python getattr函数资料翻译

该函数用于得到一个对象的属性，使用字符串而非标示符表示属性，简而言之，下面两条语句等效

value = obj.attributevalue = getattr(obj, "attribute")

如果对象存在该属性，则属性值被返回，如果不存在该属性，则返回AttributeError异常

该函数可以用于函数，模块，对象，可以用于任意支持.操作的上述类型

下面给出几个例子

path = getattr(sys, "path")doc = getattr(len, "__doc__")

result = obj.method(args)func = getattr(obj, "method")result = func(args)

理论上也可以放在一行

result = getattr(obj, "method")(args)

但是这样做容易导致异常不易检查

因此建议使用荣块捕获异常

try:    func = getattr(obj, "method")except AttributeError:    ... deal with missing method ...else:    result = func(args)

下面的2个例子则进行了函数存在检查和参数检查

func = getattr(obj, "method", None)if func:    func(args)func = getattr(obj, "method", None)if callable(func):    func(args)

总结：个人觉得该函数对于实现工厂类有较大帮助，没有遇到相关代码之前，可能不太好理解该函数作用

下面附上原文：

Python’s getattr function is used to fetch an attribute from an object, using a string object instead of an identifier to identify the attribute. In other words, the following two statements are equivalent:


value = obj.attribute
value = getattr(obj, "attribute")
If the attribute exists, the corresponding value is returned. If the attribute does not exist, you get an AttributeError exception instead.


The getattr function can be used on any object that supports dotted notation (by implementing the __getattr__ method). This includes class objects, modules, and even function objects.


path = getattr(sys, "path")
doc = getattr(len, "__doc__")
The getattr function uses the same lookup rules as ordinary attribute access, and you can use it both with ordinary attributes and methods:


result = obj.method(args)


func = getattr(obj, "method")
result = func(args)
or, in one line:


result = getattr(obj, "method")(args)
Calling both getattr and the method on the same line can make it hard to handle exceptions properly. To avoid confusing AttributeError exceptions raised by getattr with similar exceptions raised inside the method, you can use the following pattern:


try:
    func = getattr(obj, "method")
except AttributeError:
    ... deal with missing method ...
else:
    result = func(args)
The function takes an optional default value, which is used if the attribute doesn’t exist. The following example only calls the method if it exists:


func = getattr(obj, "method", None)
if func:
    func(args)
Here’s a variation, which checks that the attribute is indeed a callable object before calling it.


func = getattr(obj, "method", None)
if callable(func):
    func(args)