9.7. OOP Stringify Objects

9.7.1. Rationale

• str(obj) -> obj.__str__()

• repr(obj) -> obj.__repr__()

• format(obj, name) -> obj.__format__(name)

• print(obj) -> str(obj) -> obj.__str__()

>>> import datetime
>>> date = datetime.datetime(1961, 4, 12, 6, 7)
>>>
>>> str(date)
'1961-04-12 06:07:00'
>>> repr(date)
'datetime.datetime(1961, 4, 12, 6, 7)'
>>> format(date, '%Y-%m-%d')
'1961-04-12'
>>> print(date)
1961-04-12 06:07:00

9.7.2. String

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

• Calling function print(obj) calls str(obj), which calls obj.__str__()

• Method obj.__str__() must return str

• for end-user

>>> class Astronaut:
...     pass
>>>
>>> astro = Astronaut()
>>> str(astro)  # doctest: +ELLIPSIS
'<Astronaut object at 0x...>'

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

>>> class Astronaut:
...     def __init__(self, name):
...         self.name = name
>>>
>>>
>>> astro = Astronaut('José Jiménez')
>>>
>>> print(astro)  # doctest: +ELLIPSIS
<Astronaut object at 0x...>
>>> str(astro)  # doctest: +ELLIPSIS
'<Astronaut object at 0x...>'
>>> astro.__str__()  # doctest: +ELLIPSIS
'<Astronaut object at 0x...>'

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

>>> class Astronaut:
...     def __init__(self, name):
...         self.name = name
...
...     def __str__(self):
...         return f'My name... {self.name}'
>>>
>>>
>>> astro = Astronaut('José Jiménez')
>>>
>>> print(astro)
My name... José Jiménez
>>> str(astro)
'My name... José Jiménez'
>>> astro.__str__()
'My name... José Jiménez'

9.7.3. Representation

• Calling function repr(obj) calls obj.__repr__()

• Method obj.__repr__() must return str

• for developers

• object representation

• copy-paste for creating object with the same values

• useful for debugging

• printing list will call __repr__() method on each element

>>> class Astronaut:
...     pass
>>>
>>> astro = Astronaut()
>>> repr(astro)  # doctest: +ELLIPSIS
'<Astronaut object at 0x...>'

Using __repr__() on a class:

>>> class Astronaut:
...     def __init__(self, name):
...         self.name = name
...
...     def __repr__(self):
...         return f'Astronaut(name="{self.name}")'
>>>
>>>
>>> astro = Astronaut('José Jiménez')
>>>
>>> repr(astro)
'Astronaut(name="José Jiménez")'
>>> astro
Astronaut(name="José Jiménez")

Printing list will call __repr__() method on each element:

>>> class Astronaut:
...     def __init__(self, name):
...         self.name = name
>>>
>>> crew = [Astronaut('Jan Twardowski'),
...         Astronaut('Mark Watney'),
...         Astronaut('Melissa Lewis')]
>>>
>>> print(crew)  # doctest: +ELLIPSIS
[<Astronaut object at 0x...>, <Astronaut object at 0x...>, <Astronaut object at 0x...>]

Printing list will call __repr__() method on each element:

>>> class Astronaut:
...     def __init__(self, name):
...         self.name = name
...
...     def __repr__(self):
...         return f'{self.name}'
>>>
>>> crew = [Astronaut('Jan Twardowski'),
...         Astronaut('Mark Watney'),
...         Astronaut('Melissa Lewis')]
>>>
>>> print(crew)
[Jan Twardowski, Mark Watney, Melissa Lewis]

9.7.4. Format

• Calling function format(obj, fmt) calls obj.__format__(fmt)

• Method obj.__format__() must return str

• Used for advanced formatting

>>> class Astronaut:
...     def __init__(self, name):
...         self.name = name
...
...     def __format__(self, mood):
...         if mood == 'happy':
...             return f"Yuppi, we're going to space!"
...         elif mood == 'scared':
...             return f"I hope we don't crash"
>>>
>>>
>>> jose = Astronaut('José Jiménez')
>>>
>>> print(f'{jose:happy}')
Yuppi, we're going to space!
>>> print(f'{jose:scared}')
I hope we don't crash

>>> SECOND = 1
>>> MINUTE = 60 * SECOND
>>> HOUR = 60 * MINUTE
>>> DAY = 24 * HOUR
>>>
>>>
>>> class Duration:
...     def __init__(self, seconds):
...         self.seconds = seconds
...
...     def __format__(self, unit):
...         if unit == 'minutes':
...             result = self.seconds / MINUTE
...         elif unit == 'hours':
...             result = self.seconds / HOUR
...         elif unit == 'days':
...             result = self.seconds / DAY
...         return str(round(result, 2))
>>>
>>> duration = Duration(seconds=3600)
>>>
>>> print(f'Duration was {duration:minutes} min')
Duration was 60.0 min
>>> print(f'Duration was {duration:hours} hour')
Duration was 1.0 hour
>>> print(f'Duration was {duration:days} day')
Duration was 0.04 day

>>> SECOND = 1
>>> MINUTE = 60 * SECOND
>>> HOUR = 60 * MINUTE
>>> DAY = 24 * HOUR
>>>
>>>
>>> class Duration:
...     def __init__(self, seconds):
...         self.seconds = seconds
...
...     def __format__(self, unit):
...         if unit in ('s', 'sec', 'seconds'):
...             result = self.seconds / SECOND
...         elif unit in ('m', 'min', 'minutes'):
...             result = self.seconds / MINUTE
...         elif unit in ('h', 'hr', 'hours'):
...             result = self.seconds / HOUR
...         elif unit in ('d', 'days'):
...             result = self.seconds / DAY
...         return str(round(result, 2))
>>>
>>>
>>> duration = Duration(seconds=3600)
>>>
>>> print(f'Duration: {duration:s} seconds')
Duration: 3600.0 seconds
>>> print(f'Duration: {duration:m} minutes')
Duration: 60.0 minutes
>>> print(f'Duration: {duration:h} hours')
Duration: 1.0 hours
>>> print(f'Duration: {duration:d} days')
Duration: 0.04 days

>>> class Temperature:
...     def __init__(self, kelvin):
...         self.kelvin = kelvin
...
...     def to_fahrenheit(self):
...         return (self.kelvin-273.15) * 1.8 + 32
...
...     def to_celsius(self):
...         return self.kelvin - 273.15
...
...     def __format__(self, unit):
...         if unit == 'kelvin':
...             value = self.kelvin
...         elif unit == 'celsius':
...             value = self.to_celsius()
...         elif unit == 'fahrenheit':
...             value = self.to_fahrenheit()
...         return f'{value:.2f}'
>>>
>>>
>>> temp = Temperature(309.75)
>>>
>>> print(f'Temperature is {temp:kelvin} K')
Temperature is 309.75 K
>>> print(f'Temperature is {temp:celsius} C')
Temperature is 36.60 C
>>> print(f'Temperature is {temp:fahrenheit} F')
Temperature is 97.88 F

>>> class Point:
...     def __init__(self, x, y, z=0):
...         self.x = x
...         self.y = y
...         self.z = z
...
...     def __format__(self, name):
...
...         if name == 'in_2D':
...             result = f"Point(x={self.x}, y={self.y})"
...         elif name == 'in_3D':
...             result = f"Point(x={self.x}, y={self.y}, z={self.z})"
...         elif name == 'as_dict':
...             result = self.__dict__
...         elif name == 'as_tuple':
...             result = tuple(self.__dict__.values())
...         elif name == 'as_json':
...             import json
...             result = json.dumps(self.__dict__)
...         return str(result)
>>>
>>>
>>> point = Point(x=1, y=2)
>>>
>>> print(f'{point:in_2D}')
Point(x=1, y=2)
>>> print(f'{point:in_3D}')
Point(x=1, y=2, z=0)
>>> print(f'{point:as_tuple}')
(1, 2, 0)
>>> print(f'{point:as_dict}')
{'x': 1, 'y': 2, 'z': 0}
>>> print(f'{point:as_json}')
{"x": 1, "y": 2, "z": 0}

9.7.5. Assignments

Code 9.14. Solution

"""
* Assignment: OOP Stringify Str
* Complexity: easy
* Lines of code: 3 lines
* Time: 5 min

English:
    1. Use code from "Given" section (see below)
    2. While printing object show: species name and a sum of `self.features`
    3. Result of sum round to one decimal place
    4. Compare result with "Tests" section (see below)

Polish:
    1. Użyj kodu z sekcji "Given" (patrz poniżej)
    2. Przy wypisywaniu obiektu pokaż: nazwę gatunku i sumę `self.features`
    3. Wynik sumowania zaokrąglij do jednego miejsca po przecinku
    4. Porównaj wyniki z sekcją "Tests" (patrz poniżej)

Tests:
    >>> for *features, label in DATA:
    ...    iris = Iris(features, label)
    ...    print(iris)
    setosa 9.4
    versicolor 16.3
    virginica 19.3
"""


# Given
DATA = [(4.7, 3.2, 1.3, 0.2, 'setosa'),
        (7.0, 3.2, 4.7, 1.4, 'versicolor'),
        (7.6, 3.0, 6.6, 2.1, 'virginica')]


class Iris:
    def __init__(self, features, label):
        self.features = features
        self.label = label

Code 9.15. Solution

"""
* Assignment: OOP Stringify Repr
* Complexity: easy
* Lines of code: 3 lines
* Time: 5 min

English:
    1. Use code from "Given" section (see below)
    2. Print representation of each instance with values (use `repr()`)
    3. Result of sum round to two decimal places
    4. Compare result with "Tests" section (see below)

Polish:
    1. Użyj kodu z sekcji "Given" (patrz poniżej)
    2. Wypisz reprezentację każdej z instancji z wartościami (użyj `repr()`)
    3. Wynik sumowania zaokrąglij do dwóch miejsc po przecinku
    4. Porównaj wyniki z sekcją "Tests" (patrz poniżej)

Tests:
    >>> result = [Iris(X,y) for *X,y in DATA]
    >>> result  # doctest: +NORMALIZE_WHITESPACE
    [Iris(features=[4.7, 3.2, 1.3, 0.2], label='setosa'),
     Iris(features=[7.0, 3.2, 4.7, 1.4], label='versicolor'),
     Iris(features=[7.6, 3.0, 6.6, 2.1], label='virginica')]
"""


# Given
DATA = [(4.7, 3.2, 1.3, 0.2, 'setosa'),
        (7.0, 3.2, 4.7, 1.4, 'versicolor'),
        (7.6, 3.0, 6.6, 2.1, 'virginica')]


class Iris:
    def __init__(self, features, label):
        self.features = features
        self.label = label

Code 9.16. Solution

"""
* Assignment: OOP Stringify Format
* Complexity: easy
* Lines of code: 8 lines
* Time: 5 min

English:
    1. Use code from "Given" section (see below)
    2. Overload `__format__()` to convert length units
    3. Compare result with "Tests" section (see below)

Polish:
    1. Użyj kodu z sekcji "Given" (patrz poniżej)
    2. Przeciąż `__format__()` aby konwertował jednostki długości
    3. Porównaj wyniki z sekcją "Tests" (patrz poniżej)

Hints:
    * 1 km = 1000 m
    * 1 m = 100 cm

Tests:
    >>> result = Distance(meters=1337)
    >>> format(result, 'km')
    '1.337'
    >>> format(result, 'cm')
    '133700'
    >>> format(result, 'm')
    '1337'
"""


# Given
class Distance:
    def __init__(self, meters):
        self.meters = meters

Code 9.17. Solution

"""
* Assignment: OOP Stringify Nested
* Complexity: medium
* Lines of code: 9 lines
* Time: 21 min

English:
    1. Use code from "Given" section (see below)
    2. Overload `str` and `repr` to achieve desired printing output
    3. Compare result with "Tests" section (see below)

Polish:
    1. Użyj kodu z sekcji "Given" (patrz poniżej)
    2. Przeciąż `str` i `repr` aby osiągnąć oczekiwany rezultat wypisywania
    3. Porównaj wyniki z sekcją "Tests" (patrz poniżej)

Hints:
    * Define `Crew.__str__()`
    * Define `Astronaut.__str__()` and `Astronaut.__repr__()`
    * Define `Mission.__repr__()`

Tests:
    >>> melissa = Astronaut('Melissa Lewis')
    >>> print(f'Commander: \\n{melissa}\\n')  # doctest: +NORMALIZE_WHITESPACE
    Commander:
    Melissa Lewis

    >>> mark = Astronaut('Mark Watney', experience=[
    ...    Mission(2035, 'Ares 3')])
    >>> print(f'Space Pirate: \\n{mark}\\n')  # doctest: +NORMALIZE_WHITESPACE
    Space Pirate:
    Mark Watney veteran of [
          2035: Ares 3]

    >>> crew = Crew([
    ...     Astronaut('Jan Twardowski', experience=[
    ...         Mission(1969, 'Apollo 11'),
    ...         Mission(2024, 'Artemis 3'),
    ...     ]),
    ...     Astronaut('José Jiménez'),
    ...     Astronaut('Mark Watney', experience=[
    ...         Mission(2035, 'Ares 3'),
    ...     ]),
    ... ])

    >>> print(f'Crew: \\n{crew}')  # doctest: +NORMALIZE_WHITESPACE
    Crew:
    Jan Twardowski veteran of [
          1969: Apollo 11,
          2024: Artemis 3]
    José Jiménez
    Mark Watney veteran of [
          2035: Ares 3]
"""


# Given
class Crew:
    def __init__(self, members=()):
        self.members = list(members)


class Astronaut:
    def __init__(self, name, experience=()):
        self.name = name
        self.experience = list(experience)


class Mission:
    def __init__(self, year, name):
        self.year = year
        self.name = name