python properties

bad circle test.py

c = Circle(1)
print c.area  # prints 3.141
print c.circumference  # prints 6.282
c.radius = 2
print c.area  # prints 3.141 which is incorrect
print c.circumference  # prints 6.282 which is incorrect

bad circle.py

PI = 3.141
class Circle:
    def init(self, radius):
        self.radius = radius
        self.area = PI * self.radius ** 2
        self.circumference = 2 * PI * self.radius

first solution.py

class ImageHolder:
    def __init__(self, image_path):
        self.image_path = image_path
    def get_image(self):
        return load_image_from_path(self.image_path)

good circle test.py

c = Circle(1)
print c.area  # prints 3.141
print c.circumference  # prints 6.282
c.radius = 2
print c.area  # prints 12.564 which is correct!
print c.circumference  # prints 12.564 which is correct!

good circle.py

PI = 3.141
class Circle:
    def init(self, radius):
        self.radius = radius
    def area(self):
        return PI * self.radius ** 2
    def circumference(self):    
        return 2 * PI * self.radius

initial.py

# The class that holds onto the image
class ImageHolder:
    def __init__(self, image_path):
        self.image = load_image_from_path(image_path)

holders = []
# The bit of code that loads the images
for image_path in large_list_of_paths:
    holders.append(ImageHolder(image_path))

# Various parts of the pipeline that do something with the images
for holder in holders:
    do_something(holder.image)

property solution.py

class ImageHolder:
    def __init__(self, image_path):
        self.image_path = image_path
@property
def image(self):
        return load_image_from_path(self.image_path)