Loading xdmf data back in

I/O
4

write saves the data at the vertices, efficiently projecting the solution to a CG-1 function space. write_checkpoint saves the data with the appropriate function space, and can therefore be read into dolfin again, to the appropriate function space.
You are not saving multiple time steps to your file as you have not changed the append kwarg in write_checkpoint (see: https://fenicsproject.org/docs/dolfin/latest/cpp/de/d71/classdolfin_1_1XDMFFile.html#a9ef9031aec7bfec8f289d5210a1c811c). The following works:

from dolfin import *
nx = ny = 8 
mesh = UnitSquareMesh(nx, ny) 
V = FunctionSpace(mesh, 'P', 1) 
    
alpha = 3; beta = 1.2 
f = Expression('1 + x[0]*x[0] + alpha*x[1]*x[1] + beta*t', degree=2, alpha=alpha, beta=beta, t=0) 
 
f_out = XDMFFile("test.xdmf") 
f_out.write_checkpoint(project(f, V), "f", 0, XDMFFile.Encoding.HDF5, False)  # Not appending to file
for j in range(1,5): 
    t = float(j/5) 
    f.t = t 
f_out.write_checkpoint(project(f,V), "f",t, XDMFFile.Encoding.HDF5, True)  #appending to file
    
f_out.close() 
     
f1 = Function(V) 
f_in =  XDMFFile("test.xdmf") 
   
f_in.read_checkpoint(f1,"f",0) 
f_in.read_checkpoint(f1,"f",1)
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