code: def test_n_variables_len(self, mapper, schema):
      assert mapper.n_variables == len(schema)
  
imports: from collections import OrderedDict
  import pytest
  from causalnex.structure import StructureModel
  from causalnex.structure.categorical_variable_mapper import VariableFeatureMapper, validate_schema
label_desc: Use image node_id 10 for calling the TestVariableFeatureMapper obj's underlying member method code with example usage: obj.test_n_variables_len(mapper, schema) without return types