code: def test_band_extract_scalar(self):
      im = pyvips.Image.black(16, 16, bands=3) + [100, 128, 130]
      x = im[1]
  
      assert x.width == 16
      assert x.height == 16
      assert x.bands == 1
      assert x.avg() == 128
  
file_path: pyvips/tests/test_constants.py
imports: import pyvips
label_desc: Use image node_id 11 for calling the TestConstants obj's underlying member method code with example usage: obj.test_band_extract_scalar() without return types