What is the difference between __str__ and __repr__ in Python?

In Python, __str__ and __repr__ are special methods used to define string representations of objects, but they serve different purposes:

1. __repr__ (Official/String Representation)

• Purpose:
• Meant for unambiguous debugging and developer clarity.
• Ideally returns a string that is a valid Python expression to recreate the object.
• Used by repr() and displayed in the interactive interpreter when you type the object.
• Example:

  class Date:
      def __init__(self, year, month, day):
          self.year = year
          self.month = month
          self.day = day

      def __repr__(self):
          return f"Date({self.year}, {self.month}, {self.day})"  # Valid Python code

  d = Date(2023, 10, 5)
  print(repr(d))  # Output: Date(2023, 10, 5)

2. __str__ (User-Friendly/String Representation)

• Purpose:
• Returns a readable, informal string for end-users.
• Used by str() and print().
• Example:

  class Date:
      def __str__(self):
          return f"{self.year}-{self.month}-{self.day}"

  d = Date(2023, 10, 5)
  print(d)  # Output: 2023-10-5 (simpler for users)

Key Differences

Feature	__repr__	__str__
Audience	Developers (debugging)	End-users (readability)
Use Case	repr(obj), interactive shell	print(obj), str(obj)
Goal	Unambiguous, recreate the object	Human-readable
Fallback Behavior	If __str__ is missing, uses __repr__	No fallback to __repr__

3. When to Implement Each

• Always define __repr__:
  Provides a clear debugging representation. If __str__ is undefined, __repr__ is used as a fallback.
• Define __str__ for user-friendly output:
  Optional but recommended for better user experience.

Example with Both Methods

class Date:
    def __init__(self, year, month, day):
        self.year = year
        self.month = month
        self.day = day

    def __repr__(self):
        return f"Date({self.year}, {self.month}, {self.day})"

    def __str__(self):
        return f"{self.year}-{self.month:02d}-{self.day:02d}"

d = Date(2023, 10, 5)
print(repr(d))  # Output: Date(2023, 10, 5) (debugging)
print(d)        # Output: 2023-10-05 (user-friendly)

4. Built-in Behavior

For many objects (e.g., lists, dictionaries), __repr__ and __str__ return the same string by default. For example:

lst = [1, 2, 3]
print(str(lst))   # [1, 2, 3]
print(repr(lst))  # [1, 2, 3] (same here)

But for strings, they differ:

s = "hello"
print(str(s))   # hello
print(repr(s))  # 'hello' (includes quotes)

Summary

• Use __repr__ for unambiguous debugging.
• Use __str__ for user-friendly output.
• If in doubt, define __repr__ (it’s a best practice).