Still can't figure out what a visitor is in Python

visitor.py

# -- Shapes -------------------------------------------------------------------

class Square:
    def __init__(self, side_length, color):
        self.name = 'square'
        self.side_length = side_length
        self.color = color

class Circle:
    def __init__(self, radius, color):
        self.name = 'circle'
        self.radius = radius
        self.color = color

class Triangle:
    def __init__(self, side_length, color):
        self.name = 'triangle'
        self.side_length = side_length
        self.color = color

square = Square(1, 'red')
circle = Circle(2, 'red')
triangle = Triangle(3, 'blue')

shapes = [square, circle, triangle]


# -- Visitor Functions --------------------------------------------------------
#

# Name is just shape.name
name_fn = lambda x: x.name

# Colours are mapped to RGB
colour_mapping = {'red': '#f00', 'blue': '#00f'}
colour_fn = lambda x: color_mapping[x.color]

# Size is defined differently per shape
side_fn = lambda x: 'side length = {0}'.format(x.side_length)
radius_fn = lambda x: 'radius = {0}'.format(x.radius)
size_mapping = {Square: side_fn, Circle: radius_fn, Triangle: side_fn}
size_fn = lambda x: size_mapping[x.__class__](x)

# Wrap them into one function
visitor = lambda x: name_fn(x) + ' (' + color_fn(x) + ', ' + size_fn(x) + ')'

# And interate through the list
[visitor(shape) for shape in shapes]
        # Outputs
        ['square (#f00, side length = 1)',
         'circle (#f00, radius = 2)',
         'triangle (#00f, side length = 3)']