code: def test_operation_block(self):
      # block all loads except jpeg
      pyvips.operation_block_set("VipsForeignLoad", True)
      pyvips.operation_block_set("VipsForeignLoadJpeg", False)
  
      image = pyvips.Image.new_from_file(JPEG_FILE)
      assert image.width == 1024
  
      # should fail
      with pytest.raises(Exception):
          _ = pyvips.Image.new_from_file(WEBP_FILE)
  
      # reenable all loads
      pyvips.operation_block_set("VipsForeignLoad", False)
  
imports: import tempfile
  import pytest
  import pyvips
  from helpers import skip_if_no, JPEG_FILE, WEBP_FILE, SVG_FILE
label_desc: Use image node_id 2 for calling the TestBlock obj's underlying member method code with example usage: obj.test_operation_block() without return types