17.8. OOP Str

• • str(obj) - object stringification

• Dedicated for end-users

• Calling function print(obj) calls str(obj)

• Calling function str(obj) calls obj.__str__()

• Calling f'{obj}' calls obj.__str__()

• Calling f'{obj!s}' calls obj.__str__()

• Method obj.__str__() must return str

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> alice = User('Alice', 'Apricot')
>>>
>>> print(alice)
<__main__.User object at 0x1087b12b0>

17.8.1. Problem

• Object without __str__() method overloaded prints their memory address

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> alice = User('Alice', 'Apricot')
>>>
>>> print(alice)
<__main__.User object at 0x10aef7450>
>>>
>>> str(alice)
'<__main__.User object at 0x10aef7450>'
>>>
>>> alice.__str__()
'<__main__.User object at 0x10aef7450>'
>>>
>>> f'{alice}'
'<__main__.User object at 0x10aef7450>'
>>>
>>> f'{alice!s}'
'<__main__.User object at 0x10aef7450>'

17.8.2. Solution

• Objects can verbose print if __str__() method is present:

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
...
...     def __str__(self):
...         return f'{self.firstname} {self.lastname}'
...
>>> alice = User('Alice', 'Apricot')
>>>
>>> print(alice)
Alice Apricot
>>>
>>> str(alice)
'Alice Apricot'
>>>
>>> alice.__str__()
'Alice Apricot'
>>>
>>> f'Hello {alice}'
'Hello Alice Apricot'
>>>
>>> f'Hello {alice!s}'
'Hello Alice Apricot'

17.8.3. Case Study

>>> import datetime
>>>
>>> today = datetime.date(2000, 1, 2)

>>> str(today)
'2000-01-02'

17.8.4. Use Case - 1

>>> class User:
...     def __init__(self, username, password):
...         self.username = username
...         self.password = password
...
...     def __str__(self):
...         return f'User {self.username}'
>>>
>>>
>>> alice = User('alice', 'secret')
>>>
>>> print(alice)
User alice

17.8.5. Assignments

# %% About
# - Name: OOP Stringification Str
# - Difficulty: easy
# - Lines: 3
# - Minutes: 2

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Modify class `User`
# 2. Define new method `__str__` which returns users firstname and lastname
#    example: 'Alice Apricot'
# 3. Run doctests - all must succeed

# %% Polish
# 1. Zmodyfikuj klasę `User`
# 2. Zdefiniuj nową metodę `__str__`, która zwraca imię i nazwisko użytkownika
#    przykład: 'Alice Apricot'
# 3. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> alice = User('Alice', 'Apricot')
# >>> str(alice)
# 'Alice Apricot'

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> from pprint import pprint
>>> from inspect import isclass

>>> assert isclass(User), \
'Object `User` has an invalid type; expected: `class`.'

>>> assert hasattr(User, '__init__'), \
'Object `User` has an invalid attribute; expected: to have an attribute `__init__`.'

>>> assert hasattr(User, '__str__'), \
'Object `User` has an invalid attribute; expected: to have an attribute `__str__`.'

>>> alice = User('Alice', 'Apricot')
>>> result = str(alice)
>>> pprint(result)
'Alice Apricot'
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports

# %% Types

# %% Data

# %% Result
class User:
    def __init__(self, firstname, lastname):
        self.firstname = firstname
        self.lastname = lastname