Hướng dẫn python update instance attributes

if updating one property due to an update on another property is what you're looking for (instead of recomputing the value of the downstream property on access) use property setters:

Nội dung chính

• Attributes don't change automatically
• We don't use getter methods
• We prefer properties over getter methods
• How do you change a class attribute in Python?
• How do you access the attributes of a class in Python?
• What is an attribute in Python class?
• How do you initialize a class attribute in Python?

Nội dung chính

• Attributes don't change automatically
• We don't use getter methods
• We prefer properties over getter methods
• How do you change a class attribute in Python?
• How do you access the attributes of a class in Python?
• What is an attribute in Python class?
• How do you initialize a class attribute in Python?

Nội dung chính

• Attributes don't change automatically
• We don't use getter methods
• We prefer properties over getter methods
• How do you change a class attribute in Python?
• How do you access the attributes of a class in Python?
• What is an attribute in Python class?
• How do you initialize a class attribute in Python?

class SomeClass(object):
    def __init__(self, n):
        self.list = range(0, n)

    @property
    def list(self):
        return self._list
    @list.setter
    def list(self, val):
        self._list = val
        self._listsquare = [x**2 for x in self._list ]

    @property
    def listsquare(self):
        return self._listsquare
    @listsquare.setter
    def listsquare(self, val):
        self.list = [int(pow(x, 0.5)) for x in val]

>>> c = SomeClass(5)
>>> c.listsquare
[0, 1, 4, 9, 16]
>>> c.list
[0, 1, 2, 3, 4]
>>> c.list = range(0,6)
>>> c.list
[0, 1, 2, 3, 4, 5]
>>> c.listsquare
[0, 1, 4, 9, 16, 25]
>>> c.listsquare = [x**2 for x in range(0,10)]
>>> c.list
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

2 minute read • Python 3.7—3.10

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Let's talk about how to make an automatically updating attribute in Python.

Attributes don't change automatically

We have a Rectangle class:

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height
        self.area = self.width * self.height

When we call this class, we'll get a Rectangle object:

>>> rect = Rectangle(3, 4)

This Rectangle object has a width, a height, and an area:

>>> rect.width
3
>>> rect.height
4
>>> rect.area
12

If we're considering acceptable to change the width or height or an existing Rectangle object, we might have problem. If we change the width of a Rectangle the area won't change automatically:

>>> rect.width = 10
>>> rect.area
12

This happens because we only assigned the area one time: in our initializer method. When we first make a Rectangle object, we set the area in our __init__ method, and that's it.

We don't use getter methods

In a lot of programming languages, it's common to fix this problem by making a getter method. Instead of an area attribute we would make a method (maybe called get_area or just area):

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area(self):
        return self.width * self.height

When we access the area method now, we'll see that it's a bound method:

>>> rect = Rectangle(3, 4)
>>> rect.area
>

We can't just access rect.area to get our area anymore. We have to call the area method in order to get a result:

Now if we update the width of our Rectangle object:

And call the area method:

Python will recompute the area and give us back the correct result.

In Python, writing getter methods is not common. Instead of using getter method, we tend to make a property.

We prefer properties over getter methods

We're using the property decorator here:

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    @property
    def area(self):
        return self.width * self.height

That's called a decorator because of that @ syntax.

When we use the property decorator, it will make what was previously a method into a property.

Now if we make a Rectangle object, just like before, and we access the area attribute:

>>> rect = Rectangle(3, 4)
>>> rect.area
12

Unlike before, we don't need to put parentheses after area! In fact, we can't put parentheses. Simply by accessing the area attribute we get our answer.

So if we change the width or the height and we access area again, it will recompute the area automatically:

>>> rect.width = 10
>>> rect.area
40

So Python isn't actually storing the area attribute anywhere. Instead, every time the area attribute is accessed, it calls area method and gives us back whatever the return value is. And it does this automatically, simply because we accessed the rect.area attribute.

Summary

If you want to make an attribute on an object in Python that updates automatically you can use a property.

Series: Properties

We don't use getter and setter methods in Python. Instead we make properties.

Properties allow us to customize what happens when you access an attribute and what happens when you assign to an attribute.

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How do you change a class attribute in Python?

But be careful, if you want to change a class attribute, you have to do it with the notation ClassName. AttributeName. Otherwise, you will create a new instance variable.

How do you access the attributes of a class in Python?

Accessing the attributes of a class getattr() − A python method used to access the attribute of a class. hasattr() − A python method used to verify the presence of an attribute in a class. setattr() − A python method used to set an additional attribute in a class.

What is an attribute in Python class?

Python class attributes are variables of a class that are shared between all of its instances. They differ from instance attributes in that instance attributes are owned by one specific instance of the class only, and are not shared between instances.

How do you initialize a class attribute in Python?

Since Python will automatically call the __init__() method immediately after creating a new object, you can use the __init__() method to initialize the object's attributes. The following defines the Person class with the __init__() method: class Person: def __init__(self, name, age): self.name = name self.