7.5. Protocol Reflection

• When accessing an attribute

• setattr(obj, 'attrname', 'new_value') -> None

• delattr(obj, 'attrname') -> None

• getattr(obj, 'attrname', 'default_value') -> Any

• hasattr(obj, 'attrname') -> bool

>>> class Point:
...     def __init__(self, x, y, z):
...         self.x = x
...         self.y = y
...         self.z = z
...
...     def __setattr__(self, attrname, attrvalue):
...         if attrname not in {'x', 'y', 'z'}:
...             raise NameError('You can set only x, y, z attributes')
...         if type(attrvalue) not in (int,float):
...             raise TypeError('Attribute value must be int or float')
...         if attrvalue < 0:
...             raise ValueError('Attribute cannot be less than 0')
...         return super().__setattr__(attrname, attrvalue)

>>> pt = Point(1,2,3)
>>>
>>> pt.x = 1
>>>
>>> pt.x = -1
Traceback (most recent call last):
ValueError: Attribute cannot be less than 0
>>>
>>> pt.x = 'one'
Traceback (most recent call last):
TypeError: Attribute value must be int or float
>>>
>>> pt.notexisting = 1
Traceback (most recent call last):
NameError: You can set only x, y, z attributes

>>> class Astronaut:
...     def __init__(self, name):
...         self.name = name
>>>
>>>
>>> astro = Astronaut('Mark Watney')
>>>
>>> if astro._salary is None:
...     astro._salary = 100
Traceback (most recent call last):
AttributeError: 'Astronaut' object has no attribute '_salary'
>>>
>>>
>>> if not hasattr(astro, '_salary'):
...     astro._salary = 100
>>>
>>> print(astro._salary)
100

>>> def input(prompt):
...     return '_salary'
>>>
>>>
>>> attrname = input('Type attribute name: ')   # _salary
>>> value = getattr(astro, attrname, 'no such attribute')
>>> print(value)  
100

>>> def input(prompt):
...     return 'notexisting'
>>>
>>>
>>> attrname = input('Type attribute name: ')  # notexisting
>>> value = getattr(astro, attrname, 'no such attribute')
>>> print(value)
no such attribute

7.5.1. Protocol

• __setattr__(self, attrname, value) -> None

• __delattr__(self, attrname) -> None

• __getattribute__(self, attrname, default) -> Any

• __getattr__(self, attrname, default) -> Any

>>> class Reflection:
...
...     def __setattr__(self, attrname, value):
...         ...
...
...     def __delattr__(self, attrname):
...         ...
...
...     def __getattribute__(self, attrname, default):
...         ...
...
...     def __getattr__(self, attrname, default):
...         ...

7.5.2. Example

>>> class Immutable:
...     def __setattr__(self, attrname, value):
...         raise PermissionError('Immutable')

>>> class Protected:
...     def __setattr__(self, attrname, value):
...         if attrname.startswith('_'):
...             raise PermissionError('Field is protected')
...         else:
...             return super().__setattr__(attrname, value)

7.5.3. Set Attribute

• Called when trying to set attribute to a value

• Call Stack:

  • astro.name = 'Mark Watney'

  • => setattr(astro, 'name', 'Mark Watney')

  • => astro.__setattr__('name', 'Mark Watney')

>>> class Astronaut:
...     def __setattr__(self, attrname, value):
...         if attrname.startswith('_'):
...             raise PermissionError('Field is protected')
...         else:
...             return super().__setattr__(attrname, value)
>>>
>>>
>>> astro = Astronaut()
>>>
>>> astro.name = 'Mark Watney'
>>> print(astro.name)
Mark Watney
>>>
>>> astro._salary = 100
Traceback (most recent call last):
PermissionError: Field is protected

7.5.4. Delete Attribute

• Called when trying to delete attribute

• Call stack:

  • del astro.name

  • => delattr(astro, 'name')

  • => astro.__delattr__(name)

>>> class Astronaut:
...     def __delattr__(self, attrname):
...         if attrname.startswith('_'):
...             raise PermissionError('Field is protected')
...         else:
...             return super().__delattr__(attrname)
>>>
>>>
>>> astro = Astronaut()
>>>
>>> astro.name = 'Mark Watney'
>>> astro._salary = 100
>>>
>>> del astro.name
>>> del astro._salary
Traceback (most recent call last):
PermissionError: Field is protected

7.5.5. Get Attribute

• Called for every time, when you want to access any attribute

• Before even checking if this attribute exists

• If attribute is not found, then raises AttributeError and calls __getattr__()

• Call stack:

  • astro.name

  • => getattr(astro, 'name')

  • => astro.__getattribute__('name')

  • if astro.__getattribute__('name') raises AttributeError

  • => astro.__getattr__('name')

>>> class Astronaut:
...     def __getattribute__(self, attrname):
...         if attrname.startswith('_'):
...             raise PermissionError('Field is protected')
...         else:
...             return super().__getattribute__(attrname)
>>>
>>>
>>> astro = Astronaut()
>>>
>>> astro.name = 'Mark Watney'
>>> print(astro.name)
Mark Watney
>>>
>>> print(astro._salary)
Traceback (most recent call last):
PermissionError: Field is protected

7.5.6. Get Attribute if Missing

• Called whenever you request an attribute that hasn't already been defined

• It will not execute, when attribute already exist

• Implementing a fallback for missing attributes

Example __getattr__():

>>> class Astronaut:
...     def __init__(self):
...         self.name = None
...
...     def __getattr__(self, attrname):
...         return 'Sorry, field does not exist'
>>>
>>>
>>> astro = Astronaut()
>>> astro.name = 'Mark Watney'
>>>
>>> print(astro.name)
Mark Watney
>>>
>>> print(astro._salary)
Sorry, field does not exist

>>> class Astronaut:
...     def __init__(self):
...         self.name = None
...
...     def __getattribute__(self, attrname):
...         print('Getattribute called... ')
...         result = super().__getattribute__(attrname)
...         print(f'Result was: "{result}"')
...         return result
...
...     def __getattr__(self, attrname):
...         print('Not found. Getattr called...')
...         print(f'Creating attribute {attrname} with `None` value')
...         super().__setattr__(attrname, None)
>>>
>>>
>>>
>>> astro = Astronaut()
>>> astro.name = 'Mark Watney'
>>>
>>> astro.name  
Getattribute called...
Result was: "Mark Watney"
'Mark Watney'
>>>
>>> astro._salary  
Getattribute called...
Not found. Getattr called...
Creating attribute _salary with `None` value
>>>
>>> astro._salary  
Getattribute called...
Result was: "None"

7.5.7. Has Attribute

• Check if object has attribute

• There is no __hasattr__() method

• Calls __getattribute__() and checks if raises AttributeError

>>> class Astronaut:
...     def __init__(self, name):
...         self.name = name
>>>
>>>
>>> astro = Astronaut('Mark Watney')
>>>
>>> hasattr(astro, 'name')
True
>>>
>>> hasattr(astro, 'mission')
False
>>>
>>> astro.mission = 'Ares3'
>>> hasattr(astro, 'mission')
True

7.5.8. Use Case - 0x01

>>> class Astronaut:
...     def __getattribute__(self, attrname):
...         if attrname.startswith('_'):
...             raise PermissionError('Field is protected')
...         else:
...             return super().__getattribute__(attrname)
...
...     def __setattr__(self, attrname, value):
...         if attrname.startswith('_'):
...             raise PermissionError('Field is protected')
...         else:
...             return super().__setattr__(attrname, value)
>>>
>>>
>>> astro = Astronaut()
>>>
>>> astro.name = 'Mark Watney'
>>> print(astro.name)
Mark Watney
>>>
>>> astro._salary = 100
Traceback (most recent call last):
PermissionError: Field is protected
>>>
>>> print(astro._salary)
Traceback (most recent call last):
PermissionError: Field is protected

7.5.9. Use Case - 0x02

>>> class Temperature:
...     kelvin: float
...
...     def __init__(self, kelvin):
...         self.kelvin = kelvin
...
...     def __setattr__(self, attrname, value):
...         if attrname == 'kelvin' and value < 0.0:
...             raise ValueError('Kelvin temperature cannot be negative')
...         else:
...             return super().__setattr__(attrname, value)
>>>
>>>
>>> t = Temperature(100)
>>>
>>> t.kelvin = 20
>>> print(t.kelvin)
20
>>>
>>> t.kelvin = -10
Traceback (most recent call last):
ValueError: Kelvin temperature cannot be negative

7.5.10. Use Case - 0x03

>>> class Temperature:
...     kelvin: float
...     celsius: float
...     fahrenheit: float
...
...     def __getattr__(self, attrname):
...         if attrname == 'kelvin':
...             return super().__getattribute__('kelvin')
...         if attrname == 'celsius':
...             return self.kelvin - 273.15
...         if attrname == 'fahrenheit':
...             return (self.kelvin-273.15) * 1.8 + 32
>>>
>>>
>>> t = Temperature()
>>> t.kelvin = 373.15
>>>
>>> print(t.kelvin)
373.15
>>> print(t.celsius)
100.0
>>> print(t.fahrenheit)
212.0

7.5.11. Use Case - 0x04

>>> class Container:
...     def __init__(self, **kwargs: dict) -> None:
...         for key, value in kwargs.items():
...             setattr(self, key, value)
>>>
>>>
>>> a = Container(firstname='Pan', lastname='Twardowski')
>>> vars(a)
{'firstname': 'Pan', 'lastname': 'Twardowski'}
>>>
>>> b = Container(color='red')
>>> vars(b)
{'color': 'red'}
>>>
>>> c = Container(min=1, max=10)
>>> vars(c)
{'min': 1, 'max': 10}

7.5.12. Assignments

Code 7.44. Solution

"""
* Assignment: Protocol Reflection Delattr
* Complexity: easy
* Lines of code: 2 lines
* Time: 3 min

English:
    1. Create class `Point` with `x`, `y`, `z` attributes
    2. Prevent deleting attributes
    3. Run doctests - all must succeed

Polish:
    1. Stwórz klasę `Point` z atrybutami `x`, `y`, `z`
    2. Zablokuj usuwanie atrybutów
    3. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> pt = Point(1, 2, 3)
    >>> pt.x, pt.y, pt.z
    (1, 2, 3)

    >>> del pt.x
    Traceback (most recent call last):
    PermissionError: Cannot delete attributes

    >>> del pt.notexisting
    Traceback (most recent call last):
    PermissionError: Cannot delete attributes
"""

from dataclasses import dataclass


@dataclass
class Point:
    x: int
    y: int
    z: int

Code 7.45. Solution

"""
* Assignment: Protocol Reflection Setattr
* Complexity: easy
* Lines of code: 4 lines
* Time: 5 min

English:
    1. Create class `Point` with `x`, `y`, `z` attributes
    2. Prevent creation of new attributes
    3. Allow to modify values of `x`, `y`, `z`
    4. Run doctests - all must succeed

Polish:
    1. Stwórz klasę `Point` z atrybutami `x`, `y`, `z`
    2. Zablokuj tworzenie nowych atrybutów
    3. Zezwól na modyfikowanie wartości `x`, `y`, `z`
    4. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> pt = Point(1, 2, 3)
    >>> pt.x, pt.y, pt.z
    (1, 2, 3)
    >>> pt.notexisting = 10
    Traceback (most recent call last):
    PermissionError: Cannot set other attributes than x, y, z
    >>> pt.x = 10
    >>> pt.y = 20
    >>> pt.z = 30
    >>> pt.x, pt.y, pt.z
    (10, 20, 30)
"""

from dataclasses import dataclass


@dataclass
class Point:
    x: int
    y: int
    z: int

Code 7.46. Solution

"""
* Assignment: Protocol Reflection Frozen
* Complexity: easy
* Lines of code: 6 lines
* Time: 5 min

English:
    1. Create class `Point` with `x`, `y`, `z` attributes
    2. Prevent creation of new attributes
    3. Allow to define `x`, `y`, `z` but only at the initialization
    4. Prevent later modification of `x`, `y`, `z`
    5. Run doctests - all must succeed

Polish:
    1. Stwórz klasę `Point` z atrybutami `x`, `y`, `z`
    2. Zablokuj tworzenie nowych atrybutów
    3. Pozwól na zdefiniowanie `x`, `y`, `z` ale tylko przy inicjalizacji
    4. Zablokuj późniejsze modyfikacje `x`, `y`, `z`
    5. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> pt = Point(1, 2, 3)
    >>> pt.x, pt.y, pt.z
    (1, 2, 3)

    >>> pt.notexisting = 10
    Traceback (most recent call last):
    PermissionError: Cannot set other attributes than x, y, z

    >>> pt.x = 10
    Traceback (most recent call last):
    PermissionError: Cannot modify existing attributes

    >>> pt.y = 20
    Traceback (most recent call last):
    PermissionError: Cannot modify existing attributes

    >>> pt.z = 30
    Traceback (most recent call last):
    PermissionError: Cannot modify existing attributes
"""

from dataclasses import dataclass


@dataclass
class Point:
    x: int
    y: int
    z: int