code: def lanczos(M, sym=True):
      r"""Return a Lanczos window also known as a sinc window.
  
      Parameters
      ----------
      M : int
          Number of points in the output window. If zero, an empty array
          is returned. An exception is thrown when it is negative.
      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 window, with the maximum value normalized to 1 (though the value 1
          does not appear if `M` is even and `sym` is True).
  
      Notes
      -----
      The Lanczos window is defined as
  
      .. math::  w(n) = sinc \left( \frac{2n}{M - 1} - 1 \right)
  
      where
  
      .. math::  sinc(x) = \frac{\sin(\pi x)}{\pi x}
  
      The Lanczos window has reduced Gibbs oscillations and is widely used for
      filtering climate timeseries with good properties in the physical and
      spectral domains.
  
      See Also
      --------
      scipy.signal.windows.lanczos
  
      """
      if _len_guards(M):
          return cupy.ones(M)
      M, needs_trunc = _extend(M, sym)
  
      # To make sure that the window is symmetric, we concatenate the right hand
      # half of the window and the flipped one which is the left hand half of
      # the window.
      def _calc_right_side_lanczos(n, m):
          return cupy.sinc(2.0 * cupy.arange(n, m) / (m - 1) - 1.0)
  
      if M % 2 == 0:
          wh = _calc_right_side_lanczos(M / 2, M)
          w = cupy.r_[cupy.flip(wh), wh]
      else:
          wh = _calc_right_side_lanczos((M + 1) / 2, M)
          w = cupy.r_[cupy.flip(wh), 1.0, wh]
  
      return _truncate(w, needs_trunc)
  
imports: import warnings
  from typing import Set
  import cupy
  import numpy
label_desc: Use image node_id 27 for calling a global function with example usage: lanczos(M, sym) and returns: _truncate, cupy, cupy