python装饰器setter实现原理_python装饰器、描述符模拟源码实现
概要
本人python理論知識遠達不到傳授級別，寫文章主要目的是自我總結，并不能照顧所有人，請見諒，文章結尾貼有相關鏈接可以作為補充
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：打印程序函數運行順序
此案例打印的結果為：
foo1 function is starting
foo2 function is starting
'''
from functools import wraps
def NoParamDec(func):
#函數在被裝飾器裝時后，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)
@NoParamDec
def foo1():
foo2()
@NoParamDec
def foo2():
pass
if __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序
此案例打印結果為：
('a', 'b', 'c')
-----------------------分割線------------------------
ERROS!!!!b must be
ERROS!!!!c must be
('a', 2, ['b', 'd'])
'''
from functools import wraps
from inspect import signature
def typeAssert(*args, **kwargs):
deco_args = args
deco_kwargs = kwargs
def factor(func):
#python標準模塊類，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:
#遍歷判斷是否實際參數值是規定參數的實例
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
@makeitalic
def say():
return "Hello"
輸出：
Hello
'''
from functools import wraps
def 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=''):
# 等價于 geet=html_deco('b')(html_deco('i)(geet))
return 'Hello' + (' ' + whom) if whom else ''
if __name__ == "__main__":
print(greet('world')) # -> Hello world
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__()，則它被認為是數據描述符。僅定義__get__()的描述器稱為非數據描述符（它們通常用于方法，但是其他用途也是可能的）。
屬性查找優先級為：
類屬性
數據描述符
實例屬性
非數據描述符
默認為__getattr__()
class Property(object):
'''
內部property是用c實現的，這里用python模擬實現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 = val
if __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.f
class E(object):
#StaticMethod=StaticMethod(f)
@StaticMethod
def f( x):
return x
if __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'))
參考資料
總結
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