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class SoftDeleteMixin(object):
    deleted_at = Column(DateTime)
    deleted = Column(types.SoftDeleteInteger, default=0)

    def soft_delete(self, session):
        """Mark this object as deleted."""
        self.deleted = self.id
        self.deleted_at = timeutils.utcnow()
        self.save(session=session)

In class SoftDeleteMixin method soft_delete, it references nonexistent self.id and self.save. Why can it do that in python?

Note: the focus is the class can reference nonexistent variable and method, not that it is a Mixin class.

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    Did you try to actually run your code and call soft_delete? You'll get an AttributeError. Commented Dec 25, 2020 at 1:41
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    It can, and if you instantiate an object of this class and try to use that method, and it will throw an AttributeError. Note, the class seems to be intended to be used as a mixin, so whatever inherits from it likely is supposed to have those attributes Commented Dec 25, 2020 at 1:42
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    @Valentino In my code, I will inherit this class SoftDeleteMixin, and other class that includes id attribute and method soft_delete in my class. It work rightly. But i can't understand it. Commented Dec 25, 2020 at 1:52
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    Have you tried it? self is just a reference to an instance, the instance that the method is being called on. Does that instance have an id? a .save? If it doesn't, it will throw an AttributeError, if it does, it will not. Commented Dec 25, 2020 at 2:02
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    "I think self in python is the same as this in java before." This is correct. self in Python is the same as this in Java. The issue at hand here is that inheritance in Python is very, very, very, very different than inheritance in Java. Commented Dec 25, 2020 at 2:30

2 Answers 2

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If you instantiate a SoftDeleteMixin class and call the soft_delete method, you'll get an AttributeError.

If as you said in the comment those attributes are instantiated somewhere else, even in a child class, and you call soft_delete on a child class instance, it works because the attribute is there at the time the method is called.

To explain it in a simple way, python is an interpreted language, and except for syntax it does not perform too much checks on the whole file when executing the code, until that actual line is actually executed.

So yes, you could think it's a bad design but it is not, it's an accepted practice (see this question for more details) and it is allowed by the laguage. You can define methods which reference attributes not defined in a __init__ method or as class attributes or whatever. The important thing is that the istance has the attribute when the method is executed. It does not matter where or when the attribute is actually defined.

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1

The word "mixin" in the class name means that this class is intended to be inherited by a class that already declares id and save(). If you try to use it by itself, it will cause errors.

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