repr in Python

Introduction

This blog post will explore the Python magic method, __repr__. This method is useful and even necessary to display a human-readable representation of an object. We’ll start with the basics and work up to more advanced concepts.

What is __repr__?

• __repr__ is a magic method in Python that allows us to define a human-readable representation of an object.
• When we call the built-in repr() function on an object, Python looks for a __repr__ method within that object’s class definition. If it finds one, it returns the result of that method; otherwise, it provides a default representation.
• It’s important to note the difference between __repr__ and __str__: while __repr__ is designed to provide an unambiguous representation of the object (which can ideally be used to recreate it), __str__ is meant to return a more user-friendly string representation. We will discuss more on this.

Creating a primary __repr__ method

Let’s make a simple class Person without a __repr__ method.

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

person = Person("Soumendra", 30)
print(person)  # Output: <__main__.Person object at 0x7f8893925df0>

Without __repr__ method, the object points to the memory address.

Let’s create a simple class Person with a __repr__ method.

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

    def __repr__(self):
        return f"{type(self).__name__}('{self.name}', {self.age})"

person = Person("Soumendra", 30)
print(person)  # Output: Person('Soumendra', 30)

• Our __repr__ method returns a string that looks like the object’s constructor, making it easy to understand and recreate the object if needed.
• type(self).__name__ is a best practice to use rather than hard coding the class name.

__str__ and __repr__

• We use __str__ method for more user-friendly logs for humans to read.

• The print statement searches for __str__ method first in a class, otherwise it goes for __repr__ method.

• For example:

  class Person:
      def __init__(self, name, age):
          self.name = name
          self.age = age
  
      def __repr__(self):
          return f"{type(self).__name__}('{self.name}', {self.age})"
  
      def __str__(self):
          return f"A {type(self).__name__} object of name, {self.name} and age, {self.age}."
  
  person = Person("Soumendra", 30)
  print(person)  # Output: A Person object of name, Soumendra and age, 30.
  print(repr(person))  # Output: Person('Soumendra', 30)
  

How to write effective __repr__ methods

When writing a __repr__ method, include all relevant object attributes, and ensure the output is unambiguous. This allows other developers to quickly understand the object’s state and reconstruct it if necessary.

• To reconstruct the object from the repr output, make sure to add all the constructors (__init__) attributes to the __repr__ method. Therefore, it becomes easy to replicate the object. For example, in the above Person class example:

  >>> person = Person("Soumendra", 30)
  >>> person
  Person('Soumendra', 30)
  >>> new_person = eval(repr(person))
  >>> new_person
  Person('Soumendra', 30)
  

  It’s easy to recreate another object of the same class.

• Dynamic repr methods: You can create dynamic __repr__ methods using class attributes. For example:

  class MyClass:
      repr_format = "MyClass({self.attribute})"
  
      def __init__(self, attribute):
          self.attribute = attribute
  
      def __repr__(self):
          return self.repr_format.format(self=self)
  

• Inheritance and repr: Use class inheritance to create custom __repr__ methods that leverage the parent class’s __repr__ implementation:

  class Employee(Person):
      def __init__(self, name, age, job_title):
          super().__init__(name, age)
          self.job_title = job_title
  
      def __repr__(self):
          return f"{super().__repr__()}, '{self.job_title}')"
  

Conclusion

Understanding Python’s __repr__ method is essential for creating informative and unambiguous object representations. By implementing custom __repr__ methods and leveraging advanced techniques, you can improve the clarity and usability of your Python code.