Getting only joined fields using PyQGIS

Question

How do I get a list of fields that have been joined to a layer, or how can I check if a specific field has been joined?

In older versions there seemed to be field.source(), but this is not available anymore.

I am not performing the join via PyQGIS. The join is done manually and I would just like to use PyQGIS to detect which fields have been joined.

Answer 1

One can detect which fields have been joined by means of the QgsFields class and its method fieldOrigin:

Returns the field's origin (value from an enumeration).

from qgis.core import QgsProject
layer = QgsProject.instance().mapLayersByName("points")[0]
joined_fields = []
if len(layer.vectorJoins()) > 0:
 
 fields = layer.fields()
 
 for field in fields:
 field_name = field.name()
 indx = fields.indexOf(field_name)
 
 if fields.fieldOrigin(indx) == fields.OriginJoin:
 # or if fields.fieldOrigin(indx) == 2:
 joined_fields.append(field_name)
 
print(joined_fields)

The above code will result in a list of fields that have been joined to a layer. In my case it will be:

['month', 'month2', 'month3']

2 = OriginJoin means that this field comes from a joined layer.

OriginJoin Field comes from a joined layer (originIndex / 1000 = index of the join, originIndex % 1000 = index within the join)

---

References:

• Getting field names of layer using PyQGIS
• Getting field index from input vector layer in PyQGIS

• BritishSteel: Works very well. Thank you for providing a very clear answer!