Dear @mekong. You are talking about two different functions.
The one you are referring to is:
https://bitbucket.org/fenics-project/dolfin/src/1c52e837eb54cc34627f90bde254be4aa8a2ae17/python/dolfin/fem/interpolation.py?at=master#interpolation.py-31,54,71,73
while I am talking about
https://bitbucket.org/fenics-project/dolfin/src/1c52e837eb54cc34627f90bde254be4aa8a2ae17/python/dolfin/function/function.py?at=master#lines-379
which interpolates an expression into the already defined function v.
The approach you are using creates a new function at each time step. Each function is given a new name (if you don’t give it an explicit name yourself), thus when outputted to paraview each time step has a unique function.