Re: How to properly import a plugin?

[Sam Thursfield <sam thursfield codethink co uk>]

Hi Chandan,

On 14/08/17 20:17, Chandan Singh wrote:
> Hello,
>
> I was trying to extend one of the Element Plugins but ran into an issue
> quite early on. I'm hoping that someone on this list might have some
> answers.
>
> I was trying to naively extend a plugin (DpkgElement specifically) like so:
>
> ```
> from buildstream.plugins.elements.dpkg_build import DpkgElement
>
> # Element implementation for the 'my_dpkg_build' kind.
> class MyDpkgElement(DpkgElement):
>      pass

Thanks for trying out BuildStream!

By design, elements from the plugins/ module can't be extended in Python 
code.

The idea is that the plugin should be flexible enough that you can 
achieve whatever you want just from the .bst files you create, i.e. by 
writing YAML only. If there's something that the DpkgElement can't do, 
we should look at improving the DpkgElement plugin itself.

We do have a few elements types which are subclassed (such as the 
BuildElement base class) but these are implemented in the core 
'buildstream' module, not as plugins.

> While the above import statement worked successfully when I was trying it
> out in a python shell, it doesn't seem to work when it is run as part of
> the `bst build` command. Upon doing that, I get an error message like so
> (truncated):
>
> ```
> ...
>
>    File "/usr/local/lib/python3.5/dist-packages/pluginbase.py", line 410, in
> plugin_import
>      fromlist, level)
>    File "/src/plugins/elements/bb_dpkg_build.py", line 1, in <module>
>      from buildstream.plugins.elements.dpkg_build import DpkgElement
>    File "/usr/local/lib/python3.5/dist-packages/pluginbase.py", line 410, in
> plugin_import
>      fromlist, level)
> ImportError: No module named
> 'pluginbase._internalspace._sp606628c88e67eb4efbaa8e0085714209.elements'
> ```

It looks like we need to make limitation this clearer in the code & 
documentation, this error message is nonsense :-) I've opened 
https://gitlab.com/BuildStream/buildstream/issues/64 to see if we can 
give a better error.

Sam

--
Sam Thursfield, Codethink Ltd.
Office telephone: +44 161 236 5575