Five magic methods related to Python properties

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foreword

Today, I want to introduce five magic methods to you. They are all related to Python attributes, involving getting, deleting and modifying attributes. Let's take a look.

__getattribute__ method

Let's start by defining a simple class with no problem accessing properties.

class User:

    def __init__(self, name, sex):
        self.name = name
        self.sex = sex


u1 = User('li', 'male')
print(u1.name)

li
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Then we add the __getattribute__ method, and the access attribute will be handed over to this method.

class User:

    def __getattribute__(self, item):
        return 6

    def __init__(self, name, sex):
        self.name = name
        self.sex = sex


u1 = User('li', 'male')
print(u1.name)

6
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What's even more amazing is that it returns 6 whether or not this property exists.

class User:

    def __getattribute__(self, item):
        return 6

    def __init__(self, name, sex):
        self.name = name
        self.sex = sex


u1 = User('li', 'male')
print(u1.test)

6
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• Triggered when: Triggered when the object member property is accessed, whether it exists or not.
• Role: Do some processing on the attribute.
• Parameters: self is the current object, item is a string to access the property name.
• Return value: property value.

It should be noted here that the return value must not use self.name, which will recurse infinitely. We can use the __getattribute__ method of object to access.

class User:

    def __getattribute__(self, item):
        return object.__getattribute__(self, item)

    def __init__(self, name, sex):
        self.name = name
        self.sex = sex


u1 = User('li', 'male')
print(u1.name)

li
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__getattr__ method

• Trigger Time: Triggered when a non-existent object property is accessed.
• Effect: When accessing a non-existent property, no error will be reported.
• Parameters: self is the current object, item is a string to access the property name.
• Return value: property value.

Let's take a simple example, that is, the user may output the attribute name, then we all return the name attribute uniformly.

class User:

    def __getattr__(self, item):
        return self.name

    def __init__(self, name, sex):
        self.name = name
        self.sex = sex


u1 = User('li', 'male')
print(u1.na2me)

li
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__setattr__ method

• Trigger Time: Triggered when adding and modifying object properties.
• Role: Restrict the operation of adding and modifying object properties.
• Parameters: self is the current object, key is the property name of the set object, and value is the set value.
• Return value: None.

For example, we allow users to change the name, but not the sex.

class User:

    sex = 'male'

    def __setattr__(self, key, value):
        if key == 'sex':
            pass
        else:
            object.__setattr__(self, key, value)

    def __init__(self, name):
        self.name = name


u1 = User('li')
u1.name = 'test'
u1.sex = 'female'
print(u1.name, u1.sex)

test male
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__delattr__ method

• Trigger Time: Triggered when an object property is deleted.
• Role: Restrict the operation of adding and modifying object properties.
• Parameters: self is the current object, item is the property name of the deleted object.
• Return value: None.

We can let users delete sex, but not name.

class User:

    def __delattr__(self, item):
        if item == 'sex':
            pass
        else:
            object.__delattr__(self, item)

    def __init__(self, name, sex):
        self.name = name
        self.sex = sex


u1 = User('li', 'male')
del u1.sex
print(u1.sex)
del u1.name
print(u1.name)

male
AttributeError: 'User' object has no attribute 'name'
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__dir__ method

This method returns a list of all member names of a class or object, which is not used much.

property access order

Finally, I list the order of attribute access to you for your reference and study.

• __getattribute__
• data descriptor
• properties of the current object
• class properties
• non-data descriptor
• parent class properties
• __getattr__

That's it for today's sharing, see you next time~