How to test for previously joined layers before run a code

Question

I've a function that join two added layers. The code works fine, but now I need to test for previously joined layers before run this function. The function code is right below:

def jointables(dict): # this dict provides [targetLayer:layerToJoin]
  for k, v in dict.items():
    target = QgsProject.instance().mapLayer(k)
    layerToJoin = QgsProject.instance().mapLayer(v)
    fieldToJoin = QgsProject.instance()
    symb = QgsVectorLayerJoinInfo()
    symb.setJoinFieldName('id_feature')
    symb.setTargetFieldName('id')
    symb.setJoinLayerId(layerToJoin.id())
    symb.setEditable(True)
    symb.setDynamicFormEnabled(True)
    symb.setUpsertOnEdit(True)
    symb.setPrefix('')
    symb.setJoinLayer(layerToJoin)
    target.addJoin(symb)

I've tried some things like this:

for k, v in dict.items():
    if QgsProject.instance().mapLayer(k).addJoin(QgsVectorLayerJoinInfo()) == True:
        break
    else:
        continue

or

for k, v in dict.items():
    if QgsVectorLayerJoinInfo().isEditable() == True:
       break
    else:
       continue

among others. I've missing something. Both conditionals has the same value True, doesnt matter if already exists an previously joined layer or not. How can I solve this?

Answer 1

If you want to check whether a QgsVectorLayer has any join you can try the following :

VL.vectorJoins()

vectorJoins() returns a list of QgsVectorLayerJoinInfo for each join set on the VL layer.

So testing for example that len(VL.vectorJoins())==0 means there is no join on the layer.