{"id":164,"date":"2024-01-25T14:10:22","date_gmt":"2024-01-25T14:10:22","guid":{"rendered":"https:\/\/learnpython.elegantwallp.com\/?p=164"},"modified":"2024-01-25T14:10:23","modified_gmt":"2024-01-25T14:10:23","slug":"python-property","status":"publish","type":"post","link":"https:\/\/learnpython.elegantwallp.com\/2024\/01\/25\/python-property\/","title":{"rendered":"Python Property"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you\u2019ll learn about the Python property<\/code> class and how to use it to define properties for a class.<\/p>\n\n\n\n

Introduction to class properties<\/h2>\n\n\n\n

The following defines a\u00a0Person<\/code>\u00a0class\u00a0that has two\u00a0attributes\u00a0name<\/code>\u00a0and\u00a0age<\/code>, and create a new instance of the\u00a0Person<\/code>\u00a0class:<\/p>\n\n\n\n

class Person: def __init__(self, name, age): self.name = name self.age = age john = Person('John', 18)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Since\u00a0age<\/code>\u00a0is the\u00a0instance attribute\u00a0of the\u00a0Person<\/code>\u00a0class, you can assign it a new value like this:<\/p>\n\n\n\n
john.age = 19<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
The following assignment is also technically valid:<\/p>\n\n\n\n
john.age = -1<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
However, the age is semantically incorrect.<\/p>\n\n\n\n
To ensure that the age is not zero or negative, you use the\u00a0if<\/a><\/code>\u00a0statement to add a check as follows:<\/p>\n\n\n\n
age = -1 if age <= 0: raise ValueError('The age must be positive') else: john.age = age<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
And you need to do this every time you want to assign a value to the age<\/code> attribute. This is repetitive and difficult to maintain.<\/p>\n\n\n\n
To avoid this repetition, you can define a pair of methods called getter and setter.<\/p>\n\n\n\n
Getter and setter<\/h2>\n\n\n\n
The getter and setter methods provide an interface for accessing an instance attribute:<\/p>\n\n\n\n
    \n
  • The getter returns the value of an attribute<\/li>\n\n\n\n
  • The setter sets a new value for an attribute<\/li>\n<\/ul>\n\n\n\n
    In our example, you can make the\u00a0age<\/code>\u00a0attribute\u00a0private\u00a0(by convention) and define a getter and a setter to manipulate the\u00a0age<\/code>\u00a0attribute.<\/p>\n\n\n\n
    The following shows the new\u00a0Person<\/code>\u00a0class with a getter and setter for the\u00a0age<\/code>\u00a0attribute:<\/p>\n\n\n\n
    class Person: def __init__(self, name, age): self.name = name self.set_age(age) def set_age(self, age): if age <= 0: raise ValueError('The age must be positive') self._age = age def get_age(self): return self._age<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
    How it works.<\/p>\n\n\n\n
    In the Person<\/code> class, the set_age()<\/code> is the setter and the get_age()<\/code> is the getter. By convention the getter and setter have the following name: get_<attribute>()<\/code> and set_<attribute>()<\/code>.<\/p>\n\n\n\n
    In the\u00a0set_age()<\/code>\u00a0method, we raise a\u00a0ValueError<\/code>\u00a0if the\u00a0age<\/code>\u00a0is less than or equal to zero. Otherwise, we assign the\u00a0age<\/code>\u00a0argument to the\u00a0_age<\/code>\u00a0attribute:<\/p>\n\n\n\n
    def set_age(self, age): if age <= 0: raise ValueError('The age must be positive') self._age = age<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
    The\u00a0get_age()<\/code>\u00a0method returns the value of the\u00a0_age<\/code>\u00a0attribute:<\/p>\n\n\n\n
    def get_age(self): return self._age<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
    In the\u00a0__init__()<\/code>\u00a0method, we call the\u00a0set_age()<\/code>\u00a0setter method to initialize the\u00a0_age<\/code>\u00a0attribute:<\/p>\n\n\n\n
    def __init__(self, name, age): self.name = name self.set_age(age)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
    The following attempts to assign an invalid value to the\u00a0age<\/code>\u00a0attribute:<\/p>\n\n\n\n
    john = Person('John', 18) john.set_age(-19)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
    And Python issued a\u00a0ValueError<\/code>\u00a0as expected.<\/p>\n\n\n\n
    ValueError: The age must be positive<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
    This code works just fine. But it has a backward compatibility issue.<\/p>\n\n\n\n
    Suppose you released the Person<\/code> class for a while and other developers have been already using it. And now you add the getter and setter, all the code that uses the Person won\u2019t work anymore.<\/p>\n\n\n\n
    To define a getter and setter method while achieving backward compatibility, you can use the property()<\/code> class.<\/p>\n\n\n\n
    The Python property class<\/h2>\n\n\n\n
    The property class returns a\u00a0property<\/code>\u00a0object. The\u00a0property()<\/code>\u00a0class has the following syntax:<\/p>\n\n\n\n
    property(fget=None, fset=None, fdel=None, doc=None)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
    The property()<\/code> has the following parameters:<\/p>\n\n\n\n
      \n
    • fget<\/code> is a function to get the value of the attribute, or the getter method.<\/li>\n\n\n\n
    • fset<\/code> is a function to set the value of the attribute, or the setter method.<\/li>\n\n\n\n
    • fdel<\/code> is a function to delete the attribute.<\/li>\n\n\n\n
    • doc<\/code> is a docstring i.e., a comment.<\/li>\n<\/ul>\n\n\n\n
      The following uses the\u00a0property()<\/code>\u00a0function to define the\u00a0age<\/code>\u00a0property for the\u00a0Person<\/code>\u00a0class.<\/p>\n\n\n\n
      class Person: def __init__(self, name, age): self.name = name self.age = age def set_age(self, age): if age <= 0: raise ValueError('The age must be positive') self._age = age def get_age(self): return self._age age = property(fget=get_age, fset=set_age)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
      In the\u00a0Person<\/code>\u00a0class, we create a new property object by calling the\u00a0property()<\/code>\u00a0and assign the property object to the age attribute. Note that the\u00a0age<\/code>\u00a0is a\u00a0class attribute, not an\u00a0instance attribute.<\/p>\n\n\n\n
      The following shows that the Person.age<\/code> is a property<\/code> object:print(Person.age)<\/code>Code language: Python (python)<\/small><\/p>\n\n\n\n
      Output:<\/p>\n\n\n\n
      <property object at 0x000001F5F5149180><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
      The following creates a new instance of the\u00a0Person<\/code>\u00a0class and access the\u00a0age<\/code>\u00a0attribute:<\/p>\n\n\n\n
      john = Person('John', 18)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
      The\u00a0john.__dict__<\/code>\u00a0stores the instance attributes of the john object. The following shows the contents of the\u00a0john.__dict__<\/code>\u00a0:<\/p>\n\n\n\n
      print(john.__dict__)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
      Output:<\/p>\n\n\n\n
      {'_age': 18, 'name': 'John'}<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
      As you can see clearly from the output, the john.__dict__<\/code> doesn\u2019t have the age<\/code> attribute.<\/p>\n\n\n\n
      The following assigns a value to the\u00a0age<\/code>\u00a0attribute of the john object:<\/p>\n\n\n\n
      john.age = 19<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
      In this case, Python looks up the age<\/code> attribute in the john.__dict__<\/code> first. Because Python doesn\u2019t find the age<\/code> attribute in the john.__dict__<\/code>, it\u2019ll then find the age<\/code> attribute in the Person.__dict__<\/code>.<\/p>\n\n\n\n
      The\u00a0Person.__dict__<\/code>\u00a0stores the class attributes of the Person class. The following shows the contents of the\u00a0Person.__dict__<\/code>:<\/p>\n\n\n\n
      pprint(Person.__dict__)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
      Output:<\/p>\n\n\n\n
      mappingproxy({'__dict__': <attribute '__dict__' of 'Person' objects>, '__doc__': None, '__init__': <function Person.__init__ at 0x000002242F5B2670>, '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'Person' objects>, 'age': <property object at 0x000002242EE39180>, 'get_age': <function Person.get_age at 0x000002242F5B2790>, 'set_age': <function Person.set_age at 0x000002242F5B2700>})<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
      Because Python finds the age<\/code> attribute in the Person.__dict__<\/code>, it\u2019ll call the age<\/code> property object.<\/p>\n\n\n\n
      When you assign a value to the\u00a0age<\/code>\u00a0object:<\/p>\n\n\n\n
      john.age = 19<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
      Python will call the function assigned to the fset<\/code> argument, which is the set_age()<\/code>.<\/p>\n\n\n\n
      Similarly, when you read from the age<\/code> property object, Python will execute the function assigned to the fget<\/code> argument, which is the get_age()<\/code> method.<\/p>\n\n\n\n
      By using the property()<\/code> class, we can add a property to a class while maintaining backward compatibility. In practice, you will define the attributes first. Later, you can add the property to the class if needed.<\/p>\n\n\n\n
      Putting it all together.<\/p>\n\n\n\n
      from pprint import pprint class Person: def __init__(self, name, age): self.name = name self.age = age def set_age(self, age): if age <= 0: raise ValueError('The age must be positive') self._age = age def get_age(self): return self._age age = property(fget=get_age, fset=set_age) print(Person.age) john = Person('John', 18) pprint(john.__dict__) john.age = 19 pprint(Person.__dict__)<\/code><\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"
      Summary: in this tutorial, you\u2019ll learn about the Python property class and how to use it to define properties for a class. Introduction to class properties The following defines a\u00a0Person\u00a0class\u00a0that has two\u00a0attributes\u00a0name\u00a0and\u00a0age, and create a new instance of the\u00a0Person\u00a0class: Since\u00a0age\u00a0is the\u00a0instance attribute\u00a0of the\u00a0Person\u00a0class, you can assign it a new value like this: The following assignment is also […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[27],"tags":[],"class_list":["post-164","post","type-post","status-publish","format-standard","hentry","category-3-property"],"_links":{"self":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/164","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/comments?post=164"}],"version-history":[{"count":1,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/164\/revisions"}],"predecessor-version":[{"id":165,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/164\/revisions\/165"}],"wp:attachment":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/media?parent=164"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/categories?post=164"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/tags?post=164"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}