safeMerge.txt

### concatenate lines with same attributes
            layer = self.dlg.selectLines.currentLayer()
            crs = self.dlg.selectLines.currentLayer().crs().toWkt()
            
            ### add id to lines
            fields = []
            fields.append(QgsField('ID', QVariant.Double))
            layer.dataProvider().addAttributes(fields)
            layer.updateFields()
            i = 0
            with edit(layer):  
                for feature in layer.getFeatures():
                    feature.setAttribute(feature.fieldNameIndex('ID'), i)
                    i += 1
                    
            ### Create the merged lines layer
            outLayer = QgsVectorLayer('Linestring?crs='+ crs, 'snapped' , 'memory')
            prov = outLayer.dataProvider()
            fields = layer.fields()
            prov.addAttributes(fields)
            outLayer.updateFields()

            already_processed = []
            for lineFeat in layer.getFeatures():
                attrs = lineFeat.attributes()
                geom = lineFeat.geometry()
                curr_id = lineFeat["ID"]
                if curr_id not in already_processed:
                    query = '"ID" = %s' % (curr_id)
                    selection = layer.getFeatures(QgsFeatureRequest().setFilterExpression(query))
                    selected_ids = [k.geometry().asPolyline() for k in selection]
                    ### for finding adjacent features
                    outlist = []
                    outinds = []
                    outset = set()
                    for j, l in enumerate(selected_ids):
                        as_set = set(l)
                        inds = []
                        for k in outset.copy():
                            if outlist[k] & as_set:
                                outset.remove(k)
                                as_set |= outlist[k]
                                inds.extend(outinds[k])
                        outset.add(j)
                        outlist.append(as_set)
                        outinds.append(inds + [j])
                    outinds = [outinds[j] for j in outset]
                    del outset, outlist
                    adjacent_feats = [[selected_ids[j] for j in k] for k in outinds]
                    
                    for f in adjacent_feats:
                        first = True
                        for x in xrange(0, len(f)):
                            geom = (QgsGeometry.fromPolyline([QgsPoint(w) for w in f[x]]))
                            if first:
                                outFeat = QgsFeature()
                                outFeat.setGeometry(geom)
                                outGeom = outFeat.geometry()
                                first = False
                            else:
                                outGeom = outGeom.combine(geom)
                        outFeat.setAttributes(attrs)
                        outFeat.setGeometry(outGeom)
                        prov.addFeatures([outFeat])
                    already_processed.append(curr_id)
                else:
                    continue

            ### Add the merge lines layer to the Layers panel
            QgsProject.instance().addMapLayer(outLayer)