Cannot create 1d mesh

General
3

Thank you, I can now read the mesh with no issues.
However, I would like to make sure that the labelled boundaries ds are correct. To do this, I read the mesh and evaluate a test function at ds on the left and right edge of the line, here is the minimal working example ‘mwe.py’:

from __future__ import print_function
from fenics import *
from mshr import *
from fenics import *
from mshr import *

L = 1.0

mesh=Mesh()
with XDMFFile("line_mesh.xdmf") as infile:
   infile.read(mesh)
mvc = MeshValueCollection("size_t", mesh, mesh.topology().dim())
with XDMFFile("line_mesh.xdmf") as infile:
    infile.read(mvc, "name_to_read")

mf = dolfin.cpp.mesh.MeshFunctionSizet(mesh, mvc)

# Define function spaces
#finite elements for sigma .... omega
P_v = VectorElement( 'P', mesh.ufl_cell(), 2 )
P_z = FiniteElement( 'P', mesh.ufl_cell(), 1 )
element = MixedElement( [P_v, P_z] )
Q = FunctionSpace(mesh, element)

Q_v = Q.sub( 0 ).collapse()
Q_z= Q.sub( 1 ).collapse()

#analytical expression for a  scalar function used to test the ds
class TestFunction( UserExpression ):
    def eval(self, values, x):
        values[0] = x[0]**2
    def value_shape(self):
        return (1,)

# test for surface elements
ds_l = Measure( "ds", domain=mesh, subdomain_data=mf, subdomain_id=2 )
ds_r = Measure( "ds", domain=mesh, subdomain_data=mf, subdomain_id=3 )
f_test_ds = Function( Q_z )
f_test_ds.interpolate( TestFunction( element=Q_z.ufl_element() ) )

integral_l = assemble( f_test_ds * ds_l )
integral_r = assemble( f_test_ds * ds_r )

print( "Integral l = ", integral_l, " should be = 0" )
print( "Integral r = ", integral_r, " should be = 1" )

and I get

$python3 mwe.py 
Integral l =  0.0  should be = 0
Integral r =  0.0  should be = 1

Shouldn’t I get the value of the function at the boundaries in integral_l and integral_r ?
Is this because the integration domain ds has zero measure in 1d? If so, how may I test that ds_l and ds_r correspond to the two extremal points of the line ?