reprstr.md

__repr__ and __str__ [Readability, Programming Efficiency]

Detectors

• Github.com

What is this?

Suppose we have the following class:

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

Not pythonic

Normally, when you try to print() an instance of the object, you would get the following output...

>>> person = Person("Aamir", 21)
>>> person
<test.Person object at 0x7fa1fab83fa0>

..., which is obviously not very useful to us.

Pythonic

We would rather have some information about the object for debugging, such as...

>>> person
"<Aamir, age 21>"

This is what the __repr__ "magic method" is for, and according to Dan Bader [3], every class should have this defined. The goal of __repr__ is to create an easily identifiable representation for debugging. One may consider this analogous to the toString() method in Java.

This can be done by defining the behavior of __repr__ as such:

class Person():
    ... 
    def __repr__(self):
        return f"<{self.name}, age {self.age}>"
    

Similarly, we also have the __str__ method, which has a similar approach, but the goal is to make the output readable.

class Person():
    ...
    def __str__(self):
        return f"{self.name}, {self.age}"

We will now see the following output, and notice how the output is slightly different:

>>> person
"<Aamir, age 21>"
>>> print(person)
"Aamir, 21"

?> The reason we say "every class should have __repr__ defined is because if __str__ is not defined, Python falls back to the implementation of __repr__.

References

[1] __repr__ Official documentation
[2] __str__ Official documentation

Books that mention this topic:

[3] Python Tricks: A Buffet of Awesome Python Features by Dan Bader
[4] Effective Python: 90 Specific Ways to Write Better Python by Brett Slatkin
[5] Python Cookbook, Third Edition by David Beazley and Brian K. Jones
[6] Writing Idiomatic Python 3 by Jeff Knupp