我想使用Mechanize抓取此网页。表单元素如下所示：

<form name="ctl00" method="post" action="PSearchResults.aspx?state=ME&amp;rp=" id="ctl00"> 
<div> 
<input type="hidden" name="__EVENTTARGET" id="__EVENTTARGET" value="" /> 
<input type="hidden" name="__EVENTARGUMENT" id="__EVENTARGUMENT" value="" /> 
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="verylongstring" /> </div> 
<input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="/wEWAgKb7POZAwK4v7ffCOmari00yJft/iuZBMdOH/zh9TDI" /> 
</div> 
</form>

我正在使用Mechanize打印控件，但只能看到其中两个。如果我运行此命令：

    br.select_form(name='ctl00')
    br.form.set_all_readonly(False) # allow changing the .value of all controls
    for control in br.form.controls:
        if not control.name:
            print " - (type) =", (control.type)
            continue  
        print " - (name, type, value) =", (control.name, control.type, br[control.name])

所有打印的是：

- (name, type, value) = ('__VIEWSTATE', 'hidden', '/wEPDwUGNDQ5NTMwD2QWAgIBD2QWAgIHD2QWCgIBDw8WAh4E...more
- (name, type, value) = ('__EVENTVALIDATION', 'hidden', '/wEWAgKb7POZAwK4v7ffCOmari00yJft/iuZBMdOH/zh9TDI')

为什么不能机械化“查看” __EVENTTARGET和__EVENTARGUMENT字段？