How to keyframe constraints on bones?

Question

I get the error

line 60, in return_rotation
TypeError: bpy.struct.keyframe_insert()
property "constraints["FollowPath"].offset_factor" not found

I really dont get what should be the problem and was hoping you guys could help out.

This is the code:

import bpy
import re
import os
import math
import linecache

cube = bpy.data.objects['Cube'] # defining the cube (TestObject)


armature = bpy.data.objects['Armature'] # Defining the armature
bpy.context.scene.objects.active = armature # Selecting the armature
bpy.ops.object.mode_set(mode='POSE') # Setting the mode to POSE
footBone = armature.pose.bones['foot'] # Defining the footBone (the big one)
toesBone = armature.pose.bones['toes'] # Defining the toesBone (the small one)
footBone.rotation_mode = 'XYZ' # Set rotation mode to Euler XYZ, easier to understand
toesBone.rotation_mode = 'XYZ' # Set rotation mode to Euler XYZ, easier to understand


bpy.ops.object.mode_set(mode='OBJECT')
# insert a keyframe


def file_len(fname): # A function which returns the number of lines in a file.
    with open(fname) as f:
        for i, l in enumerate(f):
            pass
    return i + 1 


numberOfLines = file_len('somestuff\Python Test/usedata.csv')#Getting number of lines - 10 to be safe, but safe limit will be removed later.
steps = 1/numberOfLines # How much, is needed to be increased in the followPath modifier, given that only one cycle is needed.
cube.constraints['Follow Path'].offset_factor = 0 #Setting offset to 0, Todo find the right offset. Find how much a cycle is.

def return_rotation(start_value, end_value):
    steps = 1 / (end_value-start_value) # this ensures a complete cycle when all the lines are read
    counter = 0 # This is the frameNumber
    i = start_value # The line number from which to start reading from
    while i < end_value: #end_value is the line to be read to
        line = linecache.getline('somestuff\Python Test/usedata.csv', i).split(',') # fetching a single line

        # setting rotation angles. It only accepts radians, hence the conversion.
        angleX = math.radians(float(line[0]))
        angleY = math.radians(float(line[1]))
        angleZ = math.radians(float(line[2]))
        print(angleX)



        # TODO: check if toesBone is under the floor

        #----------------FootBone-------------------------------------------------------------------------------------------------#
        # Rotate Bones
        footBone.rotation_euler.rotate_axis('X', angleX)
        footBone.rotation_euler.rotate_axis('Y', angleY)
        footBone.rotation_euler.rotate_axis('Z', angleZ)

        # keyframe everything
        footBone.keyframe_insert(data_path="rotation_euler" ,frame = counter) #Insert rotation keyframe
        footBone.constraints["Follow Path"].offset_factor += steps # increasing offset factor

The problem

        footBone.keyframe_insert(data_path= 'constraints["Follow Path"].offset_factor', frame = counter) # Keyframing the offset factor.

End of problem

        # Checking if the offset_factor is above 1 or below zero, and correts this
        if(footBone.constraints['Follow Path'].offset_factor >= 1 or footBone.constraints['Follow Path'].offset_factor < 0): 
            footBone.constraints['Follow Path'].offset_factor = 0

        #----------------cube-----------------------------------------------------------------------------------------------------#
        cube.rotation_mode = 'XYZ'

        cube.rotation_euler[0] += angleX
        cube.rotation_euler[1] += angleY
        cube.rotation_euler[2] += angleZ
        print(float(line[0]))
        print(cube.rotation_euler[0])

        cube.keyframe_insert(data_path='rotation_euler',frame = counter)
        cube.constraints["Follow Path"].offset_factor += steps #Stepping through cycle
        cube.keyframe_insert(data_path= 'constraints["Follow Path"].offset_factor', frame=counter) # Keyframing the offset factor.

        #Checking if the follow path cycle is above one. Might be removed, if not using offset_factor from 0-1 but instead -99999 - 99999
        if(cube.constraints['Follow Path'].offset_factor >= 1 or cube.constraints['Follow Path'].offset_factor < 0): 
            cube.constraints['Follow Path'].offset_factor = 0


        i += 1
        counter += 1
    linecache.clearcache()

return_rotation(1,280)

Answer 1

If I add an armature manually, add a Follow Path constraint in Pose mode, enable Fixed Position and keyframe the offset slider, the data_path of the created f-curve is:

'pose.bones["Bone"].constraints["Follow Path"].offset_factor'

So it's referenced by the "owning" object, and the data path fully qualifies the pose bone and its constraint.