docstr: test creating fvec with negative length fails (pure python) 
code: def test_negative_length(self):
      """test creating fvec with negative length fails (pure python)"""
      self.assertRaises(ValueError, fvec, -10)
  
imports: import numpy
  from numpy.testing import TestCase, assert_equal, assert_almost_equal
  from aubio import fvec, zero_crossing_rate, alpha_norm, min_removal
  from aubio import float_type
label_desc: Use image node_id 1 for calling the aubio_fvec_wrong_values obj's underlying member method code with example usage: obj.test_negative_length() without return types