000000000000c0d0 (__TEXT,__text) external _initexample

I had removed all instances of 'example' from my setup.py and .pyx file, but this persisted even after removing the extension from site-packages, removing the build and dist folders, etc. I finally found that the .cpp file being generated from my .pyx file was still referring to the class name in the example. Once I reran my setup.py, import works, and indeed the .so file includes

000000000000c0a0 (__TEXT,__text) external _initmyproj

Gertlex
  • 275
  • 1
  • 2
  • 10
0

I've also ran into this problem. Make sure that your Cython file contains at least one of the following:

  • a normal Python def
  • a normal Python class (not cdef class)
  • a line of Python initialization, eg. a=None or a logger load

Otherwise Cython won't generate a PyInit routine needed to load the module, and as such the module won't be importable by Python.

Piotr Maślanka
  • 62
  • 7
  • I can't get it to fail even with an empty pyx file. If you have an example of something that doesn't work it'd probably be useful to report it as a bug as https://github.com/cython/cython/issues. – DavidW Dec 20 '20 at 19:57
-1

This is solved by adding a doc-string to your functions.

GetLastError
  • 1
  • 1
-1

I had the same error and was solved by running the main .py script in "Execute in a dedicated console " mode. Available in Tools - Preferences - Run.

aishwarya selvaraj
  • 67
  • 1
  • 8
-1

I solve it by 

def cinit(self): pass

Hope it helps.

CKE
  • 1,533
  • 19
  • 18
  • 29
文佳鹏
  • 49
  • 2
  • This sounds like you had a problem with a `cdef class` while this question is about modules. I'd be surprised if this was correct. Also note `__cinit__` not `cinit` - the double underscores are used for special methods in Python. – DavidW Nov 19 '19 at 08:54