Inheritance V Composition in chapter 8.2.2

[mrbazzan]

Thanks for this great work.

In chapter 8.2.2 (Substitutability), you gave the following example,

practices-of-the-python-pro/ch08/gastropods.py

Lines 1 to 22 in 98bd0a1

	class Slug:
	def __init__(self, name):
	self.name = name
	def crawl(self):
	print('slime trail!')
	class Snail(Slug): # <1>
	def __init__(self, name, shell_size): # <2>
	super().__init__(name)
	self.name = name
	self.shell_size = shell_size
	def race(gastropod_one, gastropod_two):
	gastropod_one.crawl()
	gastropod_two.crawl()
	race(Slug('Geoffrey'), Slug('Ramona')) # <3>
	race(Snail('Geoffrey'), Snail('Ramona')) # <4>

and further noted the following words:

You could pull more tricks out of your sleeve to make this work,
but consider that this might be a better case for composition.
A snail *has-a* shell, after all. 

Is this code below along the lines of what you were thinking?
Instead of making Snail inherit from Slug, the shell
behavior should be composed into the snail class, and there
shouldn't be any need for inheritance.

class Shell:
    size = None

class Snail:
    def __init__(self, name, shell):
        self.name = name
        self.shell_size = shell.size

Snail("Hafsoh", Shell())

[daneah]

Hey @mrbazzan, I converted this to a discussion so we can better chat about this topic. A great question and thanks for reaching out!

What you've shown here is roughly the idea I meant to convey, yes. It's a contrived example, but for thoroughness I would probably do something like this:

class Shell:
    def __init__(self, size):
        self.size = size

class Snail:
    def __init__(self, name, shell):
        self.name = name
        self.shell = shell

    @property
    def shell_size(self):
        return self.shell.size

Snail("Hafsoh", Shell(3))

And later on, if you find that Snail starts wanting to know a lot of different things about Shell, you might have feature envy, which could be a case for folding these two objects into a single object after all.