Python 3.7: from __future__ import annotations

Hi,

I’m on Windows 10, Python 3.7.1, Cython 0.29.7.

I have some pure python code that I wanted to compile that uses from __future__ import annotations, but compiling it throws:

Error compiling Cython file:
------------------------------------------------------------
...
# -*- coding: utf-8 -*-

from __future__ import annotations
                      ^
------------------------------------------------------------

app.py:6:23: future feature annotations is not defined
Traceback (most recent call last):
  File "D:\app\compile_with_cython.py", line 59, in <module>
    main()
  File "D:\app\compile_with_cython.py", line 54, in main
    language_level=3,
  File "C:\Python37\lib\site-packages\Cython\Build\Dependencies.py", line 1097, in cythonize
    cythonize_one(*args)
  File "C:\Python37\lib\site-packages\Cython\Build\Dependencies.py", line 1220, in cythonize_one
    raise CompileError(None, pyx_file)
Cython.Compiler.Errors.CompileError: app/app.py

It is used to type annotate some self method parameters that need to reference itself. I’m guessing that this is a not yet supported feature of CPython 3.7? Is there a way to bypass this without removing the from __future__ import annotations code?

Matic

[da-woods]

@shaolo1 Glad to know you have it fixed.

Just to try to clarify: the effect of “binding” is to make functions defined in Cython look more like a regular Python function. I think it’s necessary for __annotations __ to appear as an attribute of the function (but I’m not completely sure). It’s enabled by default on def functions anyway.

It isn’t required for the __future__ statement to work.

[da-woods]

@McSinyx The problem is that from __future__ import annotations should still be valid code. Although the feature is always enabled in Cython (post https://github.com/cython/cython/pull/3285) the __future__ statement shouldn’t break anything.

However @shaolo1 as far as I can tell it didn’t make it in to 0.29.15. It’s in the changelog for Cython 3.0. Given that 0.29.x doesn’t support to, generating an error message is the correct behaviour. If you download the “master” branch it’ll work.