python實現裝飾器、描述符，python裝飾描述符

python實現裝飾器、描述符，python裝飾描述符

概要

本人python理論知識遠達不到傳授層級，寫文章主要目的是自我總結，並不能照顧所有人，請見諒，文章結尾貼有相關連結可以作為補充

全文分為三個部分裝飾器理論知識、裝飾器應用、裝飾器延申

• 裝飾理基礎：無參裝飾器、有參裝飾器、functiontools、裝飾器鏈
• 裝飾器進階：property、staticmethod、classmethod源碼分析（python代碼實現）

裝飾器基礎

無參裝飾器

'''假定有一個需求是：列印程式函數運行順序此案例列印的結果為：  foo1 function is starting  foo2 function is starting'''from functools import wrapsdef NoParamDec(func):  #函數在被裝飾器裝時後，其函數屬性也會改變，wraps作用就是保證被裝飾函數屬性不變  @wraps(func)  def warpper(*args, **kwargs):    print('{} function is starting'.format(func.__name__))    return func(*args, **kwargs)    return warpper#python黑魔法省略了NoParamDec=NoParamDec(foo1)@NoParamDecdef foo1():  foo2()@NoParamDecdef foo2():  passif __name__ == "__main__":  foo1()

有參裝飾器

'''假定有一個需求是：檢查函數參數的類型，只允許匹配正確的函數通過程式此案例列印結果為：('a', 'b', 'c')-----------------------分割線------------------------ERROS!!!!b must be <class 'str'> ERROS!!!!c must be <class 'str'> ('a', 2, ['b', 'd'])  '''from functools import wrapsfrom inspect import signaturedef typeAssert(*args, **kwargs):  deco_args = args  deco_kwargs = kwargs    def factor(func):    #python標準模組類，可以用來檢查函數參數類型，只允許特定類型通過    sig = signature(func)    #將函數形式參數和規定類型進行綁定    check_bind_args = sig.bind_partial(*deco_args, **deco_kwargs).arguments        @wraps(func)    def wrapper(*args, **kwargs):      #將實際參數值和形式參數進行綁定      wrapper_bind_args = sig.bind(*args, **kwargs).arguments.items()      for name, obj in wrapper_bind_args:        #遍曆判斷是否實際參數值是規定參數的執行個體        if not isinstance(obj, check_bind_args[name]):          try:            raise TypeError('ERROS!!!!{arg} must be {obj} '.format(**{'arg': name, 'obj': check_bind_args[name]}))          except Exception as e:            print(e)      return func(*args, **kwargs)        return wrapper    return factor@typeAssert(str, str, str)def inspect_type(a, b, c):  return (a, b, c)if __name__ == "__main__":  print(inspect_type('a', 'b', 'c'))  print('{:-^50}'.format('分割線'))  print(inspect_type('a', 2, ['b', 'd']))

裝飾器鏈

'''假定有一個需求是：輸入類似代碼：@makebold@makeitalicdef say():  return "Hello"輸出：<b><i>Hello</i></b>'''from functools import wrapsdef html_deco(tag):  def decorator(fn):    @wraps(fn)    def wrapped(*args, **kwargs):      return '<{tag}>{fn_result}<{tag}>'.format(**{'tag': tag, 'fn_result': fn(*args, **kwargs)})        return wrapped    return decorator@html_deco('b')@html_deco('i')def greet(whom=''):  # 等價於 geet=html_deco('b')(html_deco('i)(geet))  return 'Hello' + (' ' + whom) if whom else ''if __name__ == "__main__":  print(greet('world')) # -> <b><i>Hello world</i></b>

裝飾器進階

property 原理

通常，描述符是具有“綁定行為”的對象屬性，其屬性訪問已經被描述符協議中的方法覆蓋。這些方法是__get__()、__set__()和__delete__()。如果一個對象定義這些方法中的任何一個，它被稱為一個描述符。如果對象定義__get__()和__set__()，則它被認為是資料描述符。僅定義__get__()的描述器稱為非資料描述符（它們通常用於方法，但是其他用途也是可能的）。

屬性尋找優先順序為：

• 類屬性
• 資料描述符
• 執行個體屬性
• 非資料描述符
• 預設為__getattr__()

class Property(object):  '''  內部property是用c實現的，這裡用python類比實現property功能  代碼參考官方doc文檔  '''  def __init__(self, fget=None, fset=None, fdel=None, doc=None):    self.fget = fget    self.fset = fset    self.fdel = fdel    self.__doc__ = doc  def __get__(self, obj, objtype=None):    if obj is None:      return self    if self.fget is None:      raise (AttributeError, "unreadable attribute")    print('self={},obj={},objtype={}'.format(self,obj,objtype))    return self.fget(obj)  def __set__(self, obj, value):    if self.fset is None:      raise (AttributeError, "can't set attribute")    self.fset(obj, value)  def __delete__(self, obj):    if self.fdel is None:      raise (AttributeError, "can't delete attribute")    self.fdel(obj)  def getter(self, fget):    return type(self)(fget, self.fset, self.fdel, self.__doc__)  def setter(self, fset):    return type(self)(self.fget, fset, self.fdel, self.__doc__)  def deleter(self, fdel):    return type(self)(self.fget, self.fset, fdel, self.__doc__)class Student( object ):  @Property  def score( self ):    return self._score  @score.setter  def score( self, val ):    if not isinstance( val, int ):      raise ValueError( 'score must be an integer!' )    if val > 100 or val < 0:      raise ValueError( 'score must between 0 ~ 100!' )    self._score = valif __name__ == "__main__":  s = Student()  s.score = 60    s.score     

staticmethod 原理

@staticmethod means: when this method is called, we don't pass an instance of the class to it (as we normally do with methods). This means you can put a function inside a class but you can't access the instance of that class (this is useful when your method does not use the instance).

class StaticMethod(object):  "python代碼實現staticmethod原理"    def __init__(self, f):    self.f = f    def __get__(self, obj, objtype=None):    return self.fclass E(object):  #StaticMethod=StaticMethod(f)  @StaticMethod  def f( x):    return xif __name__ == "__main__":  print(E.f('staticMethod Test'))

classmethod

@staticmethod means: when this method is called, we don't pass an instance of the class to it (as we normally do with methods). This means you can put a function inside a class but you can't access the instance of that class (this is useful when your method does not use the instance).

class ClassMethod(object):  "python代碼實現classmethod原理"    def __init__(self, f):    self.f = f    def __get__(self, obj, klass=None):    if klass is None:      klass = type(obj)        def newfunc(*args):      return self.f(klass, *args)        return newfunc  class E(object):  #ClassMethod=ClassMethod(f)  @ClassMethod  def f(cls,x):    return x  if __name__ == "__main__":  print(E().f('classMethod Test'))