class_docstr: For providing a custom update method
  
  This block allows you to write a custom update method to provide
  functionality not available with other blocks.
  
  Parameters
  ----------
  func : callable
      This function will be called once for each frame of the animation.
      The first argument to this function must be an integer
      representing the frame number. It should return a matplotlib
      artist.
  length : int
      The number of frames to display.
  fargs : list, optional
      A list of arguments to pass into func.
  ax : matplotlib.axes.Axes, optional
      The matplotlib axes to which the block is attached.
      Defaults to matplotlib.pyplot.gca()
  
  Attributes
  ----------
  ax : matplotlib.axes.Axes
      The matplotlib axes to which the block is attached.
code: def __len__(self):
      return self.length
  
file_path: animatplot/animatplot/blocks/update.py
imports: from .base import Block
label_desc: Use image node_id 3 for calling the Update obj's underlying member method code with example usage: obj.__len__() and returns: self