[Tutor] Cannot understand object initiation

[Alan Gauld]

On 12/10/13 16:01, Kush Goyal wrote:
> Hi,
>
> I am learning web development by using flask framework. The below code
> is used to create a class LoginForm which inherits from the class
> Form.
>
> In the method 'validate' of class 'LoginForm' the variable 'user' is
> initialized by using:
>
> "user = User.query.filter_by ( username = self.username.data ).first()"
>
> now this I do not understand.
>
> Can a object be create by using a method of a class instead of the
> class consturctor? I mean shouldn't it be:
>
> user = User()
>

An object can be returned  by any function or method.
Consider:

 >>> class C:
...    pass
...
 >>> def f():
...    return C()
...
 >>> c1 = C()
 >>> c2 = f()
 >>> isinstance(c1,C)
True
 >>> isinstance(c2,C)
True
 >>>

> Later in the code "user.check_password(self.password.data)" is used.
> So here also how can check_password be used when the object 'user' has
> not been initiated using a constructor?

It has, but it was done inside the  User.query.filter_by() method.
I don't know Flask but I assume this method initializes a
new empty User instance then populates it with data retrieved
from a database before returning the instance to its caller.

> Also, 'User' is a class which inherits from 'db.Model'. And db is
> initialized as: db = SQLAlchemy(app)
>
> So, here is db is an object and Model is a variable of class
> SQLAlchemy. Can someone please explain what is db.Model and can
> SQLAlchemy.Model be used in place of db.Model?

That requires a wee bit more detailed knowledge of SQLAlchemy.
My guess is that db.Model is a class. Classes are objects too
and can be stored as variables. Again, continuing the session
above:

 >>> myclass = C
 >>> c3 = myclass()
 >>> isinstance(c3,C)
True

HTH

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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