5.4. Decorator Function with Mth

5.4.1. Rationale

  • mydecorator is a decorator name

  • method is a method name

  • self is an instance

  • args arbitrary number of positional arguments

  • kwargs arbitrary number of keyword arguments

Syntax:
class MyClass:
    @mydecorator
    def mymethod(self, *args, **kwargs):
        ...


obj = MyClass()
obj.mymethod()
Is equivalent to:
class MyClass:
    def mymethod(self, *args, **kwargs):
        ...


obj = MyClass()
obj.mymethod = mydecorator(obj.mymethod)

5.4.2. Syntax

  • mydecorator is a decorator name

  • mymethod is a method name

  • self is an instance

  • args arbitrary number of positional arguments

  • kwargs arbitrary number of keyword arguments

Definition:

def mydecorator(method):
    def wrapper(self, *args, **kwargs):
        return method(self, *args, **kwargs)
    return wrapper

Decoration:

class MyClass:

    @mydecorator
    def mymethod(self):
        ...

Usage:

my = MyClass()
my.mymethod()

5.4.3. Example

def run(method):
    def wrapper(self, *args, **kwargs):
        return method(self, *args, **kwargs)
    return wrapper


class Astronaut:
    @run
    def hello(self, name):
        return f'My name... {name}'


astro = Astronaut()
astro.hello('José Jiménez')
# 'My name... José Jiménez'

5.4.4. Use Cases

def if_allowed(method):
    def wrapper(self, *args, **kwargs):
        if self._is_allowed:
            return method(self, *args, **kwargs)
        else:
            print('Sorry, Permission Denied')
    return wrapper


class MyClass:
    def __init__(self):
        self._is_allowed = True

    @if_allowed
    def do_something(self):
        print('Doing...')

    @if_allowed
    def do_something_else(self):
        print('Doing something else...')


my = MyClass()

my.do_something()           # Doing...
my.do_something_else()      # Doing something else...

my._is_allowed = False

my.do_something()           # Sorry, you cannot do anything
my.do_something_else()      # Sorry, you cannot do anything
def paragraph(method):
    def wrapper(self, *args, **kwargs):
        result = method(self, *args, **kwargs)
        return f'<p>{result}</p>'
    return wrapper


class HTMLReport:

    @paragraph
    def first(self, *args, **kwargs):
        return 'First'

    @paragraph
    def second(self, *args, **kwargs):
        return 'Second'


x = HTMLReport()

x.first()
# '<p>First</p>'

x.second()
# '<p>Second</p>'

5.4.5. Assignments

Code 5.33. Solution
"""
* Assignment: Decorator Method Syntax
* Complexity: easy
* Lines of code: 5 lines
* Time: 5 min

English:
    1. Create method decorator `mydecorator`
    2. Decorator should have `wrapper` with `self`, `*args` and `**kwargs` parameters
    3. Wrapper should call original method with it's original parameters,
       and return its value
    4. Decorator should return `wrapper` method
    5. Run doctests - all must succeed

Polish:
    1. Stwórz dekorator metod `mydecorator`
    2. Dekorator powinien mieć `wrapper` z parametrami `*args` i `**kwargs`
    3. Wrapper powinien wywoływać oryginalną funkcję z jej oryginalnymi
       parametrami i zwracać jej wartość
    4. Decorator powinien zwracać metodę `wrapper`
    5. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0
    >>> from inspect import isfunction

    >>> assert isfunction(mydecorator)
    >>> assert isfunction(mydecorator(lambda: None))

    >>> class MyClass:
    ...     @mydecorator
    ...     def echo(text):
    ...         return text

    >>> my = MyClass()
    >>> my.echo('hello')
    'hello'
"""


Code 5.34. Solution
"""
* Assignment: Decorator Method Alive
* Complexity: easy
* Lines of code: 5 lines
* Time: 13 min

English:
    1. Create `if_alive` method decorator
    2. Decorator will allow running `make_damage` method
       only if `current_health` is greater than 0
    3. Run doctests - all must succeed

Polish:
    1. Stwórz dekorator metody `if_alive`
    2. Dekorator pozwoli na wykonanie metody `make_damage`,
       tylko gdy `current_health` jest większe niż 0
    3. Uruchom doctesty - wszystkie muszą się powieść

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

    >>> class Hero:
    ...    def __init__(self, name):
    ...        self.name = name
    ...        self.current_health = 100
    ...
    ...    @if_alive
    ...    def make_damage(self):
    ...        return 10

    >>> hero = Hero('Jan Twardowski')
    >>> hero.make_damage()
    10
    >>> hero.current_health = -10
    >>> hero.make_damage()
    Traceback (most recent call last):
    RuntimeError: Hero is dead and cannot make damage
"""

def if_alive(method):
    def wrapper(hero, *args, **kwargs):
        return method(hero, *args, **kwargs)
    return wrapper