python decorators

Transcript

1. Decorators Hulpa Yura 2016, NULP Python course
2. Definition according to GOF Pa erns 1994 Intent A ach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. Also Known As Wrapper Motivation Sometimes we want to add responsibilities to individual objects, not to an entire class. A graphical user interface toolkit, for example, should let you add properties like borders or behaviors like scrolling to any user interface component.
3. PEP 0318 Decorators for Functions and Methods To manipulate data in human readable form def foo(self): perform method operation foo = classmethod(foo)
4. Why to use Add some functionality to existing function, method
5. Simple case # basic decoration with functions def decorate_with_sword(func): def func_wrapper(name): return '{} with a SWORD'.format(func(name)) return func_wrapper def orc_attack(name): """function that hit's you! """ return "{} , I am, hitting you".format(name) decorated_with_sword = decorate_with_sword( orc_attack) print decorated_with_sword('Artes') print decorated_with_sword.__doc__ print decorated_with_sword.__name__ >>Artes , I am, hitting you with a SWORD >>None >>func_wrapper
6. Let’s add some sugar # decorate with syntax sugar def decorate_with_sword(func): def func_wrapper(name): return '{} with a SWORD'.format(func(name)) return func_wrapper @decorate_with_sword def orc_attack(name): """function that hit's you! """ return "{}, I am, hitting you".format(name) print orc_attack('Lich King') print orc_attack.__doc__ print orc_attack.__name__ >>Lich King, I am, hitting you with a SWORD >>None >>func_wrapper
7. Small issue when wrapping into decorator we are loosing wrapped function signature Do we care? For debugging for example it would be good to save it
8. Funtrools wrap decorator from functools import wraps def decorate_with_sword(func): @wraps(func) def func_wrapper(name): # some code
9. Wrap it all # adding wraps decorator from functools from functools import wraps def decorate_with_sword(func): @wraps(func) def func_wrapper(name): return '{} with a SWORD'.format(func(name)) return func_wrapper @decorate_with_sword def orc_attack(name): """function that hit's you! """ return "{} , I am, hitting you".format(name) print orc_attack('Rengar') print orc_attack.__doc__ print orc_attack.__name__ >>Rengar , I am, hitting you with a SWORD >>function that hits you >>orc_attack
10. Class and Static method decorators
11. Syntax class ClassName(object): @classmethod def string_compare(cls): # code here print 'I am classmethod' @staticmethod def string_compare(): # code here print 'I am staticmethod'
12. test it ClassName.string_compare_class => <bound method type.string_compare_class of <class 'ClassName'>> ClassName.string_compare_static => <function string_compare_static at 0x168b5c0>
13. Cls argument class Orc(object): ugly = False @classmethod def get_instance(cls): # code here print cls() class UglyOrc(Orc): ugly = True Orc.get_instance() <Orc object at 0x1304da0> UglyOrc.get_instance() <UglyOrc object at 0x13057e0>
14. Our example code class Date(object): day = 0 month = 0 year = 0 def __init__(self, day=0, month=0, year=0): self.day = day self.month = month self.year = year
15. We want to parse and create instances of date day, month, year = map(int, string_date.split('-')) date1 = Date(day, month, year)
16. Usa case of class method class Date(object): day = 0 month = 0 year = 0 def __init__(self, day=0, month=0, year=0): self.day = day self.month = month self.year = year @classmethod def from_string(cls, date_as_string): day, month, year = map(int, date_as_string. split('-')) date1 = cls(day, month, year) return date1 date2 = Date.from_string('11-09-2012')
17. Static method class Date(object): day = 0 month = 0 year = 0 def __init__(self, day=0, month=0, year=0): self.day = day self.month = month self.year = year @staticmethod def is_date_valid(date_as_string): day, month, year = map(int, date_as_string. split('-')) return day <= 31 and month <= 12 and year < = 3999 # usage: is_date = Date.is_date_valid('11-09-2012')
18. Real life example: Bitcoin class BitCoinTransaction: def __init__(self, date, tid, price, amount): self.date = date self.tid = tid self.price = price self.amount = amount @classmethod def dictionary_to_instance(cls, start_dict): """ Return a BitCoinTransaction class instance """ date = datetime.datetime.fromtimestamp(int( start_dict["date"])) tid = int(start_dict["tid"]) price = float(start_dict["price"]) amount = float(start_dict["amount"]) return cls(date, tid, price, amount)