19.1. OOP Str

  • Calling print(obj) calls str(obj)

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

  • Method obj.__str__() must return str

  • Intended for end-users of your class

19.1.1. Default

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

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> mark = User('Mark', 'Watney')
>>>
>>> print(mark)  
<__main__.User object at 0x...>
>>>
>>> str(mark)  
'<__main__.User object at 0x...>'
>>>
>>> mark.__str__()  
'<__main__.User object at 0x...>'

19.1.2. Overloaded

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'Hello {self.firstname} {self.lastname}'
>>>
>>>
>>> mark = User('Mark', 'Watney')
>>>
>>> print(mark)
Hello Mark Watney
>>>
>>> str(mark)
'Hello Mark Watney'
>>>
>>> mark.__str__()
'Hello Mark Watney'

19.1.3. Assignments

# %% 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

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

# %% About
# - Name: Operator String Str
# - Difficulty: easy
# - Lines: 2
# - Minutes: 2

# %% English
# 1. Modify `User` to overwrite `__str__()` method
# 2. Printing object should return firstname and lastname,
#    example: Mark Watney
# 3. Run doctests - all must succeed

# %% Polish
# 1. Zmodyfikuj klasę `User` aby nadpisać metodę `__str__()`
# 2. Wypisanie obiektu powinno zwrówić imię i nazwisko,
#    przykład: Mark Watney
# 3. Uruchom doctesty - wszystkie muszą się powieść

# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'

>>> mark = User('Mark', 'Watney')
>>> print(mark)
Mark Watney

>>> melissa = User('Melissa', 'Lewis')
>>> print(melissa)
Melissa Lewis
"""

# Modify `User` to overwrite `__str__()` method
# Printing object should return firstname and lastname,
# Example: Mark Watney
class User:
    def __init__(self, firstname, lastname):
        self.firstname = firstname
        self.lastname = lastname



# %% 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

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

# %% About
# - Name: Operator String Str
# - Difficulty: easy
# - Lines: 3
# - Minutes: 3

# %% English
# 1. Modify class `Iris` to overwrite `__str__()` method
# 2. While printing object show: species name and a sum of `self.values`,
#    example: `species='setosa', total=9.4`
# 3. Result of sum round to one decimal place
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zmodyfikuj klasę `Iris` aby nadpisać metodę `__str__()`
# 2. Przy wypisywaniu obiektu pokaż: nazwę gatunku i sumę `self.values`,
#    przykład: `species='setosa', total=9.4`
# 3. Wynik sumowania zaokrąglij do jednego miejsca po przecinku
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Hints
# - `f'{var=:.1f}'`

# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'

>>> iris = Iris(
...     values=[4.7, 3.2, 1.3, 0.2],
...     species='setosa',
... )
>>> print(iris)
species='setosa', total=9.4

>>> iris = Iris(
...     values=[7.0, 3.2, 4.7, 1.4],
...     species='versicolor',
... )
>>> print(iris)
species='versicolor', total=16.3
"""

# Modify class `Iris` to overwrite `__str__()` method
# While printing object show: species name and a sum of `self.values`,
# Example: `species='setosa', total=9.4`
# Result of sum round to one decimal place
class Iris:
    values: list
    species: str

    def __init__(self, values, species):
        self.values = values
        self.species = species

    def __str__(self):
        ...