error in function approximation

fem_errors/fem_errors_2.py

+import dolfin as df
+import numpy as np
+import matplotlib.pyplot as plt
+
+f = df.Expression('2*x[0] - x[0]*x[0]', degree=1)
+Nx , L = 32,1
+mesh = df.IntervalMesh(Nx, 0, L)
+V = df.FunctionSpace(mesh, 'P', 1)
+u = df.interpolate(f, V)
+error = df.errornorm(f,u,'L2')
+print(error)