How to choose a specific quadrature rule?

General
6

For example:

from mpi4py import MPI
from dolfinx import fem, mesh
import ufl

domain = mesh.create_interval(MPI.COMM_WORLD, 1, [0, 1])

# I want to intergate f=x^2 from 0 to 1
x = ufl.SpatialCoordinate(domain)
f = x[0]**2
for i in range(5):
    metadata = {"quadrature_degree": i}
    a = f*ufl.dx(domain=domain, metadata=metadata)
    a_ = fem.form(a)
    b = fem.assemble_scalar(a_)
    print(i, f"{b:.5e}")

gives:

0 2.50000e-01
1 2.50000e-01
2 3.33333e-01
3 3.33333e-01
4 3.33333e-01

Note that you have a typo in