Question: I found this:

Use Python output:

```
print ‘Content-type: text/x-json\n\n’
print json.dumps([{'title':arr['title']}])

```

and get json string with Jquery:

```
$ajax(   
   success: function(msg){
      if(msg[0].title) alert(msg[0].title);
   }
)

```

It works, who can tell me why it is? Thanks~

Answer: 
jQuery calls JSON.parse internally on modern browsers that have it if the Content-Type is json

```
        return window.JSON && window.JSON.parse ?
            window.JSON.parse( data ) :
            (new Function("return " + data))();

```
I believe jQuery is able to determine the response type based on the header you are sending and automatically evaluate it as JSON.