python装饰器、描述符模拟源码实现

摘要：如果对象定义和，则它被认为是数据描述符。属性查找优先级为类属性数据描述符实例属性非数据描述符默认为内部是用实现的，这里用模拟实现功能代码参考官方文档原理代码实现原理代码实现原理参考资料，知乎如何理解装饰器，

本人python理论知识远达不到传授级别，写文章主要目的是自我总结，并不能照顾所有人，请见谅，文章结尾贴有相关链接可以作为补充

全文分为三个部分装饰器理论知识、装饰器应用、装饰器延申

装饰理基础：无参装饰器、有参装饰器、functiontools、装饰器链

装饰器进阶：property、staticmethod、classmethod源码分析（python代码实现）

"""
假定有一个需求是：打印程序函数运行顺序
此案例打印的结果为：
    foo1 function is starting
    foo2 function is starting
"""
from functools import wraps


def NoParamDec(func):
    #函数在被装饰器装时后，其函数属性也会改变，wraps作用就是保证被装饰函数属性不变
    @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()

"""
假定有一个需求是：检查函数参数的类型，只允许匹配正确的函数通过程序
此案例打印结果为：
("a", "b", "c")
-----------------------分割线------------------------
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标准模块类，可以用来检查函数参数类型，只允许特定类型通过
        sig = signature(func)
        #将函数形式参数和规定类型进行绑定
        check_bind_args = sig.bind_partial(*deco_args, **deco_kwargs).arguments
        
        @wraps(func)
        def wrapper(*args, **kwargs):
            #将实际参数值和形式参数进行绑定
            wrapper_bind_args = sig.bind(*args, **kwargs).arguments.items()
            for name, obj in wrapper_bind_args:
                #遍历判断是否实际参数值是规定参数的实例
                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("分割线"))
    print(inspect_type("a", 2, ["b", "d"]))

"""
假定有一个需求是：
输入类似代码：
@makebold
@makeitalic
def say():
   return "Hello"

输出：
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=""):
    # 等价于 geet=html_deco("b")(html_deco("i)(geet))
    return "Hello" + (" " + whom) if whom else ""


if __name__ == "__main__":
    print(greet("world"))  # -> Hello world

通常，描述符是具有“绑定行为”的对象属性，其属性访问已经被描述符协议中的方法覆盖。这些方法是__get__()、__set__()和__delete__()。如果一个对象定义这些方法中的任何一个，它被称为一个描述符。如果对象定义__get__()和__set__()，则它被认为是数据描述符。仅定义__get__()的描述器称为非数据描述符（它们通常用于方法，但是其他用途也是可能的）。

属性查找优先级为：

类属性

数据描述符

实例属性

非数据描述符

默认为__getattr__()

class Property(object):
    """
    内部property是用c实现的，这里用python模拟实现property功能
    代码参考官方doc文档
    """

    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 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代码实现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"))

@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代码实现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"))

1, statckoverflow: how to make a chain of decorators

2, python doc:how to descriptor

3，知乎：如何理解装饰器

4, difference-between-staticmethod-and-classmethod-in-python

5，meaning-of-classmethod-and-staticmethod-for-beginner