Introduction
In this article, you’ll learn about the special methods __str__() and __repr__() that are defined in the Python data model. The __str__() and __repr__() methods can be helpful in debugging Python code by logging or printing useful information about an object.
Python special methods begin and end with a double underscore and are informally known as dunder methods. Dunder methods are the underlying methods for Python’s built-in operators and functions. You should avoid calling dunder methods directly, and instead implement the dunder methods in your class and then use the built-in functions that call them, such as str() and repr().
What’s the difference between __str__() and __repr__()?
The __str__() method returns a human-readable, or informal, string representation of an object. This method is called by the built-in print(), str(), and format() functions. If you don’t define a __str__() method for a class, then the built-in object implementation calls the __repr__() method instead.
The __repr__() method returns a more information-rich, or official, string representation of an object. This method is called by the built-in repr() function. If possible, the string returned should be a valid Python expression that can be used to recreate the object. In all cases, the string should be informative and unambiguous.
In general, the __str__() string is intended for users and the __repr__() string is intended for developers.
__str__() and __repr__() Examples Using a Built-In Class
The examples in this section call the __str__() and __repr__() methods directly for demonstration purposes.
The datetime.datetime class is a built-in Python class which has a default implementation of the __str__() and __repr__() methods.
The following example code shows the strings returned by the default implementation of the __str__() and __repr__() methods for a datetime.datetime object:
import datetimemydate = datetime.datetime.now()print("__str__() string: ", mydate.__str__())print("str() string: ", str(mydate))print("__repr__() string: ", mydate.__repr__())print("repr() string: ", repr(mydate))The output is:
Output
__str__() string: 2023-01-27 09:50:37.429078str() string: 2023-01-27 09:50:37.429078__repr__() string: datetime.datetime(2023, 1, 27, 9, 50, 37, 429078)repr() string: datetime.datetime(2023, 1, 27, 9, 50, 37, 429078)The output shows that the str() function calls __str__() and returns a human-friendly string, while the repr() function calls __repr__() and returns a more information-rich string that can be used to recreate the object. In fact, you can use the repr() function with the eval() function to create a new object from the string:
import datetimemydate1 = datetime.datetime.now()mydate2 = eval(repr(mydate1))print("mydate1 repr() string: ", repr(mydate1))print("mydate2 repr() string: ", repr(mydate2))print("the values of the objects are equal: ", mydate1==mydate2)The output is:
Output
mydate1 repr() string: datetime.datetime(2023, 1, 26, 9, 43, 24, 479635)mydate2 repr() string: datetime.datetime(2023, 1, 26, 9, 43, 24, 479635)the values of the objects are equal: TrueThe preceding example code creates the mydate2 object from the repr() string for mydate1, and then verifies that the values of both objects are equal.
__str__() and __repr__() Examples Using a New Class
When you create a class, you should implement at least the ___repr__() method so that useful information is returned when built-in functions use __repr__().
The following class doesn’t implement the __str__() or __repr()__ methods:
class Ocean: def __init__(self, sea_creature_name, sea_creature_age): self.name = sea_creature_name self.age = sea_creature_agec = Ocean('Jellyfish', 5)print(str(c))print(repr(c))The output when you use str() and repr() is:
Output
<__main__.Ocean object at 0x102892860><__main__.Ocean object at 0x102892860>The preceding example demonstrates that the default implementation of __repr()__ for the object returns a string with only the class and the object id in hexadecimal format, which is not very useful. Note that str() and repr() return the same value, because str() calls __repr__() when __str__() isn’t implemented.
Update the Ocean class with implementations of the __str__() and __repr__() methods:
class Ocean: def __init__(self, sea_creature_name, sea_creature_age): self.name = sea_creature_name self.age = sea_creature_age def __str__(self): return f'The creature type is {self.name} and the age is {self.age}' def __repr__(self): return f'Ocean(\'{self.name}\', {self.age})'c = Ocean('Jellyfish', 5)print(str(c))print(repr(c))The output is:
Output
The creature type is Jellyfish and the age is 5Ocean('Jellyfish', 5)The implementation of __str__() in the preceding example returns an easy-to-read string that provides the relevant details of the object for a user. The implementation of __repr__() returns a string that’s a valid Python expression which could be used to recreate the object: Ocean('Jellyfish', 5). The example uses f-string formatting for the strings, but you can format the strings using any format supported by Python.
Conclusion
In this article, you explored the differences between the __str__() and the __repr__() methods and implemented these special methods in a class, so that you didn’t need to call them directly. Learn more about working with strings in Python through our Python string tutorials.
