[Qgis-developer] Change feature via Python

[Martin Dobias]

On Mon, Sep 5, 2011 at 10:45 AM,  <bernhard.stroebl at jena.de> wrote:
> Dear devs,
>
> my situation is as follows:
> I am working on some Python code. I have a non-spatial layer in the project
> created from a PostgreSQL table.
> For this layer I can (via Python):
> 1) create a new feature, manipulate its attribute values and add it to the
> layer
> 2) access an existing feature and its attribute values by this code:
> feat = QgsFeature()
> ok = layer.featureAtId(fid, feat, False, True)
> 3) open the feature form and have the user manipulate the attribute values
> of either 1) or 2)
>
> However I could not find a way to access an existing feature and manipulate
> its attribute values so that the changed values are written back to the
> layer just like the feature form does.
> I understand that the feature is not part of the layer as soon as it is
> accessed by the code given above. Is there a way to replace a feature in a
> layer or change its attribute values other than via feature form?

Sure. There are two options.

You can use layer's changeAttributeValue() method to tell the vector
layer about the changes. For this the layer has to be in editing mode
- either user can start it or you can start it from plugin with
startEditing() and stop with commitChanges() or rollBack(). In the
editing mode the changes are stored in memory and stored to a
file/database when editing mode is left with commitChanges() method.
This is equivalent to the use of feature form.

The other option is to use vector data provider's method
changeAttributeValues(). The difference is that the changes are
immediately written to the backend and the editing mode is not
considered. That method is actually called from vector layer when
committing changes from the editing mode.

Martin