code: def set_alpha(self, alpha):
      GraphicsContextBase.set_alpha(self, alpha)
      _alpha = self.get_alpha()
      rgb = self._rgb
      if self.get_forced_alpha():
          self.ctx.set_source_rgba(rgb[0], rgb[1], rgb[2], _alpha)
      else:
          self.ctx.set_source_rgba(rgb[0], rgb[1], rgb[2], rgb[3])
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import six
  import gzip
  import sys
  import warnings
  import numpy
  from matplotlib.backend_bases import _Backend, FigureCanvasBase, FigureManagerBase, GraphicsContextBase, RendererBase
  from matplotlib.mathtext import MathTextParser
  from matplotlib.path import Path
  from matplotlib.transforms import Affine2D
  from matplotlib.font_manager import ttfFontProperty
label_desc: Use image node_id 3 for calling the GraphicsContextCairo obj's underlying member method code with example usage: obj.set_alpha(alpha) without return types