[轉]Python中的getattr()函數詳解

轉自：http://www.cnblogs.com/chenjianhong/p/4144453.html

最近看Dive into python第四章自省中提到getattr()函數,做爲一個內建函數平時本身沒怎麼用過因此也不太理解這個函數的一些用法

看了下函數自己的doc

getattr(object, name[, default]) -> value

Get a named attribute from an object; getattr(x, 'y') is equivalent to x.y.   When a default argument is given, it is returned when the attribute doesn't exist; without it, an exception is raised in that case.

解釋的很抽象 告訴我這個函數的做用至關因而

object.name

試了一下getattr(object,name)確實和object.name是同樣的功能.只不過這裏能夠把name做爲一個變量去處理

書上的例子很好的說明了這個函數的功用

使用getattr能夠輕鬆實現工廠模式。

例：一個模塊支持html、text、xml等格式的打印，根據傳入的formate參數的不一樣，調用不一樣的函數實現幾種格式的輸出

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1 2 3 4

import statsout  def output(data, format = "text" ):                                 output_function = getattr (statsout, "output_%s" % format )       return output_function(data)

這個例子中能夠根據傳入output函數的format參數的不一樣 去調用statsout模塊不一樣的方法(用格式化字符串實現output_%s)

返回的是這個方法的對象 就能夠直接使用了 若是要添加新的格式 只須要在模塊中寫入新的方法函數 在調用output函數時使用新的參數就能夠使用不一樣的格式輸出

確實很方便

 

爲了加深對getattr函數的理解 轉載一篇英文的說明

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)