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A Social Network with Flask

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Class Method

4:07 with Kenneth Love

Sometimes it just doesn't make sense to make an instance of a class before we call a method on it. Python gives us a decorator named `@classmethod` that allows us to create an instance of the class from inside of it. Let's see how to use this to make user creation easier.

@classmethod

Let's talk more about @classmethod. Let's make a class to represent an email.

class Email:
    to = None
    from = None
    subject = None
    content = None

If I want to make a new Email using the class constructor, that's easy. email = Email() and then fill in the attributes. Assuming there's a __init__() that handles setting the attributes, I can probably do that in one step.

But what if I want a method for immediately creating and sending the email? I either have to create an instance and then call .send() on the instance or I need a @classmethod way of generating one.

class Email:
    to = None
    from = None
    subject = None
    content = None

    @classmethod
    def create_and_send(cls, to, from, subject, content):
        cls(to=to, from=from, subject=subject, content=content).send()

This won't be a benefit to every class you create, but it's often a better way of approaching use cases where you don't need the class to hang around longer than needed to perform some action.

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    Okay, so for models.py we need to bring in our stuff from flask bcrypt. 0:02
    So from flask.ext.bycrpt import generate_passord_hash 0:07
    because that's what we're gonna have to generate, right? 0:12
    Just to bring home this point, we're using flask.ext again. 0:18
    All right, so now, let's add a method to our user model, 0:22
    that we can use to generate new users. 0:29
    Okay. 0:33
    So let's say def create_user. 0:33
    And I'm gonna pass in cls instead of self, and I'll explain why in just a minute. 0:37
    We're gonna take a name, and let's call it username. 0:42
    An email and a password. 0:47
    And then we'll say is_admin, or admin, which is gonna be false. 0:49
    Okay, so inside here, let's do a try. 0:54
    And we'll do cls.create. 1:01
    And we're gonna say username=username, 1:03
    email=email, password=generate_password_hash. 1:07
    Cuz we don't ever want to hold onto the password. 1:12
    And then is_admin=admin. 1:15
    All right, [SOUND] and then we're gonna catch this IntegrityError. 1:18
    And the IntegrityError will be thrown if the user name of 1:26
    the email are not actually unique. 1:29
    So if somebody tries to write another email, or 1:32
    an account with an email that's already used, or 1:34
    an account where the user name is already used, it'll throw this IntegrityError. 1:36
    And if that happens, we want to raise a ValueError. 1:39
    Because one of the values is bad. 1:43
    And we're gonna say User already exists. 1:46
    Nothing crazy fancy. 1:50
    Okay. 1:52
    So, we're not done with this yet. 1:53
    Because we have this problem here of, why did I say problem? 1:56
    It's intentional we have this cls instead of self. 2:01
    And if you think about it, it would be kind of weird 2:05
    to have to create a user instance in order to call the create user method. 2:09
    Because if it's a normal method it's an instance method. 2:15
    It starts with self and you have to do it with an instance. 2:17
    So I'd have to create a user instance, and 2:21
    then use that user instance to call, create user, to make a new user instance. 2:23
    That's weird. 2:29
    So, what we're gonna do, 2:30
    is we're actually gonna decorate this with a decorator named classmethod. 2:31
    Classmethod describes a method 2:36
    that belongs to a class that can create the class it belongs to. 2:40
    Talk about this again. 2:44
    The way it works if we don't have classmethod we have to create a user 2:45
    instance to call create user which will create a user instance. 2:50
    That's weird. 2:55
    That's messy. 2:56
    With classmethod on here and 2:56
    with cls as this first argument, so the class is going to get passed into it. 2:59
    It will create the user model instance when it runs this method and 3:05
    then use that here. 3:11
    So, cls here, ends up being user. 3:13
    And cls here, ends up being user. 3:17
    So it just calls user.create, on the model, which will create a new instance. 3:20
    It's a little weird, I admit. 3:25
    Think about it for a minute, it makes sense. 3:27
    This cls is a pretty common convention that you'll see in Python code, 3:28
    where you're referring back to the class that the method belongs to. 3:32
    That makes me feel a lot better. 3:36
    If you've ever gotten an email from a site with your password in plain text, 3:38
    you know how badly people handle passwords. 3:41
    Not us though, we never store the plain password at all. 3:44
    Class methods are a great solution to building classes from nothing. 3:47
    We'll be sure and always use our create user method whenever we need to 3:51
    create a new user from now on. 3:53
    We have a lot of our data model finished. 3:56
    And we're well prepared for users to actually sign up and log into our site. 3:58
    I think we're to a point where we can actually start 4:02
    on the user facing parts of the site. 4:03
    Let's do that in the next stage 4:05

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