[QGIS-Developer] Common PyQGIS functions for QGIS 3

[Nyall Dawson]

On Thu, 21 Feb 2019 at 00:51, Thomas Baumann <rdbath.regiodata at gmail.com> wrote:
>
> Hi Nyall,
>
> thanks for your feedback.
> You are right that some of my examples were not ideal to explain what I meant, so I try to explain it a bit better this time ;-)

Thanks - it's much appreciated!

> create a vectorlayer
>
> Something like:
>
> https://gist.github.com/rdbath/bbba7640cf98ab09dc9bb6018011f05e#file-pyqgis_common_functions_examples-py-L24

Ah, that one IS a source of pain. QGIS 3.0 makes it easier with

   new_empty_memory_layer = QgsMemoryDataProvider.createMemoryLayer( ... )

   and

   new_memory_layer_with_copy_of_features = source_layer.materialize(
QgsFeatureRequest().setFilterExpression( ... ) )

You'd still need to follow these up with a call to QgsVectorFileWriter
to save as a disk based format.

There's some trickiness here because with some disk formats it's not
possible to create an empty layer upfront -- e.g. geojson. You have to
have some features ready to initially populate that layer with at time
of creation.

I'd be interested to hear any ideas for "ultimate wishlist" api here?

> Example2: For loading layer I meant something like
>
> https://gist.github.com/rdbath/bbba7640cf98ab09dc9bb6018011f05e#file-pyqgis_common_functions_examples-py-L7:
...

This approach is somewhat of a hack, which is why I wouldn't like to
see it as part of any "officially supported" python library. Really,
API for doing this belongs in core QGIS API itself (covered by
appropriate documentation + unit tests, of course!)

> Example3: Iterate through layers:
...
> So if there would be a pyqgis-commons library where I would know that I just have to call a function "get_visible_layers" instead of  using [l.layer() for l in
> QgsProject.instance().layerTreeRoot().findLayers() if l.isVisible()]  I probably could use this in my next plugin without having to search/google how to do this.

Right - but again, this doesn't belong in a Python specific library.
What we need here is a core API addition "visibleLayers":

    visible_layers = QgsProject.instance().layerTreeRoot().visibleLayers()

> Example4: show messages / log messages:

> One other example would be the function "display_warning_message_bar" from the inasafe-utilities where I can show a short message and also provide a button to show more details about a warning:
> https://github.com/inasafe/inasafe/blob/bd00bfeac510722b427544b186bfa10861749e51/safe/utilities/qgis_utilities.py#L132

Once again, I think this could easily be in the c++ API, so that both
Python and c++ code could take advantage of it. I don't see anything
Python-specific here.

>  Example5: Settings:

> Here I hat something in mind like the qgis-settingsmanager of opengis ( https://github.com/opengisch/qgissettingmanager ).
> I know it is not only a collection of python-/pyqt-functions but a whole python module but I still it is an example of how functionalities are provided to make the development of plugins easier.

Yes, that one *is* nice. I think Denis originally intended that to be
made available in the default PyQGIS library, but I suspect just ran
out of time.

> So it is always about providing some helper functions that I could also write by myself but which will save time during development and can help me to do the things in a consistent way in all of my plugins.

Right, but I think it's very important to differentiate here between:

1. API additions which are useful in all contexts (c++ and Python)
2. Python specific API additions, which make use of Python only
functionality like decorators, exceptions, and context managers


>
>
>>
>> I'm not saying we can't improve the PyQGIS experience, but I don't
>> personally see these as good candidates.
>>
>> I think we have a LOT of work to do with things like:
>> - throwing proper exceptions instead of crashing/returning "None"
>> values/returning "invalid" objects (e.g. QgsGeometry.fromWkt should
>> ideally raise an exception for invalid wkt instead of returning a null
>> geometry)
>> - providing Python-esque things like __repr__ methods, __len__, []
>> operators (which behave in a python style, e.g. with -1 returning the
>> last item), iterators, etc
>> - providing more "context managers" (e.g. with .... : #something )
>
>
> I don't have such a deep insight in the QGIS-codebase but I guess this would probably have to be done in the C++-part of QGIS, wouldn't it?

Sometimes, or sometimes it's done like this:
https://github.com/qgis/QGIS/blob/master/python/core/__init__.py.in

> I agree. Even if the PyQGIS-Documentation is now much better than in the past for me often some short examples like the ones from Thomas Gratier are really helpful to know how to proceed:
> https://webgeodatavore.github.io/pyqgis-samples/

I'd love to see discussion on how we can merge this awesome resource
into the new PyQGIS documentation. Together they'd make an awesome
resource!

Nyall



>
> So I see three ideas here:
>
> - provide a collection of common PyQGIS-functions
> - provide nice PyQGIS-examples
> - improve the PyQGIS experience in generall (your examples like throwing proper exception)
>
>
> regards,
> Thomas
>
>
>
>> Nyall
>>
>> >
>> >
>> >
>> > Wouldn't it be possible to provide such a collection not only from privat persons/projects but from the QGIS-project itself so users could add common functions?
>> > I think the chances would be higher that such a "official" collection would be used in the long run and constantly extended.
>> >
>> > Apart from the fact that developers could save time while writing their plugins this could perhaps also help to improve the overall quality of the qgis-plugins as there would probably be several persons who could/would countercheck these common functions to make sure they are well written.
>> >
>> > regards,
>> > Thomas
>> >
>> > PS: I asked something similar also on gis.stackexchange recently:
>> >
>> > https://gis.stackexchange.com/questions/311755/looking-for-common-pyqgis-functions-for-qgis-3
>> >
>> >
>> >
>> >
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