6.5. Property¶
6.5.1. Rationale¶
Disable attribute modification
Logging value access
Check boundary
Raise exceptions (TypeError)
Check argument type
6.5.2. Problem¶
>>> class Point:
... x: int
...
... def get_x(self): pass
... def set_x(self, newvalue): pass
... def del_x(self): pass
>>>
>>>
>>> pt = Point()
>>> pt.set_x(1)
>>> class Point:
... x: int
... y: int
...
... def get_x(self): pass
... def set_x(self, newvalue): pass
... def del_x(self): pass
... def get_y(self): pass
... def set_y(self, newvalue): pass
... def del_y(self): pass
>>>
>>>
>>> pt = Point()
>>> pt.set_x(1)
>>> pt.set_y(1)
>>> class Point:
... x: int
... y: int
... z: int
...
... def get_x(self) -> int: pass
... def get_x(self): pass
... def set_x(self, newvalue): pass
... def del_x(self): pass
... def get_y(self): pass
... def set_y(self, newvalue): pass
... def del_y(self): pass
... def get_z(self): pass
... def set_z(self, newvalue): pass
... def del_z(self): pass
>>>
>>>
>>> pt = Point()
>>> pt.set_x(1)
>>> pt.set_y(1)
>>> pt.set_z(1)
6.5.3. What if...¶
>>> class Point:
... x: int
... y: int
... z: int
...
... def set_position(self, x, y, z):
... self.x = x
... self.y = y
... self.z = z
>>>
>>> pt = Point()
>>> pt.set_position(1, 2, 3)
Works for point. How about astronauts
>>> class Astronaut:
... firstname: str
... middlename: str
... lastname: str
...
... def set_name(self, name):
... first, mid, last = name.split()
... self.firstname = first
... self.middlename = mid
... self.lastname = last
Do everyone have a middle name? Do everyone have first or lastname?
6.5.4. Solution¶
>>> class Point:
... x: int
... y: int
... z: int
>>>
>>>
>>> pt = Point()
>>> pt.x = 1
>>> pt.y = 2
>>> pt.z = 3
But what if we want to make validation:
>>> class Point:
... x: int
... y: int
... z: int
...
... def set_x(self, newvalue):
... if newvalue > 0:
... self.x = newvalue
... else:
... raise ValueError
...
... def set_y(self, newvalue):
... if newvalue > 0:
... self.y = newvalue
... else:
... raise ValueError
...
... def set_z(self, newvalue):
... if newvalue > 0:
... self.z = newvalue
... else:
... raise ValueError
We can refactor this code:
>>> class Point:
... x: int
... y: int
... z: int
...
... def _is_valid(self, value):
... if newvalue > 0:
... return value
... else:
... raise ValueError
...
... def set_x(self, newvalue):
... self.x = self._valid(newvalue)
...
... def set_y(self, newvalue):
... self.y = self._valid(newvalue)
...
... def set_z(self, newvalue):
... self.z = self._valid(newvalue)
But problem persist.
What if all parameters can have different ranges:
age between 0 and 130
height between 150 and 210
name first capital letter, then lowercased letters
6.5.5. Protocol¶
myattribute = property()
- creates property@myattribute.getter
- getter for attribute@myattribute.setter
- setter for attribute@myattribute.deleter
- deleter for attributeMethod name must be the same as attribute name
myattribute
has to beproperty
@property
- creates property and a getter
>>> class MyClass:
... myattribute = property()
...
... @myattribute.getter
... def myattribute(self):
... return ...
...
... @myattribute.setter
... def myattribute(self):
... ...
...
... @myattribute.deleter
... def myattribute(self):
... ...
6.5.6. Example¶
>>> class KelvinTemperature:
... value: float
>>>
>>> t = KelvinTemperature()
>>> t.value = -2 # Should raise ValueError('Kelvin cannot be negative')
>>> class KelvinTemperature:
... value: float
...
... def __init__(self, initialvalue):
... self.value = initialvalue
>>>
>>> t = KelvinTemperature(-1) # Should raise ValueError('Kelvin cannot be negative')
>>> t.value = -2 # Should raise ValueError('Kelvin cannot be negative')
>>> class KelvinTemperature:
... value: float
...
... def __init__(self, initialvalue):
... if initialvalue < 0:
... raise ValueError('Negative Kelvin Temperature')
... self.value = initialvalue
>>>
>>>
>>> t = KelvinTemperature(1)
>>> t.value = -1 # Should raise ValueError('Kelvin cannot be negative')
>>> class KelvinTemperature:
... _value: float
...
... def __init__(self, initialvalue):
... self.set_value(initialvalue)
...
... def set_value(self, newvalue):
... if newvalue < 0:
... raise ValueError('Negative Kelvin Temperature')
... self._value = newvalue
>>> class KelvinTemperature:
... _value: float
... value = property()
...
... def __init__(self, initialvalue):
... self.value = initialvalue
...
... @value.setter
... def value(self, newvalue):
... if newvalue < 0:
... raise ValueError('Negative Kelvin Temperature')
... self._value = newvalue
6.5.7. Use Cases¶
>>> class Astronaut:
... def __init__(self, firstname, lastname):
... self._firstname = firstname
... self._lastname = lastname
...
... @property
... def name(self):
... return f'{self._firstname} {self._lastname[0]}.'
>>>
>>>
>>> astro = Astronaut('Mark', 'Watney')
>>> print(astro.name)
Mark W.
>>> class Astronaut:
... name = property()
...
... def __init__(self, firstname, lastname):
... self._firstname = firstname
... self._lastname = lastname
...
... @name.getter
... def name(self):
... return f'{self._firstname} {self._lastname[0]}.'
>>>
>>>
>>> astro = Astronaut('Mark', 'Watney')
>>> print(astro.name)
Mark W.
>>> class Temperature:
... kelvin = property()
... __value: float
...
... def __init__(self, kelvin=None):
... self.__value = kelvin
...
... @kelvin.setter
... def kelvin(self, newvalue):
... if newvalue < 0:
... raise ValueError('Negative Kelvin Temperature')
... else:
... self.__value = newvalue
>>>
>>>
>>> t = Temperature()
>>> t.kelvin = 10
>>> t.kelvin = -1
Traceback (most recent call last):
ValueError: Negative Kelvin Temperature
6.5.8. Attribute Access¶
Java way: Setter and Getter
Pythonic way: Properties, Reflection, Descriptors
Accessing class fields using setter and getter:
>>> class Astronaut:
... def __init__(self, name=None):
... self._name = name
...
... def set_name(self, name):
... self._name = name
...
... def get_name(self):
... return self._name
>>>
>>>
>>> astro = Astronaut()
>>> astro.set_name('Mark Watney')
>>> print(astro.get_name())
Mark Watney
Accessing class fields. Either put name
as an argument for __init__()
or create dynamic field in runtime:
>>> class Astronaut:
... def __init__(self, name=None):
... self.name = name
>>>
>>>
>>> astro = Astronaut()
>>> astro.name = 'Jan Twardowski'
>>> print(astro.name)
Jan Twardowski
6.5.9. Property class¶
Property's arguments are method pointers
get_name
,set_name
,del_name
and a docstringNot recommended
>>> class Astronaut:
... def __init__(self, name=None):
... self._name = name
...
... def get_name(self):
... return self._name
...
... def set_name(self, value):
... self._name = value
...
... def del_name(self):
... del self._name
...
... name = property(get_name, set_name, del_name, "I am the 'name' property.")
6.5.10. Property Descriptor¶
Prefer
name = property()
>>> class Astronaut:
... name = property()
...
... def __init__(self, name=None):
... self._name = name
...
... @name.getter
... def name(self):
... return self._name
...
... @name.setter
... def name(self, value):
... self._name = value
...
... @name.deleter
... def name(self):
... del self._name
6.5.11. Property Decorator¶
Typically used when, there is only getter and no setter and deleter methods
>>> class Astronaut:
... def __init__(self, name=None):
... self._name = name
...
... @property
... def name(self):
... return self._name
...
... @name.setter
... def name(self, value):
... self._name = value
...
... @name.deleter
... def name(self):
... del self._name
6.5.12. Use Case 1¶
>>> class Astronaut:
... def __init__(self):
... self._name = None
...
... def set_name(self, name):
... self._name = name.title()
...
... def get_name(self):
... if self._name:
... firstname, lastname = self._name.split()
... return f'{firstname} {lastname[0]}.'
...
... def del_name(self):
... self._name = None
>>>
>>>
>>> astro = Astronaut()
>>>
>>> astro.set_name('JaN TwARdoWskI')
>>> print(astro.get_name())
Jan T.
>>>
>>> astro.del_name()
>>> print(astro.get_name())
None
>>> class Astronaut:
... name = property()
...
... def __init__(self):
... self._name = None
...
... @name.getter
... def name(self):
... if self._name:
... firstname, lastname = self._name.split()
... return f'{firstname} {lastname[0]}.'
...
... @name.setter
... def name(self, name):
... self._name = name.title()
...
... @name.deleter
... def name(self):
... self._name = None
>>>
>>>
>>> astro = Astronaut()
>>>
>>> astro.name = 'JAN TwARdoWski'
>>> print(astro.name)
Jan T.
>>>
>>> del astro.name
>>> print(astro.name)
None
6.5.13. Use Case 2¶
Calculate age
from dataclasses import dataclass
from datetime import date
DAY = 1
YEAR = 365.2425 * DAYS
@dataclass
class Astronaut:
firstname: str
lastname: str
date_of_birth: date
@property
def age(self):
age = date.today() - self.date_of_birth
return round(age.days/YEAR, 1)
astro = Astronaut('Mark', 'Watney', date(1969, 7, 21))
print(astro.age)
6.5.14. Use Case 3¶
Cached Property
>>> from dataclasses import dataclass, field
>>> from datetime import date
>>>
>>> YEAR = 365.2425
>>>
>>>
>>> @dataclass
... class Astronaut:
... firstname: str
... lastname: str
... date_of_birth: date
... __cache: dict = field(default_factory=dict)
...
... @property
... def age(self):
... if 'age' not in self.__cache:
... age = (date.today() - self.date_of_birth).days / YEAR
... self.__cache['age'] = round(age, 1)
... return self.__cache['age']
>>>
>>>
>>> astro = Astronaut('Mark', 'Watney', date(1969, 7, 21))
>>> print(astro.age)
51.6
6.5.15. Use Case 4¶
>>> class Temperature:
... def __init__(self, initial_temperature):
... self._protected = initial_temperature
...
... @property
... def value(self):
... print('You are trying to access a value')
... return self._protected
>>>
>>>
>>> t = Temperature(100)
>>>
>>> print(t.value)
You are trying to access a value
100
>>> class Temperature:
... def __init__(self, initial_temperature):
... self._protected = initial_temperature
...
... @property
... def value(self):
... return self._protected
...
... @value.setter
... def value(self, new_value):
... if new_value < 0.0:
... raise ValueError('Kelvin Temperature cannot be negative')
... else:
... self._protected = new_value
>>>
>>>
>>> t = Temperature(100)
>>> t.value = -10
Traceback (most recent call last):
ValueError: Kelvin Temperature cannot be negative
>>> class Temperature:
... def __init__(self, initial_temperature):
... self._protected = initial_temperature
...
... @property
... def value(self):
... return self._protected
...
... @value.deleter
... def value(self):
... print('Resetting temperature')
... self._protected = 0.0
>>>
>>>
>>> t = Temperature(100)
>>>
>>> del t.value
Resetting temperature
>>>
>>> print(t.value)
0.0
6.5.16. Assignments¶
"""
* Assignment: Protocol Property Getter
* Complexity: easy
* Lines of code: 4 lines
* Time: 5 min
English:
1. Use data from "Given" section (see below)
2. Define class `Point` with `x`, `y`, `z` attributes
3. Define property `position` in class `Point`
4. Accessing `position` returns `(x, y, z)`
5. Compare result with "Tests" section (see below)
Polish:
1. Użyj danych z sekcji "Given" (patrz poniżej)
2. Zdefiniuj klasę `Point` z atrybutami `x`, `y`, `z`
3. Zdefiniuj property `position` w klasie `Point`
4. Dostęp do `position` zwraca `(x, y, z)`
5. Porównaj wyniki z sekcją "Tests" (patrz poniżej)
Tests:
>>> pt = Point(x=1, y=2, z=3)
>>> pt.x, pt.y, pt.z
(1, 2, 3)
>>> pt.position
(1, 2, 3)
"""
# Given
class Point:
def __init__(self, x, y, z):
self.x = x
self.y = y
self.z = z
"""
* Assignment: Protocol Property Setter
* Complexity: easy
* Lines of code: 9 lines
* Time: 5 min
English:
1. Use data from "Given" section (see below)
2. Define class `Point` with `x`, `y`, `z` attributes
3. Define property `position` in class `Point`
4. Setting `position`:
a. If argument is not list, tuple, set raise Type Error
b. If argument has length other than 3, raise Value
b. Else sets `x`, `y`, `z` attributes from sequence
5. Compare result with "Tests" section (see below)
Polish:
1. Użyj danych z sekcji "Given" (patrz poniżej)
2. Zdefiniuj klasę `Point` z atrybutami `x`, `y`, `z`
3. Zdefiniuj property `position` w klasie `Point`
4. Ustawianie `position`:
a. Jeżeli argument nie jest list, tuple, set podnieś TypeError
b. Jeżeli argument nie ma długości 3, podnieś ValueError
b. W przeciwnym wypadku ustaw kolejne atrybuty `x`, `y`, `z` z sekwencji
5. Porównaj wyniki z sekcją "Tests" (patrz poniżej)
Tests:
>>> pt = Point(x=1, y=2, z=3)
>>> pt.x, pt.y, pt.z
(1, 2, 3)
>>> pt.position = 4, 5, 6
>>> pt.x, pt.y, pt.z
(4, 5, 6)
>>> pt.position = [7, 8, 9]
>>> pt.x, pt.y, pt.z
(7, 8, 9)
>>> pt.position = 1, 2
Traceback (most recent call last):
ValueError
>>> pt.position = {'a':1, 'b':2}
Traceback (most recent call last):
TypeError
"""
# Given
class Point:
def __init__(self, x, y, z):
self.x = x
self.y = y
self.z = z
"""
* Assignment: Protocol Property Deleter
* Complexity: easy
* Lines of code: 6 lines
* Time: 5 min
English:
1. Use data from "Given" section (see below)
2. Define class `Point` with `x`, `y`, `z` attributes
3. Define property `position` in class `Point`
4. Deleting `position` sets all attributes to 0 (`x=0`, `y=0`, `z=0`)
5. Compare result with "Tests" section (see below)
Polish:
1. Użyj danych z sekcji "Given" (patrz poniżej)
2. Zdefiniuj klasę `Point` z atrybutami `x`, `y`, `z`
3. Zdefiniuj property `position` w klasie `Point`
4. Usunięcie `position` ustawia wszystkie atrybuty na 0 (`x=0`, `y=0`, `z=0`)
5. Porównaj wyniki z sekcją "Tests" (patrz poniżej)
Tests:
>>> pt = Point(x=1, y=2, z=3)
>>> pt.x, pt.y, pt.z
(1, 2, 3)
>>> del pt.position
>>> pt.x, pt.y, pt.z
(0, 0, 0)
"""
# Given
class Point:
def __init__(self, x, y, z):
self.x = x
self.y = y
self.z = z