code: def bartlett(M, sym=True):
      r"""
      Return a Bartlett window.
  
      The Bartlett window is very similar to a triangular window, except
      that the end points are at zero.  It is often used in signal
      processing for tapering a signal, without generating too much
      ripple in the frequency domain.
  
      Parameters
      ----------
      M : int
          Number of points in the output window. If zero or less, an empty
          array is returned.
      sym : bool, optional
          When True (default), generates a symmetric window, for use in filter
          design.
          When False, generates a periodic window, for use in spectral analysis.
  
      Returns
      -------
      w : ndarray
          The triangular window, with the first and last samples equal to zero
          and the maximum value normalized to 1 (though the value 1 does not
          appear if `M` is even and `sym` is True).
  
      See Also
      --------
      triang : A triangular window that does not touch zero at the ends
  
      Notes
      -----
      The Bartlett window is defined as
  
      .. math:: w(n) = \frac{2}{M-1} \left(
                \frac{M-1}{2} - \left|n - \frac{M-1}{2}\right|
                \right)
  
      Most references to the Bartlett window come from the signal
      processing literature, where it is used as one of many windowing
      functions for smoothing values.  Note that convolution with this
      window produces linear interpolation.  It is also known as an
      apodization (which means"removing the foot", i.e. smoothing
      discontinuities at the beginning and end of the sampled signal) or
      tapering function. The Fourier transform of the Bartlett is the product
      of two sinc functions.
      Note the excellent discussion in Kanasewich. [2]_
  
      References
      ----------
      .. [1] M.S. Bartlett, "Periodogram Analysis and Continuous Spectra",
             Biometrika 37, 1-16, 1950.
      .. [2] E.R. Kanasewich, "Time Sequence Analysis in Geophysics",
             The University of Alberta Press, 1975, pp. 109-110.
      .. [3] A.V. Oppenheim and R.W. Schafer, "Discrete-Time Signal
             Processing", Prentice-Hall, 1999, pp. 468-471.
      .. [4] Wikipedia, "Window function",
             https://en.wikipedia.org/wiki/Window_function
      .. [5] W.H. Press,  B.P. Flannery, S.A. Teukolsky, and W.T. Vetterling,
             "Numerical Recipes", Cambridge University Press, 1986, page 429.
  
      Examples
      --------
      Plot the window and its frequency response:
  
      >>> from scipy import signal
      >>> from scipy.fftpack import fft, fftshift
      >>> import matplotlib.pyplot as plt
  
      >>> window = signal.bartlett(51)
      >>> plt.plot(window)
      >>> plt.title("Bartlett window")
      >>> plt.ylabel("Amplitude")
      >>> plt.xlabel("Sample")
  
      >>> plt.figure()
      >>> A = fft(window, 2048) / (len(window)/2.0)
      >>> freq = np.linspace(-0.5, 0.5, len(A))
      >>> response = 20 * np.log10(np.abs(fftshift(A / abs(A).max())))
      >>> plt.plot(freq, response)
      >>> plt.axis([-0.5, 0.5, -120, 0])
      >>> plt.title("Frequency response of the Bartlett window")
      >>> plt.ylabel("Normalized magnitude [dB]")
      >>> plt.xlabel("Normalized frequency [cycles per sample]")
  
      """
      # Docstring adapted from NumPy's bartlett function
      if _len_guards(M):
          return np.ones(M)
      M, needs_trunc = _extend(M, sym)
  
      n = np.arange(0, M)
      w = np.where(
          np.less_equal(n, (M - 1) / 2.0),
          2.0 * n / (M - 1),
          2.0 - 2.0 * n / (M - 1),
      )
  
      return _truncate(w, needs_trunc)
  
imports: from __future__ import division, print_function, absolute_import
  import operator
  import warnings
  import numpy
  from scipy import fftpack, linalg, special
  from scipy._lib.six import string_types
label_desc: Use image node_id 13 for calling a global function with example usage: bartlett(M, sym) and returns: _truncate, np