gistfile1.py

# The Single Responsibility of this method is to edit the canvas
# given a brush and the point where the user interacted.
def editCanvas(canvas, pointer, brush):
    # The canvas being read only is an exceptional situation
    # that can be handled by the editCanvas method.
    # The situation is handled using a guard clause
    if canvas.isReadOnly:
        return
        
    # This line is a part of the Normal Flow because it is not
    # contained within a conditional.
    # The behavior of this line belongs to the Main Flow.
    # Further, editCanvas has multiple flows it can use to achieve
    # it's responsibility. This line expresses the Alternate Flow
    # using polymorphism.
    brush.apply(canvas, pointer)