How to use self in Python

Before introducing the usage of self in Python, let’s first introduce the classes and instances in Python
We know that the most important concepts of object-oriented are classes and instances. Classes are abstract templates. For example, an abstract thing like a student can be represented by a Student class. Instances are specific "objects" created based on classes. Each object inherits the same methods from the class, but its data may be different.
1. Take the Student class as an example. In Python, the class is defined as follows:

class Student(object):
    pass

(Object) indicates which class the class inherits from. The Object class is a class that all classes will inherit.

2. Instance: After the class is defined, an instance of Student can be created through the Student class. The instance is created through the class name ():

student = Student()

3. Since the class serves as a template Therefore, when creating an instance, we can forcefully fill in the attributes that we think must be bound. A built-in method in Python, the __init__ method, is used here. For example, in the Student class, tie attributes such as name and score to it:

class Student(object):
    def __init__(self, name, score):
        self.name = name
        self.score = score

Note here: (1), ## The first parameter of the #__init__ method is always self, which represents the created class instance itself. Therefore, within the __init__ method, you can put each This property is bound to self, because self points to the created instance itself. (2). With the __init__ method, when creating an instance, you cannot pass in empty parameters. You must pass in parameters that match the __init__ method, but self is not required. Pass, the Python interpreter will pass the instance variables in by itself:

>>>student = Student("Hugh", 99)
>>>student.name
"Hugh"
>>>student.score
99

In addition, here

self refers to the class itself, self.name is Student# The attribute variables of the ## class are owned by the Student class. The name is an external parameter, not one that comes with the Student class. Therefore, self.name = name means to assign the value of the external parameter name to the Student class’s own attribute variable self.name. 4. Compared with ordinary numbers, there is only one difference when defining a function in a class, that is,

The first parameter

is always the instance variable of the class itselfself , and when calling, there is no need to pass this parameter. In addition, class methods (functions) are no different from ordinary functions. You can use default parameters, variable parameters or keyword parameters (*args is variable parameters, args What is received is a tuple, **kw is a keyword parameter, and what kw receives is a dict). 5. Since the

Student

class instance itself owns these data, then to access these data, there is no need to access it from external functions, but you can directly define access within the Student class. Functions (methods) of data, so that "data" can be encapsulated. These functions that encapsulate data are associated with the Student class itself, and are called class methods:

class Student(obiect):
    def __init__(self, name, score):
        self.name = name
        self.score = score
    def print_score(self):
        print "%s: %s" % (self.name, self.score)

>>>student = Student("Hugh", 99)
>>>student.print_score
Hugh: 99

In this way, when we look at the Student class from the outside, we only need to know what needs to be given to create an instance. name and score. How to print is defined inside the Student class.

These data and logic are encapsulated, making it easy to call, but the details of the internal implementation are not known.

If you want to prevent internal attributes from being accessed externally, you can add two underscores

before the name of the attribute. In Python, if the variable name of an instance starts with

, It has become a private variable (private), which can only be accessed internally and not externally. Therefore, we changed the Student class:

class Student(object):

    def __init__(self, name, score):
        self.__name = name
        self.__score = score
    def print_score(self):
        print "%s: %s" %(self.__name,self.__score)

After the change, there is no change for the external code, but Instance variable

.__name

and instance variable .__score can no longer be accessed from the outside: <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>&gt;&gt;&gt; student = Student(&amp;#39;Hugh&amp;#39;, 99) &gt;&gt;&gt; student.__name Traceback (most recent call last): File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt; AttributeError: &amp;#39;Student&amp;#39; object has no attribute &amp;#39;__name&amp;#39;</pre><div class="contentsignin">Copy after login</div></div> This ensures that external code cannot modify the internal state of the object at will, so Code is more robust through the protection of access restrictions.

But what if the external code wants to get the name and score? You can add methods like get_name and get_score to the Student class:

class Student(object):
    ...

    def get_name(self):
        return self.__name

    def get_score(self):
        return self.__score

What if you want to allow external code to modify the score? You can add the set_score method to the Student class:

class Student(object):
    ...

    def set_score(self, score):
        self.__score = score

It should be noted that in Python, variable names are similar to

__xxx__

, that is, they start with a double underscore and end with a double underscore. It is a special variable. Special variables can be accessed directly and are not private variables. Therefore, variable names such as __name__ and __score__ cannot be used. Sometimes, you will see instance variable names starting with an underscore, such as _name. Such instance variables can be accessed externally. However, according to the convention, when you see such a variable When, it means, "Although I can be accessed, please treat me as a private variable and do not access it at will."

Another benefit of encapsulation is that you can add new methods

to the Student class at any time, such as: get_grade:<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>class Student(object): ... def get_grade(self): if self.score &gt;= 90: return &amp;#39;A&amp;#39; elif self.score &gt;= 60: return &amp;#39;B&amp;#39; else: return &amp;#39;C&amp;#39;</pre><div class="contentsignin">Copy after login</div></div>Similarly, ## The #get_grade

method can be called directly on the instance variable without knowing the internal implementation details:

>>> student.get_grade()
'A'

6、self的仔细用法
(1)、self代表类的实例，而非类。

class Test:
    def ppr(self):
        print(self)
        print(self.__class__)

t = Test()
t.ppr()
执行结果：
<__main__.Test object at 0x000000000284E080>
<class &#39;__main__.Test&#39;>

从上面的例子中可以很明显的看出，self代表的是类的实例。而self.__class__则指向类。
注意：把self换成this，结果也一样，但Python中最好用约定俗成的self。
（2）、self可以不写吗？
在Python解释器的内部，当我们调用t.ppr()时，实际上Python解释成Test.ppr(t)，也就是把self替换成了类的实例。

class Test:
    def ppr():
        print(self)

t = Test()
t.ppr()

运行结果如下：

Traceback (most recent call last):
File "cl.py", line 6, in
t.ppr()
TypeError: ppr() takes 0 positional arguments but 1 was given

运行时提醒错误如下：ppr在定义时没有参数，但是我们运行时强行传了一个参数。

由于上面解释过了t.ppr()等同于Test.ppr(t)，所以程序提醒我们多传了一个参数t。

这里实际上已经部分说明了self在定义时不可以省略。

当然，如果我们的定义和调用时均不传类实例是可以的，这就是类方法。

class Test:
    def ppr():
        print(__class__)

Test.ppr()

运行结果：
<class &#39;__main__.Test&#39;>

（3）、在继承时，传入的是哪个实例，就是那个传入的实例，而不是指定义了self的类的实例。

class Parent:
    def pprt(self):
        print(self)

class Child(Parent):
    def cprt(self):
        print(self)
c = Child()
c.cprt()
c.pprt()
p = Parent()
p.pprt()

运行结果：

<__main__.Child object at 0x0000000002A47080>
<__main__.Child object at 0x0000000002A47080>
<__main__.Parent object at 0x0000000002A47240>

解释：
运行c.cprt()时应该没有理解问题，指的是Child类的实例。
但是在运行c.pprt()时，等同于Child.pprt(c)，所以self指的依然是Child类的实例，由于self中没有定义pprt()方法，所以沿着继承树往上找，发现在父类Parent中定义了pprt()方法，所以就会成功调用。

（4）、在描述符类中，self指的是描述符类的实例

class Desc:
    def __get__(self, ins, cls):
        print('self in Desc: %s ' % self )
        print(self, ins, cls)
class Test:
    x = Desc()
    def prt(self):
        print('self in Test: %s' % self)
t = Test()
t.prt()
t.x

运行结果如下：

self in Test: <__main__.Test object at 0x0000000002A570B8>
self in Desc: <__main__.Desc object at 0x000000000283E208>
<__main__.Desc object at 0x000000000283E208> <__main__.Test object at 0x0000000002A570B8>

这里主要的疑问应该在：Desc类中定义的self不是应该是调用它的实例t吗？怎么变成了Desc类的实例了呢？
因为这里调用的是t.x，也就是说是Test类的实例t的属性x，由于实例t中并没有定义属性x，所以找到了类属性x，而该属性是描述符属性，为Desc类的实例而已，所以此处并没有顶用Test的任何方法。

那么我们如果直接通过类来调用属性x也可以得到相同的结果。

下面是把t.x改为Test.x运行的结果。

self in Test: <__main__.Test object at 0x00000000022570B8>
self in Desc: <__main__.Desc object at 0x000000000223E208>
<__main__.Desc object at 0x000000000223E208> None <class &#39;__main__.Test&#39;>

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