can't build python part with C modules

[Alberto Donato]

Building python packages that have C modules fails with snapcraft as C headers are not found.

This is a sample snap that fails building on Bionic:

name: test
version: '1.0'
summary: test
description: test
grade: stable
confinement: strict
base: core18

apps:
  uwsgi:
    command: uwsgi

parts:
  uwsgi:
    plugin: python
    source: .
    python-packages:
      - uWSGI
    build-packages:
      - python3-dev
    stage-packages:
      - libpython3.6

exporting C_INCLUDE_PATH=/usr/include/python3.6 makes it work.

Tested with both stable (2.43.1) and candidate (3.0) snaps.

Also, removing the core18 base and using libpython3.5 when building on xenial shows the same issue

[Alberto Donato (ack)]

This is a bigger issue when building with VMs in snapcraft 3, as there's no way to export C_INCLUDE_PATH.

FWIW I also tried

override-build: |
   export C_INCLUDE_PATH=/usr/include/python3.6
   snapcraftctl build

but that didn't work for me either

[林博仁(Buo-ren, Lin) (buo-ren-lin)]

> This is a bigger issue when building with VMs in snapcraft 3, as there's no way to export C_INCLUDE_PATH.

You can use the `build-environment` key, refer https://github.com/snapcore/snapcraft/pull/2322 for more info.

For legacy Snapcraft syntax you have to override the build step and build the part manually, or [write a local plugin](https://forum.snapcraft.io/t/writing-local-plugins/5125) to replace/complement [the `python` plugin](https://github.com/snapcore/snapcraft/blob/master/snapcraft/plugins/python.py).

[林博仁(Buo-ren, Lin) (buo-ren-lin)]

Here's the part recipe that fixes the problem:

```
parts:
  uwsgi:
    plugin: python
    source: .
    python-packages:
      - uWSGI
    build-packages:
      - python3-dev
    build-environment:
      - C_INCLUDE_PATH: /usr/include/python3.6
    stage-packages:
      - libpython3.6
```

I noticed that "/usr/include/*/Python.h" isn't in any of the default GCC preprocessor search paths though, is the regular non-snap build really can't reproduce this issue?

[Alberto Donato (ack)]

Thanks, that's much nicer that having to export it outside of snapcraft!

Still, I think this bug is valid as snapcraft should pass that export itself in the python plugin, as it knows the python version that's being used.