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

Mở Lib/xml/etree/ElementTree.py và kiểm tra vòng lặp elem.iter() của add_qname. Kiểm tra thứ tự nhánh được đề xuất cho các tag string và QName, sau đó xác minh rằng việc tuần tự hóa XML vẫn chính xác và so sánh hiệu năng trên một cây đã được phân tích tương tự ví dụ 300kb được báo cáo. Được xem là hoàn tất khi trường hợp tag string được cải thiện mà không làm hỏng việc xử lý QName hoặc tag không hợp lệ.