blocks|key|1102675|text|下面是一个简单明了的示例，说明一个类如何根据使用getattr()执行保存方法的操作系统来触发不同版本的保存方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1102676|import+os

class+Log(object):
++++def+__init__(self):
++++++++self.os+=+os.name
++++def+__getattr__(self,+name):
++++++++"""+look+for+a+'save'+attribute,+or+just+
++++++++++return+whatever+attribute+was+specified+"""
++++++++if+name+==+'save':
++++++++++++try:
++++++++++++++++#+try+to+dynamically+return+a+save+
++++++++++++++++#+method+appropriate+for+the+user's+system
++++++++++++++++return+getattr(self,+self.os)
++++++++++++except:
++++++++++++++++#+bail+and+try+to+return+
++++++++++++++++#+a+default+save+method
++++++++++++++++return+getattr(self,+'_save')
++++++++else:
++++++++++++return+getattr(self,+name)

++++#+each+of+these+methods+could+have+save+logic+specific+to+
++++#+the+system+on+which+the+script+is+executed
++++def+posix(self):+print+'saving+on+a+posix+machine'
++++def+nt(self):+print+'saving+on+an+nt+machine'
++++def+os2(self):+print+'saving+on+an+os2+machine'
++++def+ce(self):+print+'saving+on+a+ce+machine'
++++def+java(self):+print+'saving+on+a+java+machine'
++++def+riscos(self):+print+'saving+on+a+riscos+machine'
++++def+_save(self):+print+'saving+on+an+unknown+operating+system'

++++def+which_os(self):+print+os.name|code-block|syntax|javascript|1102677|现在让我们在一个示例中使用这个类：|1102678|logger+=+Log()

#+Now+you+can+do+one+of+two+things:
save_func+=+logger.save
#+and+execute+it,+or+pass+it+along+
#+somewhere+else+as+1st+class:
save_func()

#+or+you+can+just+call+it+directly:
logger.save()

#+other+attributes+will+hit+the+else+
#+statement+and+still+work+as+expected
logger.which_os()|1102679|entityMap^0|O|9|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|P|$]]

Here's a quick and dirty example of how a class could fire different versions of a save method depending on which operating system it's being executed on using <code>getattr()</code>.

<pre><code>import os

class Log(object):
 def __init__(self):
 self.os = os.name
 def __getattr__(self, name):
 """ look for a 'save' attribute, or just 
 return whatever attribute was specified """
 if name == 'save':
 try:
 # try to dynamically return a save 
 # method appropriate for the user's system
 return getattr(self, self.os)
 except:
 # bail and try to return 
 # a default save method
 return getattr(self, '_save')
 else:
 return getattr(self, name)

 # each of these methods could have save logic specific to 
 # the system on which the script is executed
 def posix(self): print 'saving on a posix machine'
 def nt(self): print 'saving on an nt machine'
 def os2(self): print 'saving on an os2 machine'
 def ce(self): print 'saving on a ce machine'
 def java(self): print 'saving on a java machine'
 def riscos(self): print 'saving on a riscos machine'
 def _save(self): print 'saving on an unknown operating system'

 def which_os(self): print os.name
</code></pre>

Now let's use this class in an example:

<pre><code>logger = Log()

# Now you can do one of two things:
save_func = logger.save
# and execute it, or pass it along 
# somewhere else as 1st class:
save_func()

# or you can just call it directly:
logger.save()

# other attributes will hit the else 
# statement and still work as expected
logger.which_os()
</code></pre>

blocks|key|887902|text|getattr的一个非常常见的用例是将数据映射到函数。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|887903|例如，在像Django或Pylons这样的web框架中，getattr使得将web请求的URL映射到将要处理它的函数变得简单。举个例子，如果你仔细看一下Pylons的路由，你会发现(至少在默认情况下)它会砍掉一个请求的URL，比如：|887904|http://www.example.com/customers/list|code-block|syntax|javascript|887905|分成“客户”和“列表”。然后，它搜索一个名为CustomerController的控制器类。假设它找到了这个类，它会创建这个类的一个实例，然后使用getattr来获取它的list方法。然后，它调用该方法，将请求作为参数传递给它。|887906|一旦掌握了这个概念，扩展web应用程序的功能就变得非常容易:只需向控制器类添加新方法，然后在页面中创建使用这些方法的适当URL的链接。所有这些都是由getattr实现的。|887907|entityMap^0|0|7|0|S|7|0|0|M|I|21|7|2D|4|0|22|7|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|Z|8|@$9|10|A|11|B|C]|$9|12|A|13|B|C]|$9|14|A|15|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|16|8|@$9|17|A|18|B|C]]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|R|$]]

A pretty common use case for <code>getattr</code> is mapping data to functions. 

For instance, in a web framework like Django or Pylons, <code>getattr</code> makes it straightforward to map a web request's URL to the function that's going to handle it. If you look under the hood of Pylons's routing, for instance, you'll see that (by default, at least) it chops up a request's URL, like:

<pre><code>http://www.example.com/customers/list
</code></pre>

into "customers" and "list". Then it searches for a controller class named <code>CustomerController</code>. Assuming it finds the class, it creates an instance of the class and then uses <code>getattr</code> to get its <code>list</code> method. It then calls that method, passing it the request as an argument.

Once you grasp this idea, it becomes really easy to extend the functionality of a web application: just add new methods to the controller classes, and then create links in your pages that use the appropriate URLs for those methods. All of this is made possible by <code>getattr</code>.

blocks|key|887951|text|Python中的对象可以有属性--数据属性和使用这些属性的函数(方法)。实际上，每个对象都有内置属性(可以在Python控制台中尝试使用dir(None)、dir(True)、dir(...)、dir(dir)+)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|887952|例如，您有一个对象person，它有几个属性：name、gender等。|887953|您可以访问这些属性(可以是方法或数据对象)，通常可以这样写：person.name、person.gender、person.the_method()等。|887954|但是，如果您在编写程序时不知道属性的名称，该怎么办？例如，属性的名称存储在一个名为attr_name的变量中。|887955|如果|887956|attr_name+=+'gender'|code-block|syntax|javascript|887957|然后，不是写|887958|gender+=+person.gender|887959|你可以写|887960|gender+=+getattr(person,+attr_name)|887961|一些实践：|887962|Python+3.4.0+(default,+Apr+11+2014,+13:05:11)

>>>+class+Person():
...+++++name+=+'Victor'
...+++++def+say(self,+what):
...+++++++++print(self.name,+what)
...+
>>>+getattr(Person,+'name')
'Victor'
>>>+attr_name+=+'name'
>>>+person+=+Person()
>>>+getattr(person,+attr_name)
'Victor'
>>>+getattr(person,+'say')('Hello')
Victor+Hello|887963|如果对象中不存在具有给定名称的属性，则getattr将引发AttributeError：|887964|>>>+getattr(person,+'age')
Traceback+(most+recent+call+last):
++File+"<stdin>",+line+1,+in+<module>
AttributeError:+'Person'+object+has+no+attribute+'age'|887965|但是您可以传递一个默认值作为第三个参数，如果该属性不存在，则会返回该参数：|887966|>>>+getattr(person,+'age',+0)
0|887967|您可以结合使用getattr和dir来遍历所有属性名称并获取它们的值：|887968|>>>+dir(1000)
['__abs__',+'__add__',+...,+'__trunc__',+'__xor__',+'bit_length',+'conjugate',+'denominator',+'from_bytes',+'imag',+'numerator',+'real',+'to_bytes']

>>>+obj+=+1000
>>>+for+attr_name+in+dir(obj):
...+++++attr_value+=+getattr(obj,+attr_name)
...+++++print(attr_name,+attr_value,+callable(attr_value))
...+
__abs__+<method-wrapper+'__abs__'+of+int+object+at+0x7f4e927c2f90>+True
...
bit_length+<built-in+method+bit_length+of+int+object+at+0x7f4e927c2f90>+True
...

>>>+getattr(1000,+'bit_length')()
10|887969|这样做的一个实际用途是查找名称以test和call+them开头的所有方法。|887970|与getattr类似，还有setattr，它允许您设置具有其名称的对象的属性：|887971|>>>+setattr(person,+'name',+'Andrew')
>>>+person.name++#+accessing+instance+attribute
'Andrew'
>>>+Person.name++#+accessing+class+attribute
'Victor'
>>>|887972|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.python.org/3/library/functions.html#getattr|1|https://docs.python.org/3/library/functions.html#dir|2|https://docs.python.org/3/library/unittest.html#organizing-test-code|3|4|https://docs.python.org/3/library/functions.html#setattr^0|1W|9|26|9|2G|8|2P|8|0|9|6|N|4|S|6|0|U|B|16|D|1K|J|0|15|9|0|0|0|0|0|0|0|0|0|J|7|T|E|J|7|0|0|0|0|0|7|7|F|3|F|3|1|0|0|G|4|L|9|2|0|1|7|D|7|1|7|3|D|7|4|0|0^^$0|@$1|2|3|4|5|6|7|21|8|@$9|22|A|23|B|C]|$9|24|A|25|B|C]|$9|26|A|27|B|C]|$9|28|A|29|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|2A|8|@$9|2B|A|2C|B|C]|$9|2D|A|2E|B|C]|$9|2F|A|2G|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|2H|8|@$9|2I|A|2J|B|C]|$9|2K|A|2L|B|C]|$9|2M|A|2N|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|2O|8|@$9|2P|A|2Q|B|C]]|D|@]|E|$]]|$1|L|3|M|5|6|7|2R|8|@]|D|@]|E|$]]|$1|N|3|O|5|P|7|2S|8|@]|D|@]|E|$Q|R]]|$1|S|3|T|5|6|7|2T|8|@]|D|@]|E|$]]|$1|U|3|V|5|P|7|2U|8|@]|D|@]|E|$Q|R]]|$1|W|3|X|5|6|7|2V|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|P|7|2W|8|@]|D|@]|E|$Q|R]]|$1|10|3|11|5|6|7|2X|8|@]|D|@]|E|$]]|$1|12|3|13|5|P|7|2Y|8|@]|D|@]|E|$Q|R]]|$1|14|3|15|5|6|7|2Z|8|@$9|30|A|31|B|C]|$9|32|A|33|B|C]]|D|@$9|34|A|35|1|36]]|E|$]]|$1|16|3|17|5|P|7|37|8|@]|D|@]|E|$Q|R]]|$1|18|3|19|5|6|7|38|8|@]|D|@]|E|$]]|$1|1A|3|1B|5|P|7|39|8|@]|D|@]|E|$Q|R]]|$1|1C|3|1D|5|6|7|3A|8|@$9|3B|A|3C|B|C]|$9|3D|A|3E|B|C]]|D|@$9|3F|A|3G|1|3H]]|E|$]]|$1|1E|3|1F|5|P|7|3I|8|@]|D|@]|E|$Q|R]]|$1|1G|3|1H|5|6|7|3J|8|@$9|3K|A|3L|B|C]]|D|@$9|3M|A|3N|1|3O]]|E|$]]|$1|1I|3|1J|5|6|7|3P|8|@$9|3Q|A|3R|B|C]|$9|3S|A|3T|B|C]]|D|@$9|3U|A|3V|1|3W]|$9|3X|A|3Y|1|3Z]]|E|$]]|$1|1K|3|1L|5|P|7|40|8|@]|D|@]|E|$Q|R]]|$1|1M|3|-4|5|6|7|41|8|@]|D|@]|E|$]]]|1N|$1O|$5|1P|1Q|1R|E|$1S|1T]]|1U|$5|1P|1Q|1R|E|$1S|1V]]|1W|$5|1P|1Q|1R|E|$1S|1X]]|1Y|$5|1P|1Q|1R|E|$1S|1T]]|1Z|$5|1P|1Q|1R|E|$1S|20]]]]

Objects in Python can have attributes -- data attributes and functions to work with those (methods). Actually, every object has built-in attributes (try <code>dir(None)</code>, <code>dir(True)</code>, <code>dir(...)</code>, <code>dir(dir)</code> in Python console).
For example you have an object <code>person</code>, that has several attributes: <code>name</code>, <code>gender</code>, etc.
You access these attributes (be it methods or data objects) usually writing: <code>person.name</code>, <code>person.gender</code>, <code>person.the_method()</code>, etc.
But what if you don't know the attribute's name at the time you write the program? For example you have attribute's name stored in a variable called <code>attr_name</code>.
if
<pre><code>attr_name = 'gender'
</code></pre>
then, instead of writing
<pre><code>gender = person.gender
</code></pre>
you can write
<pre><code>gender = getattr(person, attr_name)
</code></pre>
Some practice:
<pre><code>Python 3.4.0 (default, Apr 11 2014, 13:05:11)

&gt;&gt;&gt; class Person():
... name = 'Victor'
... def say(self, what):
... print(self.name, what)
... 
&gt;&gt;&gt; getattr(Person, 'name')
'Victor'
&gt;&gt;&gt; attr_name = 'name'
&gt;&gt;&gt; person = Person()
&gt;&gt;&gt; getattr(person, attr_name)
'Victor'
&gt;&gt;&gt; getattr(person, 'say')('Hello')
Victor Hello
</code></pre>
<a href="https://docs.python.org/3/library/functions.html#getattr" rel="nofollow noreferrer"><code>getattr</code></a> will raise <code>AttributeError</code> if attribute with the given name does not exist in the object:
<pre><code>&gt;&gt;&gt; getattr(person, 'age')
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
AttributeError: 'Person' object has no attribute 'age'
</code></pre>
But you can pass a default value as the third argument, which will be returned if such attribute does not exist:
<pre><code>&gt;&gt;&gt; getattr(person, 'age', 0)
0
</code></pre>
You can use <code>getattr</code> along with <a href="https://docs.python.org/3/library/functions.html#dir" rel="nofollow noreferrer"><code>dir</code></a> to iterate over all attribute names and get their values:
<pre><code>&gt;&gt;&gt; dir(1000)
['__abs__', '__add__', ..., '__trunc__', '__xor__', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']

&gt;&gt;&gt; obj = 1000
&gt;&gt;&gt; for attr_name in dir(obj):
... attr_value = getattr(obj, attr_name)
... print(attr_name, attr_value, callable(attr_value))
... 
__abs__ &lt;method-wrapper '__abs__' of int object at 0x7f4e927c2f90&gt; True
...
bit_length &lt;built-in method bit_length of int object at 0x7f4e927c2f90&gt; True
...

&gt;&gt;&gt; getattr(1000, 'bit_length')()
10
</code></pre>
A practical use for this would be to find all methods whose names start with <code>test</code> and <a href="https://docs.python.org/3/library/unittest.html#organizing-test-code" rel="nofollow noreferrer">call them</a>.
Similar to <a href="https://docs.python.org/3/library/functions.html#getattr" rel="nofollow noreferrer"><code>getattr</code></a> there is <a href="https://docs.python.org/3/library/functions.html#setattr" rel="nofollow noreferrer"><code>setattr</code></a> which allows you to set an attribute of an object having its name:
<pre><code>&gt;&gt;&gt; setattr(person, 'name', 'Andrew')
&gt;&gt;&gt; person.name # accessing instance attribute
'Andrew'
&gt;&gt;&gt; Person.name # accessing class attribute
'Victor'
&gt;&gt;&gt;
</code></pre>

blocks|key|887978|text|#+getattr

class+hithere():

++++def+french(self):
++++++++print+'bonjour'

++++def+english(self):
++++++++print+'hello'

++++def+german(self):
++++++++print+'hallo'

++++def+czech(self):
++++++++print+'ahoj'

++++def+noidea(self):
++++++++print+'unknown+language'


def+dispatch(language):
++++try:
++++++++getattr(hithere(),language)()
++++except:
++++++++getattr(hithere(),'noidea')()
++++++++#+note,+do+better+error+handling+than+this

dispatch('french')
dispatch('english')
dispatch('german')
dispatch('czech')
dispatch('spanish')|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|887979|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code># getattr

class hithere():

 def french(self):
 print 'bonjour'

 def english(self):
 print 'hello'

 def german(self):
 print 'hallo'

 def czech(self):
 print 'ahoj'

 def noidea(self):
 print 'unknown language'


def dispatch(language):
 try:
 getattr(hithere(),language)()
 except:
 getattr(hithere(),'noidea')()
 # note, do better error handling than this

dispatch('french')
dispatch('english')
dispatch('german')
dispatch('czech')
dispatch('spanish')
</code></pre>

blocks|key|888124|text|有时，在代码中使用次要属性之前，我会使用getattr(..)来懒惰地初始化这些属性。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|888125|比较以下内容：|888126|class+Graph(object):
++++def+__init__(self):
++++++++self.n_calls_to_plot+=+0

++++#...
++++#A+lot+of+code+here
++++#...

++++def+plot(self):
++++++++self.n_calls_to_plot+%2B=+1|code-block|syntax|javascript|888127|要这样做：|888128|class+Graph(object):
++++def+plot(self):
++++++++self.n_calls_to_plot+=+1+%2B+getattr(self,+"n_calls_to_plot",+0)|888129|第二种方法的优点是n_calls_to_plot只出现在代码中使用它的地方。这对可读性很好，因为(1)当你阅读它是如何使用的时候，你可以立即看到它以什么值开始，(2)它不会给__init__(..)方法带来干扰，理想情况下，它应该是关于类的概念状态的，而不是一些实用的计数器，它只由函数的一个方法出于技术原因使用，比如优化，与对象的含义无关。|888130|entityMap^0|K|B|0|0|0|0|0|9|F|2F|C|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|Z|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|11|8|@$9|12|A|13|B|C]|$9|14|A|15|B|C]]|D|@]|E|$]]|$1|S|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|T|$]]

I sometimes use <code>getattr(..)</code> to lazily initialise attributes of secondary importance just before they are used in the code. 

Compare the following:

<pre><code>class Graph(object):
 def __init__(self):
 self.n_calls_to_plot = 0

 #...
 #A lot of code here
 #...

 def plot(self):
 self.n_calls_to_plot += 1
</code></pre>

To this:

<pre><code>class Graph(object):
 def plot(self):
 self.n_calls_to_plot = 1 + getattr(self, "n_calls_to_plot", 0)
</code></pre>

The advantage of the second way is that <code>n_calls_to_plot</code> only appears around the place in the code where it is used. This is good for readability, because (1) you can immediately see what value it starts with when reading how it's used, (2) it doesn't introduce a distraction into the <code>__init__(..)</code> method, which ideally should be about the conceptual state of the class, rather than some utility counter that is only used by one of the function's methods for technical reasons, such as optimisation, and has nothing to do with the meaning of the object.

blocks|key|1102988|text|当我从存储在类中的数据创建XML文件时，如果属性不存在或类型为None，我经常会收到错误。在这种情况下，我的问题不是不知道属性名称是什么，而是数据是否存储在该属性中。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1102989|class+Pet:
++++def+__init__(self):
++++++++self.hair+=+None
++++++++self.color+=+None|code-block|syntax|javascript|1102990|如果我使用hasattr来做这件事，即使属性值是None类型，它也会返回True，这会导致我的ElementTree+set命令失败。|1102991|hasattr(temp,+'hair')
>>True|1102992|如果属性值是None类型，getattr也会返回它，这会导致我的ElementTree+set命令失败。|1102993|c+=+getattr(temp,+'hair')
type(c)
>>+NoneType|1102994|我现在使用以下方法来处理这些情况：|1102995|def+getRealAttr(class_obj,+class_attr,+default+=+''):
++++temp+=+getattr(class_obj,+class_attr,+default)
++++if+temp+is+None:
++++++++temp+=+default
++++elif+type(temp)+!=+str:
++++++++temp+=+str(temp)
++++return+temp|1102996|这就是我使用getattr的时间和方式。|1102997|entityMap^0|V|4|0|0|5|7|O|4|10|4|1N|3|0|0|6|4|D|7|18|3|0|0|0|0|6|7|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|C]|$9|19|A|1A|B|C]|$9|1B|A|1C|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|1D|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1E|8|@$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]|$9|1J|A|1K|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|H|7|1L|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|1M|8|@]|D|@]|E|$]]|$1|U|3|V|5|H|7|1N|8|@]|D|@]|E|$I|J]]|$1|W|3|X|5|6|7|1O|8|@$9|1P|A|1Q|B|C]]|D|@]|E|$]]|$1|Y|3|-4|5|6|7|1R|8|@]|D|@]|E|$]]]|Z|$]]

Quite frequently when I am creating an XML file from data stored in a class I would frequently receive errors if the attribute didn't exist or was of type <code>None</code>. In this case, my issue wasn't not knowing what the attribute name was, as stated in your question, but rather was data ever stored in that attribute.

<pre><code>class Pet:
 def __init__(self):
 self.hair = None
 self.color = None
</code></pre>

If I used <code>hasattr</code> to do this, it would return <code>True</code> even if the attribute value was of type <code>None</code> and this would cause my ElementTree <code>set</code> command to fail.

<pre><code>hasattr(temp, 'hair')
&gt;&gt;True
</code></pre>

If the attribute value was of type <code>None</code>, <code>getattr</code> would also return it which would cause my ElementTree <code>set</code> command to fail. 

<pre><code>c = getattr(temp, 'hair')
type(c)
&gt;&gt; NoneType
</code></pre>

I use the following method to take care of these cases now:

<pre><code>def getRealAttr(class_obj, class_attr, default = ''):
 temp = getattr(class_obj, class_attr, default)
 if temp is None:
 temp = default
 elif type(temp) != str:
 temp = str(temp)
 return temp
</code></pre>

This is when and how I use <code>getattr</code>.

blocks|key|888197|text|在Python中实现switch语句时getattr()的另一个用法。它使用两个反射来获取case类型。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|888198|import+sys

class+SwitchStatement(object):
++++"""+a+class+to+implement+switch+statement+and+a+way+to+show+how+to+use+gettattr+in+Pythion"""

++++def+case_1(self):
++++++++return+"value+for+case_1"

++++def+case_2(self):
++++++++return+"value+for+case_2"

++++def+case_3(self):
++++++++return+"value+for+case_3"

++++def+case_4(self):
++++++++return+"value+for+case_4"

++++def+case_value(self,+case_type=1):
++++++++"""This+is+the+main+dispatchmethod,+that+uses+gettattr"""
++++++++case_method+=+'case_'+%2B+str(case_type)
++++++++#+fetch+the+relevant+method+name
++++++++#+Get+the+method+from+'self'.+Default+to+a+lambda.
++++++++method+=+getattr(self,+case_method,+lambda:+"Invalid+case+type")
++++++++#+Call+the+method+as+we+return+it
++++++++return+method()

def+main(_):
++++switch+=+SwitchStatement()
++++print+swtich.case_value(_)

if+__name__+==+'__main__':
++++main(int(sys.argv[1]))|code-block|syntax|javascript|888199|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Another use of getattr() in implementing a switch statement in Python. It uses both reflection to get the case type.

<pre><code>import sys

class SwitchStatement(object):
 """ a class to implement switch statement and a way to show how to use gettattr in Pythion"""

 def case_1(self):
 return "value for case_1"

 def case_2(self):
 return "value for case_2"

 def case_3(self):
 return "value for case_3"

 def case_4(self):
 return "value for case_4"

 def case_value(self, case_type=1):
 """This is the main dispatchmethod, that uses gettattr"""
 case_method = 'case_' + str(case_type)
 # fetch the relevant method name
 # Get the method from 'self'. Default to a lambda.
 method = getattr(self, case_method, lambda: "Invalid case type")
 # Call the method as we return it
 return method()

def main(_):
 switch = SwitchStatement()
 print swtich.case_value(_)

if __name__ == '__main__':
 main(int(sys.argv[1]))
</code></pre>

blocks|key|888228|text|它也在从https://www.programiz.com/python-programming/methods/built-in/getattr中澄清|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|888229|class+Person:
++++age+=+23
++++name+=+"Adam"

person+=+Person()
print('The+age+is:',+getattr(person,+"age"))
print('The+age+is:',+person.age)|code-block|syntax|javascript|888230|年龄:+23岁|888231|年龄:+23岁|888232|class+Person:
++++age+=+23
++++name+=+"Adam"

person+=+Person()

#+when+default+value+is+provided
print('The+sex+is:',+getattr(person,+'sex',+'Male'))

#+when+no+default+value+is+provided
print('The+sex+is:',+getattr(person,+'sex'))|888233|性别是:男性|888234|AttributeError：“Person”对象没有属性“”性别“”|888235|entityMap|0|LINK|mutability|MUTABLE|url|https://www.programiz.com/python-programming/methods/built-in/getattr^0|4|1X|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@$A|11|B|12|1|13]]|C|$]]|$1|D|3|E|5|F|7|14|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|15|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|16|8|@]|9|@]|C|$]]|$1|M|3|N|5|F|7|17|8|@]|9|@]|C|$G|H]]|$1|O|3|P|5|6|7|18|8|@]|9|@]|C|$]]|$1|Q|3|R|5|6|7|19|8|@]|9|@]|C|$]]|$1|S|3|-4|5|6|7|1A|8|@]|9|@]|C|$]]]|T|$U|$5|V|W|X|C|$Y|Z]]]]

It is also clarifying from <a href="https://www.programiz.com/python-programming/methods/built-in/getattr" rel="nofollow noreferrer">https://www.programiz.com/python-programming/methods/built-in/getattr</a> 

<pre><code>class Person:
 age = 23
 name = "Adam"

person = Person()
print('The age is:', getattr(person, "age"))
print('The age is:', person.age)
</code></pre>

The age is: 23

The age is: 23

<pre><code>class Person:
 age = 23
 name = "Adam"

person = Person()

# when default value is provided
print('The sex is:', getattr(person, 'sex', 'Male'))

# when no default value is provided
print('The sex is:', getattr(person, 'sex'))
</code></pre>

The sex is: Male

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

blocks|key|1103117|text|我在Python2.7.17中尝试过|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1103118|有些人已经回答了。但是，我尝试调用getattr(obj，'+set_value+')，但这并没有执行set_value方法，所以我改成了getattr(obj，'set_value')()+-->这有助于调用相同的方法。|1103119|示例代码：|1103120|示例1：|1103121|++++class+GETATT_VERIFY():
+++++++name+=+"siva"
+++++++def+__init__(self):
+++++++++++print+"Ok"
+++++++def+set_value(self):
+++++++++++self.value+=+"myself"
+++++++++++print+"oooh"
++++obj+=+GETATT_VERIFY()
++++print+getattr(GETATT_VERIFY,+'name')
++++getattr(obj,+'set_value')()
++++print+obj.value|code-block|syntax|javascript|1103122|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Q|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|R|8|@]|9|@]|A|$]]|$1|H|3|I|5|J|7|S|8|@]|9|@]|A|$K|L]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I have tried in Python2.7.17

Some of the fellow folks already answered. However I have tried to call
getattr(obj, 'set_value') and this didn't execute the set_value method, So i changed to getattr(obj, 'set_value')() --> This helps to invoke the same.

Example Code:

Example 1:

<pre><code> class GETATT_VERIFY():
 name = "siva"
 def __init__(self):
 print "Ok"
 def set_value(self):
 self.value = "myself"
 print "oooh"
 obj = GETATT_VERIFY()
 print getattr(GETATT_VERIFY, 'name')
 getattr(obj, 'set_value')()
 print obj.value
</code></pre>

blocks|key|1103152|text|我认为这个例子是不言而喻的。它运行第一个参数的方法，其名称在第二个参数中给出。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1103153|class+MyClass:
+++def+__init__(self):
++++++pass
+++def+MyMethod(self):
++++++print("Method+ran")

#+Create+an+object
object+=+MyClass()
#+Get+all+the+methods+of+a+class
method_list+=+[func+for+func+in+dir(MyClass)+if+callable(getattr(MyClass,+func))]
#+You+can+use+any+of+the+methods+in+method_list
#+"MyMethod"+is+the+one+we+want+to+use+right+now

#+This+is+the+same+as+running+"object.MyMethod()"
getattr(object,'MyMethod')()|code-block|syntax|javascript|1103154|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I think this example is self explanatory. It runs the method of first parameter, whose name is given in the second parameter.

<pre><code>class MyClass:
 def __init__(self):
 pass
 def MyMethod(self):
 print("Method ran")

# Create an object
object = MyClass()
# Get all the methods of a class
method_list = [func for func in dir(MyClass) if callable(getattr(MyClass, func))]
# You can use any of the methods in method_list
# "MyMethod" is the one we want to use right now

# This is the same as running "object.MyMethod()"
getattr(object,'MyMethod')()
</code></pre>

I've recently <a href="http://diveintopython.net/power_of_introspection/getattr.html" rel="noreferrer">read about the <code>getattr()</code> function</a>. The problem is that I still can't grasp the idea of its usage. The only thing I understand about <code>getattr()</code> is that <code>getattr(li, "pop")</code> is the same as calling <code>li.pop</code>.

I didn't understand when the book mentioned how you use it to get a reference to a function without knowing its name until run-time. Maybe this is me being a noob in programming, in general. Could anyone shed some light on the subject? When and how do I use this exactly?

What is getattr() exactly and how do I use it?

路由

Django

Python

我最近使用了。问题是我仍然不能掌握它的用法。我对getattr()唯一的理解是，getattr(li, "pop")和调用li.pop是一样的。我不理解书中提到的如何使用它来获得对函数的引用，直到运行时才知道它的名称。也许这就是我在编程方面的新手。有没有人能说明一下这个问题？我到底应该在什么时候以及如何使用它？

问getattr()到底是什么，我该如何使用它？
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问getattr()到底是什么，我该如何使用它？
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