Str() vs repr() in Python

In Python, the built-in str() and repr() functions both produce a textual representation of an object.

import datetime
today = datetime.datetime.now()

print(str(today))
print(repr(today))

Output:

2021-10-14 10:15:31.405463
datetime.datetime(2021, 10, 14, 10, 15, 31, 405463)

The difference between str() and repr() is:

  • The str() function returns a user-friendly description of an object.
  • The repr() method returns a developer-friendly string representation of an object.

In this guide, you learn:

  • What are the functions str() and repr().
  • How to override the behavior of these functuons in custom objects.

Example of str() and repr() in Python

Let’s create a datetime object for today, and inspect its string representations:

import datetime
today = datetime.datetime.now()

print(str(today))
print(repr(today))

Output:

2021-08-14 08:18:25.138663
datetime.datetime(2021, 8, 14, 8, 18, 25, 138663)
  • str() displays today’s date so that an end-user could understand it.
  • repr() prints the developer-friendly representation of the date object. This string is valid Python you can use to reconstruct the date object.

How Do str() and repr() Work Under the Hood

  • When you call str() on an object, it calls the special method __str__ of the object.
  • And when you call repr() on an object, it calls the special method __repr__ of the object.
  • Also, when you call print() on an object, it calls __str__ method of the object. If __str__ is not implemented, the __repr__ is called as a fallback.

So, the special methods __str__ and __repr__ define how the object is presented “textually”. These special methods are implemented in the class that produces the objects.

Custom Classes with String Representations

If you’re writing your own class, it may be useful to create a clear textual representation of the class for both developers and end-users.

This means you need to implement the special methods __str__ and __repr__ into the class.

For example, let’s say you have a class that represents Fruits:

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

If you create a Fruit object and print it:

banana = Fruit("Banana")
print(banana)

You get a somewhat unambiguous output:

<__main__.Fruit object at 0x7f0ece0e8d00>

This is the default string representation of an object. But this string representation is not useful for a user.

You can change the default string representation by providing an implementation to the string representation methods, __str__ and __repr__. Conventionally the __str__ should produce a user-friendly result and __repr__ a developer-friendly one.

For example:

class Fruit:
    def __init__(self, name):
        self.name = name
    
    def __str__(self):
        return f'I am a {self.name}'
    
    def __repr__(self):
        return f'Fruit("{self.name}")'

Now, if you create a Fruit object and print it:

banana = Fruit("Banana")
print(banana)

You get an output:

I am a Banana

This is because the print() function calls the __str__ method of the instance under the hood. So essentially the above is equivalent to:

print(banana.__str__())

Similarly, if you print the string representation of the fruit object using the str() function:

print(str(banana))

You get an output:

I am a Banana

Because calling str(banana) is the same as calling banana.__str__().

And using repr() produces a more developer-friendly string representation:

print(repr(banana))

Output:

Fruit("Banana")

Because calling repr(banana) is the same as banana.__repr__().

Now you know how to provide a string representation for your custom objects and what is the difference between __str__ and __repr__.

Real-life Usage of __str__ and __repr__

Let’s go back to the datetime example in the very beginning:

import datetime
today = datetime.datetime.now()

print(str(today))
print(repr(today))

Output:

2021-08-14 08:18:25.138663
datetime.datetime(2021, 8, 14, 8, 18, 25, 138663)

So str() and repr() call __str__ and __repr__ methods behind the scenes. 

Let’s take a source dive into Python’s source code. As we want to know how the datetime works, let’s inspect the datetime.py file.

It seems that the base class for datetime is date. Let’s see if we can find methods __repr__ and __str__ from the date class.

If you search for class date and scroll down, you find the __repr__ method. This is what is responsible for producing the output when calling repr() on a datetime object.

Python source code for a developer-friendly string representation of a datetime object.

If you scroll down a bit further, you also encounter the __str__ method that shows the user-friendly representation of the date:

Python source code for string representation of a datetime object.


The __str__ method, in this case, is the isoformat function. This means the default string representation of a datetime object is the date in the ISO format.

Conclusion

Both str() and repr() return a “textual representation” of a Python object.

The difference is:

  • str() gives a user-friendly representation
  • repr() gives a developer-friendly representation.

In Python, you can customize the string representations of your objects by implementing methods __str__ and __repr__.

Thanks for reading. I hope you find it useful.

Happy coding!

Further Reading


50 Python interview questions

50 buzzwords of web development

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