为什么会这样dom.firstChild.firstChild文件.nodeValue是否打印根标记内的文本?

<?xml version="1.0" encoding="utf-8"?>
<library>library-text. :D
    <book isbn="1111111111">
        <title lang="en">T1 T1 T1 T1 T1</title>
        <date>2001</date>
        <author>A1 A1 A1 A1 A1</author>     
        <price>10.00</price>
    </book>
    <book isbn="2222222222">
        <title lang="en">T2 T2 T2 T2 T2</title>
        <date>2002</date>
        <author>A2 A2 A2 A2 A2</author>     
        <price>20.00</price>
    </book>
    <book isbn="3333333333">
        <title lang="en">T3 T3 T3 T3</title>
        <date>2003</date>
        <author>A3 A3 A3 A3 A3y</author>        
        <price>30.00</price>
    </book>
</library>

import xml.dom.minidom as minidom

xml_fname = "library.xml"

dom = minidom.parse(xml_fname) 

print(dom.firstChild.tagName)
print(dom.firstChild.firstChild.nodeValue)

输出

library
library-text. :D

为什么会这样 dom.firstChild.firstChild.nodeValue 打印根标记内的文本?

dom.firstChild.nodeValue