Distance Along Line | Python Fiddle

##Points=vector point
##Only_use_selected_points=boolean False
##Line=vector line
##Only_use_selected_lines=boolean False
##step=number 1

from qgis.core import *
from qgis.PyQt.QtCore import QVariant

layer1 = processing.getObject(Points)
crs = layer1.crs().toWkt()
layer2 = processing.getObject(Line)

if Only_use_selected_lines:
    line_feats = layer2.selectedFeatures()
else:
    line_feats = layer2.getFeatures()

for ft in line_feats:
    index = QgsSpatialIndex()
    tmp_points = {}
    line_geom = ft.geometry()
    len = line_geom.length()
    current = 0

# This layer contains all the points created along the line (it isn't an output)
    temp_pts = QgsVectorLayer('Point?crs='+ crs, 'temp_pts' , 'memory')
    prov = temp_pts.dataProvider()
    # Uncomment the next line if you want to see the 'temp_pts' layer
    #QgsMapLayerRegistry.instance().addMapLayer(temp_pts)

while current < len:
        point = line_geom.interpolate(current) # Create a point along the line at the current distance
        fet = QgsFeature()
        fet.setGeometry(point)
        (result, feat) = prov.addFeatures([fet])
        tmp_points[feat[0].id()] = current
        index.insertFeature(feat[0])
        current += step # Increase the distance by the step provided

# This layer is the final output which stores the distances from the closest point along the line and the start/end of the line
    out = 'output_%s' % (ft.id())
    output = QgsVectorLayer('Point?crs='+ crs, out, 'memory') 
    prov2 = output.dataProvider()
    fields = layer1.pendingFields() # Fields from the input layer
    fields.append(QgsField('DIST_START', QVariant.Double, '', 10, 3)) # Name for the new field in the output layer
    fields.append(QgsField('DIST_END', QVariant.Double, '', 10, 3)) # Name for the new field in the output layer
    prov2.addAttributes(fields) # Add input layer fields to the outLayer
    output.updateFields()

if Only_use_selected_points:
        point_feats = layer1.selectedFeatures()
    else:
        point_feats = layer1.getFeatures()

for feat in point_feats:
        geom = feat.geometry()
        attrs = feat.attributes()
        nearest = index.nearestNeighbor(geom.asPoint(), 1)
        dist_start = tmp_points[nearest[0]]
        attrs.append(dist_start)
        dist_end = len - tmp_points[nearest[0]]
        attrs.append(dist_end)
        outGeom = QgsFeature()
        outGeom.setGeometry(geom)
        outGeom.setAttributes(attrs)
        prov2.addFeatures([outGeom])

# Add the 'output' layer to the Layers panel
    QgsMapLayerRegistry.instance().addMapLayer(output)

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