[ python ] 反射

在python中，反射包含如下幾個函數

def hasattr(*args, **kwargs): # real signature unknown
    """
    Return whether the object has an attribute with the given name.
    
    This is done by calling getattr(obj, name) and catching AttributeError.
    """
    pass

hasattr

def hasattr(*args, **kwargs): # real signature unknown
    """
    Return whether the object has an attribute with the given name.
    
    This is done by calling getattr(obj, name) and catching AttributeError.
    """
    pass

getattr

def setattr(x, y, v): # real signature unknown; restored from __doc__
    """
    Sets the named attribute on the given object to the specified value.
    
    setattr(x, 'y', v) is equivalent to ``x.y = v''
    """
    pass

setattr

def delattr(x, y): # real signature unknown; restored from __doc__
    """
    Deletes the named attribute from the given object.
    
    delattr(x, 'y') is equivalent to ``del x.y''
    """
    pass

delattr

 

 

hasattr  判斷是否含有某屬性

In [1]: class Person(object):
   ...:     def __init__(self, name, age):
   ...:         self.name = name
   ...:         self.age = age

In [2]: p = Person('hkey', 20)	# 實例化一個對象

In [3]: hasattr(p, 'name')	# 經過 hasattr 判斷 p 是否包含 name 屬性，這裏name參數必須是字符串類型
Out[3]: True

In [4]: hasattr(p, 'sex')	# 若是 p 中沒有 sex 屬性，則返回 False
Out[4]: False

 

getattr 獲取對象的屬性

 

In [5]: getattr(p, 'name')	# p.name  =  getattr(p, 'name')  
Out[5]: 'hkey'

In [6]: getattr(p, 'age')	# p.age = getattr(p, 'age')  
Out[6]: 20

In [7]: getattr(p, 'sex')	# p.sex = getattr(p, 'sex')	p 對象中不包含 sex 屬性則報錯.
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-7-36d4b75cf6b8> in <module>
----> 1 getattr(p, 'sex')

AttributeError: 'Person' object has no attribute 'sex'

 

getattr 進階用法：

當使用 getattr 獲取不存在屬性的時候，咱們不但願直接拋出異常。

getattr(對象, 'str1', '對象屬性不存在時的默認信息')

 

In [16]: getattr(p, 'sex', '不知道')	# p 對象中是沒有 sex 屬性，直接返回默認值
Out[16]: '不知道'
In [17]: getattr(p, 'sex', 'None')	# 通常來講直接設置爲 None
Out[17]: 'None'

 

setattr 設置對象的屬性

 

In [8]: setattr(p, 'sex', 'male')	# 經過 setattr 爲 p 對象設置爲 sex 屬性，屬性值爲 male

In [9]: p.sex	# 驗證 setattr 是否設置成功
Out[9]: 'male'

 

setattr 進階用法：

setattr 不只能夠爲對象添加屬性，還能夠爲對象綁定方法

In [23]: setattr(p, 'say_hello', lambda self: 'hello, ' + self.name) 	# setattr(對象名, '方法名', '匿名函數') self 表明對象自己

In [24]: p.say_hello(p)	# 調用方式必須將對象傳入 對象.方法名(對象)
Out[24]: 'hello, hkey'

 

delattr 刪除對象的屬性

 

In [10]: delattr(p, 'sex')	# 經過 delattr 刪除 p 對象的 sex 屬性

In [11]: p.sex	# 驗證 sex 屬性是否刪除成功
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-13-0e282ee1b157> in <module>
----> 1 p.sex

AttributeError: 'Person' object has no attribute 'sex'

 

反射在實際代碼中使用的很是廣泛，尤爲是在和用戶交互的時候。

實例：自行編寫一個shell，當要執行一個命令的時候使用反射來執行

import os


class Manage_cmd(object):
    def run(self):
        while True:
            cmd = input('>>>').strip()
            if not cmd: continue
            if hasattr(self, cmd):	# 經過 hasattr 來判斷對象self是否含有cmd屬性
                getattr(self, cmd)()	
            else:
                print('-bash: %s: command not found' % cmd)	

    def ls(self):
        print(os.listdir('.'))

    def exit(self):
        exit(1)


cmd = Manage_cmd()
cmd.run()

 

執行結果：