Slight ElementTree serialization performance enhancement for trees with str tags

Feature or enhancement

Proposal:

This proposal improves the performance of writing xml whose trees are made up of tag names that are predominantly strings. This comes at the cost of performance for trees with tags that are predominantly QNames

As far as I'm aware, using a str for the tag name is more common than using a QName and we should optimise for that scenario (for example, parsing an xml document with ElementTree returns Elements whose tags are all strings).

Reordering the following if block to make the isinstance(tag, str) check first gives a performance improvement of 1 - 1.5% on a tree parsed from a file that was about 300kb:

--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -827,12 +827,12 @@ def add_qname(qname):
     # populate qname and namespaces table
     for elem in elem.iter():
         tag = elem.tag
-        if isinstance(tag, QName):
-            if tag.text not in qnames:
-                add_qname(tag.text)
-        elif isinstance(tag, str):
+        if isinstance(tag, str):
             if tag not in qnames:
                 add_qname(tag)
+        elif isinstance(tag, QName):
+            if tag.text not in qnames:
+                add_qname(tag.text)
         elif tag is not None and tag is not Comment and tag is not PI:
             _raise_serialization_error(tag)
         for key, value in elem.items():

As this enhancement is within a loop that traverses the entire xml document, the larger the xml tree, the greater the performance improvement as the tree traversal starts to account for more time than other setup code.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

打开 Lib/xml/etree/ElementTree.py，检查 add_qname 的 elem.iter() 循环。检查针对 string 和 QName 标签提出的分支顺序，然后验证 XML 序列化仍然正确，并在与报告中的 300kb 示例类似的已解析树上比较性能。完成的标准是 string 标签的情况得到改进，同时不破坏 QName 或无效标签的处理。