__str__ trong python

In Python __str__ is meant to return the object’s human-friendly representation, while __repr__ should return a valid python expression that can be used to recreate the object.

In the following sections, we’ll look at some examples, and I’ll show you how to implement these magic methods correctly for your own classes.

Table of Contents

Magic Methods

Most of the “automagical” behavior - like comparision, conversion or object initialization - of Python classes are implemented as magic methods or the so-called dunder methods. The name comes from the double underscore naming conventions.

These methods are usually not called explicitly, but get triggered automatically in certain scenarios.

str() and repr() vs str() and repr()

A good example can be the __str__() method, which gets called, whenever an object is passed to the str() builtin, or it’s twin the repr() builtin which calls the object’s __repr__() dunder method.

Both return a kind of string representation of the given object, with a slight, but important difference between the two.

What’s The Goal of str()?

__str__() is meant to return a human-readable, friendly, verbose representation of the object. Think of it as a string representation that’s suitable for the end-user of your Python program.

What Is repr() For?

In contrast __repr__() should always return a unique representation of the object. If it is possible the return value should be a valid Python expression that can be used to recreate the original object.

In other words eval(repr(obj)) should equal obj.

If this is not possible, then __repr__ should be a useful description of the object enclosed in triangular brackets - usually something like

Default Implementations

If a class does not implement a custom str()0 method, the default implementation will be used, which is just falling back to __repr__ on string conversion.

For this reason it is always a good idea to implement a working, meaningful __repr__ dunder method for your classes.

Containers And Recursive Data Structures

It might be surprising at first, so it’s good to note here that all containers’ and recursive data structures’ str()0 method uses the contained objects’ __repr__ method instead of its str()0 - for readability reasons.

Summary: in this tutorial, you’ll learn how to use the Python __str__ method to make a string representation of a class.

Introduction to the Python __str__ method

Let’s start with the Person class:

class Person:
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name
        self.age = age
Code language: Python (python)

The Person class has three instance attributes including

person = Person('John', 'Doe', 25)
print(person)
Code language: Python (python)

0,

person = Person('John', 'Doe', 25)
print(person)



Code language: Python (python)

1, and

person = Person('John', 'Doe', 25)
print(person)
Code language: Python (python)

2.

The following creates a new instance of the Person class and display it:

person = Person('John', 'Doe', 25)
print(person)
Code language: Python (python)

Output:

<__main__.Person object at 0x0000023CA16D13A0>
Code language: Python (python)

When you use the

person = Person('John', 'Doe', 25)
print(person)
Code language: Python (python)

4 function to display the instance of the Person class, the

person = Person('John', 'Doe', 25)
print(person)
Code language: Python (python)

4 function shows the memory address of that instance.

Sometimes, it’s useful to have a string representation of an instance of a class. To customize the string representation of a class instance, the class needs to implement the __str__ magic method.

Internally, Python will call the __str__ method automatically when an instance calls the

person = Person('John', 'Doe', 25)
print(person)
Code language: Python (python)

9 method.

Note that the

person = Person('John', 'Doe', 25)
print(person)
Code language: Python (python)

4 function converts all non-keyword arguments to strings by passing them to the

person = Person('John', 'Doe', 25)
print(person)
Code language: Python (python)

9 before displaying the string values.

The following illustrates how to implement the __str__ method in the Person class:

class Person:
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name
        self.age = age

    def __str__(self):
        return f'Person({self.first_name},{self.last_name},{self.age})'
Code language: Python (python)

And when you use the

person = Person('John', 'Doe', 25)
print(person)
Code language: Python (python)

4 function to print out an instance of the Person class, Python calls the __str__ method defined in the Person class. For example: