python实现装饰器、描述符

yipeiwu_com5年前Python基础

概要

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)

装饰器基础

无参装饰器

'''
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()

有参装饰器

'''
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 <class 'str'> 
ERROS!!!!c must be <class 'str'> 
('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']))

装饰器链

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

输出:
<b><i>Hello</i></b>
'''
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')) # -> <b><i>Hello world</i></b>

装饰器进阶

property 原理

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 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'))

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'))

相关文章

全面理解Python中self的用法

刚开始学习Python的类写法的时候觉得很是麻烦,为什么定义时需要而调用时又不需要,为什么不能内部简化从而减少我们敲击键盘的次数?你看完这篇文章后就会明白所有的疑问。 self代表类的实...

python虚拟环境的安装配置图文教程

python虚拟环境的安装配置图文教程

使用pip安装     pip install virtualenv 因为已经安装过了,所以显示这样 在这里我想在这里推荐大家以后再安装类库时可以用豆瓣源来安装,速...

Python性能优化技巧

Python是一门非常酷的语言,因为很少的Python代码可以在短时间内做很多事情,并且,Python很容易就能支持多任务和多重处理。 py   1、关键代码可以依赖于扩展包...

Python使用os模块和fileinput模块来操作文件目录

os模块 在python编程中,我们会经常使用到文件和目录,这些操作python提供一个os模块,里面有很多操作文件和目录的函数. 下面介绍os的常用功能 1.获取当前路径 >...

python获取文件真实链接的方法,针对于302返回码

使用模块requests 方式代码如下: import requests url_string="https://******" r = requests.head(url_st...