[QGIS-Developer] Changing attributes of a newly created feature in Python form init code

[Johannes Kröger (WhereGroup)]

Hey guys,

I feel really dumb and I guess I am missing some basic concept...:

  * I have a layer with two attributes: "text" (string) and "int" (integer).
  * I used drag'n'drop form design to have a form with just the "text"
    attribute in it.
  * I added Python Init Code:

```
from functools import partial

def on_ok(layer, feature):
     field_idx = layer.fields().indexOf("int")
     attribute_changed = layer.changeAttributeValue(feature.id(), 
field_idx, 12345)
     if not attribute_changed:
         raise Exception("Attribute value could not be changed!")

def my_form_open(dialog, layer, feature):
     ok_button = dialog.findChild(QPushButton)
     ok_button.clicked.connect(partial(on_ok, layer, feature))
```

So the form does not show an input widget for the "int" field. But when 
pressing OK, the code should change the edited feature's "int" value to 
12345.

This works if I edit existing features.

But if I create a new feature, the "int" value will be empty/NULL 
instead of 12345.

Any pointers on how to properly update a new feature's attribute fields, 
that exist on a layer but are not shown the user with the QGIS' widgets 
in the attribute form?

Cheers, Hannes
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