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

開啟 Lib/xml/etree/ElementTree.py，檢查 add_qname 的 elem.iter() 迴圈。檢查針對 string 和 QName 標籤提出的分支順序，然後驗證 XML 序列化仍然正確，並在與報告中的 300kb 範例類似的已解析樹上比較效能。完成的標準是 string 標籤的情況得到改善，同時不破壞 QName 或無效標籤的處理。