docstr: Tests the `__len__` method of spatial indexes.
code: def test_len(self):
      """Tests the `__len__` method of spatial indexes."""
      assert len(self.df.sindex) == self.expected_size
  
imports: from math import sqrt
  import numpy
  import pandas
  import pytest
  import shapely
  from numpy.testing import assert_array_equal
  from shapely.geometry import GeometryCollection, LineString, MultiPolygon, Point, Polygon, box
  import geopandas
  from geopandas import GeoDataFrame, GeoSeries, read_file
  from geopandas import _compat
label_desc: Use image node_id 27 for calling the TestShapelyInterface obj's underlying member method code with example usage: obj.test_len() without return types