python中getattr函數使用方法 getattr實現原廠模式_python

看了下函數本身的doc

複製代碼 代碼如下:

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 告訴我這個函數的作用相當於是

object.name

試了一下getattr(object,name)確實和object.name是一樣的功能.只不過這裡可以把name作為一個變數去處理書上的例子很好的說明了這個函數的功用，使用getattr可以輕鬆實現原廠模式。

例：一個模組支援html、text、xml等格式的列印，根據傳入的formate參數的不同，調用不同的函數實現幾種格式的輸出

複製代碼 代碼如下:

import statsout
def output(data, format="text"):                          
    output_function = getattr(statsout, "output_%s" %format)
    return output_function(data)
[code]
這個例子中可以根據傳入output函數的format參數的不同 去調用statsout模組不同的方法(用格式化字串實現output_%s)

返回的是這個方法的對象 就可以直接使用了 如果要添加新的格式 只需要在模組中寫入新的方法函數 在調用output函數時使用新的參數就可以使用不同的格式輸出

確實很方便

為了加深對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:

[code]
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)