摘要：項目地址本篇主要關(guān)于三個常用內(nèi)置方法，，在語言的設(shè)計中，通常的語法操作最終都會轉(zhuǎn)化為方法調(diào)用，例如相當(dāng)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(zhuǎn)換為方法調(diào)用的協(xié)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(guān)于三個常用內(nèi)置方法：property()，staticmethod()，classmethod()

在 Python 語言的設(shè)計中，通常的語法操作最終都會轉(zhuǎn)化為方法調(diào)用，例如：

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

# 相當(dāng)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(zhuǎn)換為方法調(diào)用的協(xié)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(diào)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(zhuǎn)化為描述符協(xié)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(shè)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(zhuǎn)換為方法調(diào)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(nèi)置函數(shù)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(dāng)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(guān)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(tài)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(xiàn)：

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(tài)方法與類和實例無關(guān)，也就不再（不能）需要 self 關(guān)鍵詞；與之相反，當(dāng)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(diào)用來實現(xiàn)屬性（方法）獲取、賦值等操作的規(guī)則，通過這一規(guī)則可以方便我們深入程序內(nèi)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(xiàn)的，如果想要進一步深入了解其實現(xiàn)原理，可以訪問參考鏈接的教程，其中包括了這三個內(nèi)置方法的 Python 實現(xiàn)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