str vs repr

Overview

Python uses two main dunder methods (special methods starting and ending with double underscores) to convert objects to strings: `__str__` for readable output and `__repr__` for unambiguous developer representations. By default, both print `<__main__.MyClass object at 0x...>`, but overriding them gives meaningful descriptions. Think of `__str__` as the 'display name' for a user and `__repr__` as the 'technical ID card' for a developer.

It solves the problem of opaque object representations. Clear string representations make logging, debugging, and user interfaces significantly easier by avoiding memory addresses in terminal output.

Where used: Logging configuration, Pydantic error messages, REPL debugging

Why learn this

Makes debugging faster by displaying object state instead of memory addresses in the REPL.
Allows custom string formatting in logging or UI without writing extra `to_string()` methods.

Code walkthrough

class Config:
  def __init__(self, env):
    self.env = env
  def __repr__(self):
    return f'Config(env={self.env!r})'

c = Config('prod')
print(c)
print([c])

Focus: print([c])

Aha moment

class Task:
  def __init__(self, name):
    self.name = name
  def __str__(self):
    return self.name

t = Task('build')
print([t])

Prediction: What does this print?

Common guess: ['build']

Collections like lists always use `__repr__` to format their elements, even if you print the list using `str()`. Since `Task` lacks a `__repr__`, you get the default memory address.

Common mistakes

Implementing only __str__: If you only implement `__str__`, collections like lists or dicts will still use `__repr__` when printed, showing memory addresses. Fix: Implement `__repr__` first; Python uses it as a fallback for `__str__`.

Glossary

unambiguous: Clear and exact, leaving no doubt about what something is or what it means. Example: `User(name='Alice', role='admin')`.
REPL: Read-Eval-Print Loop; an interactive programming environment where you can type commands and see the results immediately. Example: `repr(obj)`.

Recall questions

What is the primary difference in purpose between `__str__` and `__repr__`?
If an object only implements `__repr__`, what happens when you call `str(obj)`?

Understanding checks

What does this code output?

Box()

When `__str__` is not explicitly defined, Python falls back to calling the object's `__repr__` method.

Why is it important to implement `repr` for an object instead of only `str`?

Because collections like lists and dictionaries use `__repr__` to display their elements, even if you print them using `str()`.

If you only implement `__str__`, printing a list of your objects will still show unhelpful memory addresses.

Practice tasks

Basic User Representation

Create a `User` class with `name` and `role` attributes. Implement `__repr__` to return a string like `User(name='Alice', role='admin')` and `__str__` to return `Alice (admin)`.

Challenge

Logging Context

Write an `APIRequest` class taking `method` and `url`. Implement `__repr__` so that logging a list of requests shows them clearly as `APIRequest(method='GET', url='/users')`. Do not implement `__str__`.

Return to Python Backend Roadmap

__str__ vs __repr__