摘要：項目地址本篇主要關于三個常用內置方法，，在語言的設計中，通常的語法操作最終都會轉化為方法調用，例如相當于中的描述符就是將對象屬性的獲取賦值以及刪除等行為轉換為方法調用的協議例如我們要獲取一個對象的屬性，可以通過的方式取得而通過的

項目地址：https://git.io/pytips

本篇主要關于三個常用內置方法：property()，staticmethod()，classmethod()

在 Python 語言的設計中，通常的語法操作最終都會轉化為方法調用，例如：

a = 1
b = 2
print("a + b = {}".format(a+b))

# 相當于
print("a.__add__(b) = {}".format(a.__add__(b)))

a + b = 3
a.__add__(b) = 3

Python 中的描述符（Descriptor）就是將對象屬性的獲取、賦值以及刪除等行為轉換為方法調用的協議：

descr.__get__(self, obj, type=None) --> value

descr.__set__(self, obj, value) --> None

descr.__delete__(self, obj) --> None

例如我們要獲取一個對象的屬性，可以通過o.x的方式取得：

class Int:
    ctype = "Class::Int"
    def __init__(self, val):
        self._val = val
        
a = Int(1)
print(a.ctype)

Class::Int

而通過.的方式尋找屬性的值實際上調用了object.__getattribute__(self, name)方法：

class Int:
    ctype = "Class::Int"
    def __init__(self, val):
        self._val = val
    def __getattribute__(self, name):
        print("? doesn"t want to give `{}" to you!".format(name))
        return "?"
a = Int(2)
print(a.ctype)

? doesn"t want to give `ctype" to you!
?

而這里的__getattribute__(self, name)方法實際上就是將.的屬性獲取方法轉化為描述符協議定義的descr.__get__(self, key)：

class Str:
    def __init__(self, val):
        self._val = val
    def __get__(self, name, ctype=None):
        print("You can __get__ anything from here!")
        return self._val
class Int:
    ctype = Str("Class::Int")
    def __init__(self, val):
        self._val = val
    def __getattribute__(self, name):
        return type(self).__dict__[name].__get__(None, type(self))
a = Int(2)
print(a.ctype)

You can __get__ anything from here!
Class::Int

這里的 a.ctype = (Int.__dict__["ctype"]).__get__(None, Int)，即通過描述符的方式獲取了 ctype 屬性的值。同樣的道理，你也可以通過 descr.__set__(self, obj, val) 設置屬性的值：

class Str:
    def __init__(self, val):
        self._val = val
    def __get__(self, name, ctype=None):
        print("You can __get__ anything from here!")
        return self._val
    def __set__(self, name, val):
        print("You can __set__ anything to me!")
        self._val = val
class Int:
    ctype = Str("Class::Int")
    def __init__(self, val):
        self._val = val
a = Int(3)
print(a.ctype)
a.ctype = "Class::Float"
print(a.ctype)

You can __get__ anything from here!
Class::Int
You can __set__ anything to me!
You can __get__ anything from here!
Class::Float

將這些取值、賦值的操作轉換為方法調用讓我們有辦法在做這些操作的過程中插入一些小動作，這么好用的東西自然是已加入豪華內置函數陣容，正是我們常見的

property()

classmethod()

staticmethod()

property

property(fget=None, fset=None, fdel=None, doc=None) 方法簡化了上面的操作：

class Int:
    def __init__(self, val):
        self._val = val
        self._ctype = None
        
    def get_ctype(self):
        print("INFO: You can get `ctype`")
        return self._ctype
    def set_ctype(self, val):
        print("INFO: You"re setting `ctype` =", val)
        self._ctype=val
    ctype = property(fget=get_ctype, fset=set_ctype, doc="Property `ctype`")
    
a = Int(4)
print(a.ctype)
a.ctype = "Class::Int"
print(a.ctype)

INFO: You can get `ctype`
None
INFO: You"re setting `ctype` = Class::Int
INFO: You can get `ctype`
Class::Int

顯然，更方便一些的用法是將 property 當做修飾器：

class Int:
    _ctype = None
    def __init__(self, val):
        self._val = val
    @property
    def ctype(self):
        print("INFO: You can get `ctype` from me!")
        return self._ctype
    @ctype.setter
    def ctype(self, val):
        print("INFO: You"re setting `ctype` =", val)
        self._ctype = val
a = Int(5)
print(a.ctype)
a.ctype = "Class::Int"
print(a.ctype)

INFO: You can get `ctype` from me!
None
INFO: You"re setting `ctype` = Class::Int
INFO: You can get `ctype` from me!
Class::Int

staticmethod & classmethod

顧名思義，property 是關于屬性的全部操作，如果是要獲取類中的方法，則需要用到 staticmethod 和 classmethod。顧名思義，staticmethod 將方法變成靜態方法，即類和實例都可以訪問，如果不用 staticmethod 我們可以用下面這種別扭的方法實現：

class Int:
    def __init__(self, val):
        self._val = val
    def _get_ctype(self=None):
        print("INFO: You can get `ctype` from here!")
        return "Class::Int"
    
    @staticmethod
    def get_ctype():
        print("INFO: You can get `ctype` from here!")
        return "Class::StaticInt"       
    
a = Int(6)
print(a._get_ctype())
print(Int._get_ctype())

print(a.get_ctype())
print(Int.get_ctype())

INFO: You can get `ctype` from here!
Class::Int
INFO: You can get `ctype` from here!
Class::Int
INFO: You can get `ctype` from here!
Class::StaticInt
INFO: You can get `ctype` from here!
Class::StaticInt

可以看到，靜態方法與類和實例無關，也就不再（不能）需要 self 關鍵詞；與之相反，當我們需要在方法中保留類（而非實例）的引用時，則需要用 classmethod：

class Int:
    _ctype = ""
    def __init__(self, val):
        self._val = val
        
    @classmethod
    def set_ctype(klass, t):
        klass._ctype = t
        return "{}.ctype = {}".format(klass.__name__, t)
a = Int(7)
print(a.set_ctype("Class::Int"))
print(Int.set_ctype("Class::Float"))
b = Int(8)
print(b._ctype)

Int.ctype = Class::Int
Int.ctype = Class::Float
Class::Float

Python 的描述符給出一種通過方法調用來實現屬性（方法）獲取、賦值等操作的規則，通過這一規則可以方便我們深入程序內部并實施操控，因此 property/staticmethod/classmethod 在 Python 是通過底層（如 CPython 中的 C）實現的，如果想要進一步深入了解其實現原理，可以訪問參考鏈接的教程，其中包括了這三個內置方法的 Python 實現版本，我也把它們 copy 過來方便查看。

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Descriptor HowTo Guide

class Property(object):
    "Emulate PyProperty_Type() in Objects/descrobject.c"

    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel
        if doc is None and fget is not None:
            doc = fget.__doc__
        self.__doc__ = doc

    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        if self.fget is None:
            raise AttributeError("unreadable attribute")
        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 StaticMethod(object):
    "Emulate PyStaticMethod_Type() in Objects/funcobject.c"

    def __init__(self, f):
        self.f = f
        
    def __get__(self, obj, objtype=None):
        return self.f

class ClassMethod(object):
    "Emulate PyClassMethod_Type() in Objects/funcobject.c"
    
    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