python - 向 lxml 中的多个子项添加相同的元素

Question

背景:我正在尝试使用从 Web 服务端点检索的元数据充实 XML 报告。报告列出了文本模块和图形，每个图形都有几种分辨率。我无法为每个分辨率添加元数据。

问题:这是简化的问题。

from lxml import etree as ET

myxml = """\
<report>
    <object id="foo">
        <reportitems>
            <reportitem id="1"/>
            <reportitem id="2"/>
            <reportitem id="3"/>
        </reportitems>
    </object>
</report>
"""

report = ET.fromstring(myxml)
test = ET.Element("test", foo="bar")

for r in report.findall("object/reportitems/reportitem"):
    r.append(test)

我得到这个输出:

<report>
    <object id="foo">
        <reportitems>
            <reportitem id="1"/>
            <reportitem id="2"/>
            <reportitem id="3"><test foo="bar"/></reportitem>
        </reportitems>
    </object>
</report>

现在，如果我像这样修改代码(使用相同的 XML 片段):

report = ET.fromstring(myxml)
myElements = [ET.Element("test1"), ET.Element("test2"), ET.Element("test3")]

counter = 0
for r in report.findall("object/reportitems/reportitem"):
    r.append(myElements[counter])
    counter += 1

...然后我得到这个输出:

<report>
    <object id="foo">
        <reportitems>
            <reportitem id="1"><test1/></reportitem>
            <reportitem id="2"><test2/></reportitem>
            <reportitem id="3"><test3/></reportitem>
        </reportitems>
    </object>
</report>

为什么我不能将相同(相同)的元素作为子元素添加到我迭代的多个元素中？

Answer 1

此行为在 lxml tutorial 中进行了描述:

There is another important case where the behaviour of Elements in lxml (in 2.0 and later) deviates from that of lists and from that of the original ElementTree (prior to version 1.3 or Python 2.7/3.2):

>>> for child in root:
...     print(child.tag)
child0 child1 child2 child3
>>> root[0] = root[-1]  # this moves the element in lxml.etree!
>>> for child in root:
...     print(child.tag)
child3 child1 child2

In this example, the last element is moved to a different position, instead of being copied, i.e. it is automatically removed from its previous position when it is put in a different place. In lists, objects can appear in multiple positions at the same time, and the above assignment would just copy the item reference into the first position, so that both contain the exact same item:

>>> l = [0, 1, 2, 3]
>>> l[0] = l[-1]
>>> l
[3, 1, 2, 3]

Note that in the original ElementTree, a single Element object can sit in any number of places in any number of trees, which allows for the same copy operation as with lists. The obvious drawback is that modifications to such an Element will apply to all places where it appears in a tree, which may or may not be intended. The upside of this difference is that an Element in lxml.etree always has exactly one parent, which can be queried through the getparent() method. This is not supported in the original ElementTree.

>>> root is root[0].getparent()  # lxml.etree only!
True

If you want to copy an element to a different position in lxml.etree, consider creating an independent deep copy using the copy module from Python's standard library:

>>> from copy import deepcopy

>>> element = etree.Element("neu")
>>> element.append( deepcopy(root[1]) )

>>> print(element[0].tag)
child1
>>> print([ c.tag for c in root ])
['child3', 'child1', 'child2']