code: def _np_stft(data, fft_length, hop_length, window_length):
      frames = SpectralOpsTest._np_frame(
          data, window_length, hop_length
      )
      window = SpectralOpsTest._np_hann_periodic_window(window_length)
      return np.fft.rfft(frames * window, fft_length)
  
imports: import itertools
  from absl.testing import parameterized
  import numpy
  from tensorflow.python.eager import context
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import test_util
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import gradient_checker_v2
  from tensorflow.python.ops import gradients_impl
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops.signal import spectral_ops
  from tensorflow.python.ops.signal import window_ops
  from tensorflow.python.platform import test
label_desc: Use image node_id 3 for calling the SpectralOpsTest obj's underlying member method code with example usage: obj._np_stft(data, fft_length, hop_length, window_length) and returns: np