code: def test_default_updates_partial_graph(self):
      a = shared(0)
      a.default_update = a + 1  # Increment a each time it is used
      b = 2 * a
      # Use only the tip of the graph, a is not used
      f = pfunc([b], b)
      assert a.get_value() == 0
      f(21)
      assert a.get_value() == 0
  
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 26 for calling the TestPfunc obj's underlying member method code with example usage: obj.test_default_updates_partial_graph() without return types