hasattr、getattr、setattr、delattr、反射

In doing program development, we often encounter such a demand: the need to perform an object in a certain way, or need to call the object a variable, but for various reasons we can not determine the method or variable exists, it is we need to use a special method or mechanism to access and manipulate this unknown method or variable, in which it is called reflex mechanism

Reflection: a string of ways to access the object's properties, methods, call the object , Python are objects in all, you can use reflection .

1. hasattr (obj, name): Check whether the object obj contains property or method called name.
2. getattr (object, name [, default]): Gets object in the Object attribute called name attribute value.
3. setattr (obj, name, value, /): The name attribute of the object obj set value.

Hsattr

Determining whether the object has a method name or attribute name. There are returns True, no returns False

class Person(object):
    def __init__(self,name):
        self.name = name
    def talk(self):
        print("%s正在交谈"%self.name)

p = Person("santa")
print(hasattr(p,"talk"))    # True。因为存在talk方法
print(hasattr(p,"name"))    # True。因为存在name变量
print(hasattr(p,"abc"))     # False。因为不存在abc方法或变量

getattr

Acquiring an object property or method, if there is print. hasattrAnd getattrsupporting the use of

class Person(object):
    def __init__(self,name):
        self.name = name
    def talk(self):
        print("%s正在交谈"%self.name)
p = Person("santa")

n = getattr(p,"name")   # 获取name变量的内存地址
print(n)                # 此时打印的是:laowang

f = getattr(p,"talk")   # 获取talk方法的内存地址
f()                     # 调用talk方法

# 我们发现getattr有三个参数，那么第三个参数是做什么用的呢?
s = getattr(p,"abc","not find")
print(s)
#若属性不存在，则返回默认值(第三个参数)

Print Results:

Santa
Santa are talking
not find

setattr

After assigning an object to a property, if the property does not exist, create assignments

def abc(self):
    print("%s正在交谈"%self.name)

class Person(object):
    def __init__(self,name):
        self.name = name

p = Person("santa")
setattr(p,"talk",abc)   # 将abc函数添加到对象中p中，并命名为talk
p.talk(p)               # 调用talk方法，因为这是额外添加的方法，需手动传入对象
#结果
#santa正在交谈

setattr(p,"age",30)     # 添加一个变量age,赋值为30
print(p.age)            # 打印结果:30

delattr

Delete the object of a specified property. Note: The method can not be used to delete

class Person(object):
    def __init__(self,name):
        self.name = name
    def talk(self):
        print("%s正在交谈"%self.name)

p = Person("laowang")

delattr(p,"name")         # 删除name变量
print(hasattr(p,'name'))  #False

Reflected Application

# 反射应用:
根据用户输入进行动态调用
class FileControl:

    def run(self):
        while True:
            # 让用户输入上传或下载功能的命令:
            user_input = input('请输入 上传(upload) 或 下载(download) 功能:').strip()

            # 通过用户输入的字符串判断方法是否存在，然后调用相应的方法
            if hasattr(self, user_input):
                func = getattr(self, user_input)
                func()
            else:
                print('输入有误!')

    def upload(self):
        print('文件正在上传...')

    def download(self):
        print('文件正在下载...')

file_control_obj = FileControl()
file_control_obj.run()

This is the reflection of python, in fact, its core essence is to use a string to an object (module) in operation (Find / get / delete / add) members, based on a string of driving events!

__import __ (understand)

If you want to import functions for different directories, you can be: introducing dynamic module

def run():
    inp = input("请输入您想访问页面的url：  ").strip()
    modles, func = inp.split("/")
    obj = __import__("lib." + modules, fromlist=True)  # 注意fromlist参数  modles是url
    if hasattr(obj, func):
        func = getattr(obj, func)
        func()
    else:
        print("404")
 
if __name__ == '__main__':
    run()

Thinking:

Some may ask python not have two built-in functions exec and eval it? They also can perform string. such as:

exec("print('haha')")  
结果：
haha

So they will not do it directly? Have to try so hard to use getattr, __ import__ doing?

In fact, in the example above, around the central theme is how to use different string-driven events, such as import module, function calls, and so on, these are the reflection of python, is a manifestation of programming, design patterns, the combination of high cohesion, loosely coupled programming ideas, not simply be replaced with a string performed. Of course, exec and eval has its stage, are often used in the web framework.