code: def test_default_updates(self):
      x = shared(0)
      x.default_update = x + 1
  
      f = pfunc([], [x])
      f()
      assert x.get_value() == 1
  
      x.default_update = None
  
      f()
      assert x.get_value() == 2
  
      g = pfunc([], [x])
      g()
      assert x.get_value() == 2
  
imports: import numpy
  import pytest
  import pytensor.tensor
  from pytensor.compile import UnusedInputError, get_mode
  from pytensor.compile.function import function, pfunc
  from pytensor.compile.function.pfunc import rebuild_collect_shared
  from pytensor.compile.io import In
  from pytensor.compile.sharedvalue import shared
  from pytensor.configdefaults import config
  from pytensor.graph.utils import MissingInputError
  from pytensor.misc.safe_asarray import _asarray
  from pytensor.tensor.math import sum
  from pytensor.tensor.type import bscalar, bvector, dmatrices, dmatrix, dscalar, dvector, fscalar, fvector, imatrix, iscalar, ivector, ivectors, lscalar, wvector
  from pytensor.utils import PYTHON_INT_BITWIDTH
label_desc: Use image node_id 20 for calling the TestPfunc obj's underlying member method code with example usage: obj.test_default_updates() without return types