{
  "source_file": "0930",
  "source_platform": "Medium",
  "author": "Kurtis Pykes",
  "title_listed": "A Guide To Python's Dunder Methods",
  "keywords": "Python,魔术方法",
  "domain": "数据处理",
  "content_type": "深度技术解析",
  "is_paid_flag": "部分免费/会员墙",
  "url": "https://towardsdatascience.com/a-guide-to-pythons-dunder-methods-3b8104fce335/",
  "publish_date": "2022-05",
  "corpus_stage": "预训练/SFT",
  "notes": "",
  "id": "6fde9259c504",
  "collected_at": "2026-10-01T23:54:22",
  "title": "A Guide To Python's Dunder Methods | Towards Data Science",
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It also helps in adverstising on the website.\n\n\n  * Cookie\nedgebucket\n  * Duration\nsession\n  * Description\nReddit sets this cookie to save the information about a log-on Reddit user, for the purpose of advertisement recommendations and updating the content.\n\n\n  * Cookie\ndid\n  * Duration\n1 year\n  * Description\nArbor sets this cookie to show targeted ads to site visitors.This cookie expires after 2 months or 1 year.\n\n\nUncategorised\nOther uncategorised cookies are those that are being analysed and have not been classified into a category as yet.\nNo cookies to display.\nReject All Save My Preferences Accept All\nToggle Search[](https://towardsdatascience.com/)Toggle Mobile Navigation\n[](https://towardsdatascience.com/)\nPublish AI, ML & data-science insights to a global community of data professionals.\nSign in[Submit an Article](https://write.insightmediagroup.io)\n[Latest](https://towardsdatascience.com/latest/)[Editor's Picks](https://towardsdatascience.com/tag/editors-pick/)[Newsletter](https://towardsdatascience.com/tag/the-variable/)[ShipAI](https://towardsdatascience.com/shipai/)\n* * *\n[Write For TDS](https://towardsdatascience.com/submissions/)\n[LinkedIn](https://www.linkedin.com/company/towards-data-science/)[X](https://x.com/TDataScience)Toggle Search\n[Programming](https://towardsdatascience.com/category/programming/)\n# A Guide To Python's Dunder Methods\nThe Magic Behind Python\n[Kurtis Pykes](https://towardsdatascience.com/author/kurtispykes/)\nMay 7, 20225 min read\n![Photo by Chris Ried on Unsplash](https://assets.insightmediagroup.io/media/wp-content/uploads/2022/05/0_OD79hy5cwDRMI8m-scaled.jpg)\nPhoto by [Chris Ried](https://unsplash.com/@cdr6934?utm_source=medium&utm_medium=referral) on [Unsplash](https://unsplash.com?utm_source=medium&utm_medium=referral)\nPython has several magic methods - you'd typically hear practitioners refer to as _dunder methods (I'll be referring to them as both interchangeably)_. These methods carry out a procedure known as _operator overloading:_ **providing extended meaning beyond the predefined meaning to an operator** _._ We use operational overloading to add custom behaviors to our classes so we can use them with Python's operators and built-in functions.\nThe simplest way to think of a dumber method is as a contract between your implementation and the Python interpreter. One of the terms of the contract involves Python performing some actions behind the scenes under some given circumstance (i.e. trying to add an integer to a custom class).\nDunder methods begin and end with two underscores: the most popular one you may come across is the `__init__()` method. We create the `__init__()` method in a class so we can allow the class to be initialized with specific attributes of the class - _Learn more about Object-oriented programming in_[ _Getting Started with Object-Oriented Programming in Python 3_](https://medium.com/geekculture/getting-started-with-object-oriented-programming-in-python-3-e0a87d38acfc) _._\nBut `__init__()` is only one of several magic methods. In this article, we'll cover the different types of dunder methods you may come across and what they do.\nLearn this step by step with the interactive [Computer Science](https://roadmap.sh/computer-science) roadmap.\n## String representation methods\nWhenever we create a new object in Python, we implicitly create a related object since all classes inherit from `Object`. The methods defined in the `Object` are inherited by our newly created class and are used in various situations like when an object is printed.\ntext\n\n```\nclass Car: \n    pass\n```\n\ntext\n\n```\ncar = Car()\n```\n\ntext\n\n```\nprint(car)\n```\n\ntext\n\n```\n\"\"\"\n<__main__.Car object at 0x7f53e19a8d90>\n\"\"\"\n```\n\nHow did the code above know what to print? Simple. The `Object`, which is the parent of all classes in Python, has a dunder method called `__repr__()` (pronnounced dunder repper); When we call the `print()` statement it makes a call to the `__repr__()` method from our `Car` object, which was inherited from the parent class `Object`, and returns a value back to the main program.\nBut a child calss can override the parent class. All we have to do is create a method with the same name in the child class - learn more about inheritance in [_Inheritance: Getting to Grips with OOP in Python_](https://medium.com/geekculture/inheritance-getting-to-grips-with-oop-in-python-2ec35b52570).\ntext\n\n```\nclass Car: \n    def __repr__(self):\n        return f\"{self.__class__.__qualname__}\"\n\ncar = Car()\n```\n\ntext\n\n```\nprint(car)\n```\n\ntext\n\n```\n\"\"\"\nCar\n\"\"\"\n```\n\nWe can also use the `__str__()` (pronounced stir) dunder method to create a string representation. The `__str__()` method returns a human-readable string that provides more insightful information about the object.\n_**Note**_ _: Our object doesn't have much going on so we'll use the same information we did in the_` ___repr__()_`_._\ntext\n\n```\nclass Car: \n    def __str__(self):\n        return f\"{self.__class__.__qualname__}\"\n\ncar = Car()\n```\n\ntext\n\n```\nprint(car)\n```\n\ntext\n\n```\n\"\"\"\nCar\n\"\"\"\n```\n\nThe `__repr__()` method serves as a backup behavior if the `__str__()` is missing. Thus, when you call `print()` it first looks for `__str__()` to see if it's been defined otherwise it calls `__repr__()`.\n## Math dunder methods\nWhen we create expressions we use special symbols known as _operators_. If we wish to use operands in an expression then it must have a math dunder method the operator can use to evaluate the expression. Without creating a math dunder method, Python would raise a type error.\ntext\n\n```\nclass RandomNumbers: \n    def __init__(self, a, b): \n        self.a = a \n        self.b = b\n\nset_a = RandomNumbers(2, 4)\nset_b = RandomNumbers(3, 5)\n```\n\ntext\n\n```\nprint(set_a + set_b)\n```\n\ntext\n\n```\n\"\"\"\nTraceback (most recent call last):\n  File \"<string>\", line 9, in <module>\nTypeError: unsupported operand type(s) for +: 'RandomNumbers' and 'RandomNumbers'\n\"\"\"\n```\n\nOf course, we could simply create a `add_random_numbers()` method in our class, but a better way to solve this is to use the `__add__()` dunder method - this way, we can use the `+` operator with our `RandomNumbers` object.\ntext\n\n```\nclass RandomNumbers: \n    def __init__(self, a, b): \n        self.a = a \n        self.b = b\n\n    def __add__(self, other):\n        # Only permit RandomNumber objects to be added\n        if not isinstance(other, RandomNumbers): \n            return NotImplemented\n\n        return RandomNumbers(other.a + self.a, other.b + self.b)\n\n    def __repr__(self):\n        return f\"{self.__class__.__qualname__}({self.a}, {self.b})\"\n\nset_a = RandomNumbers(2, 4)\nset_b = RandomNumbers(3, 5)\n```\n\ntext\n\n```\nprint(set_a + set_b)\n```\n\ntext\n\n```\n\"\"\"\nRandomNumbers(5, 9)\n\"\"\"\n```\n\nWhen a `RandomNumbers` object is on the left side of the `+` operator then Python will call the `__add__()` method: the method on the right side of the `+` operator is passed as the `other` parameter to the `__add__()` method.\nIn our example, we prevent our object from adding objects that aren't `RandomNumber` instances. Let's see another method that will allow us to multiply our numbers with integer:\ntext\n\n```\nclass RandomNumbers: \n    def __init__(self, a, b): \n        self.a = a \n        self.b = b\n\n    def __add__(self, other):\n        if not isinstance(other, RandomNumbers):\n            return NotImplemented\n\n        return RandomNumbers(other.a + self.a, other.b + self.b)\n\n    def __mul__(self, other): \n        if not isinstance(other, int): \n            return NotImplemented \n\n        return RandomNumbers(self.a * other, self.b * other)\n\n    def __repr__(self):\n        return f\"{self.__class__.__qualname__}({self.a}, {self.b})\"\n\nset_a = RandomNumbers(2, 4)\n```\n\ntext\n\n```\nprint(set_a * 3)\n```\n\ntext\n\n```\n\"\"\"\nRandomNumbers(6, 12)\n\"\"\"\n```\n\nAgain, notice the `RandomNumbers` instance is on the left side of the `*` operator. What happens if we move it to the right?\ntext\n\n```\n-- snip -- \n```\n\ntext\n\n```\nset_a = RandomNumbers(2, 4)\n```\n\ntext\n\n```\nprint(3 * set_a)\n```\n\ntext\n\n```\n\"\"\"\nTraceback (most recent call last):\n  File \"<string>\", line 23, in <module>\nTypeError: unsupported operand type(s) for *: 'int' and 'RandomNumbers'\n\"\"\"\n```\n\nPython raises a type error.\nThe reason for this behavior is because math dunder methods are called when the object is on the left side of the math operator. If you want to extend the functionality so you can also call the object on the right side, then you'll have to have to define the _reverse dunder method_.\nLet's demonstrate this with our multiplication example:\ntext\n\n```\nclass RandomNumbers:\n    def __init__(self, a, b): \n        self.a = a \n        self.b = b\n\n    def __add__(self, other):\n        if not isinstance(other, RandomNumbers):\n            return NotImplemented\n\n        return RandomNumbers(other.a + self.a, other.b + self.b)\n\n    def __mul__(self, other): \n        if not isinstance(other, int): \n            return NotImplemented \n\n        return RandomNumbers(self.a * other, self.b * other)\n\n    def __rmul__(self, other): \n        return self.__mul__(other)\n\n    def __repr__(self):\n        return f\"{self.__class__.__qualname__}({self.a}, {self.b})\"\n\nset_a = RandomNumbers(2, 4)\n```\n\ntext\n\n```\nprint(3 * set_a)\n```\n\ntext\n\n```\n\"\"\"\nRandomNumbers(6, 12)\n\"\"\"\n```\n\nProblem solved.\nThese are only two types of dunder methods, but t[here](https://docs.python.org/2/reference/datamodel.html#special-method-names) are several - you can find a list of them all here. If you're interested in upgrading your Python skills, I recommend you check out the Python skills and career tracks available with [DataCamp](https://www.datacamp.com/learn/python).\n_Thanks for reading._\n**Connect with me:** [LinkedIn](https://www.linkedin.com/in/kurtispykes/) [Twitter](https://twitter.com/KurtisPykes) [Instagram](https://www.instagram.com/kurtispykes/)\nIf you enjoy reading stories like this one and wish to support my writing, consider [becoming a Medium member](https://kurtispykes.medium.com/membership). With a $5 a month commitment, you unlock unlimited access to stories on Medium. If you use [my sign-up link](https://kurtispykes.medium.com/membership), I'll receive a small commission.\nAlready a member? 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