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gh-158860: Make minidom Node.normalize() linear in adjacent text nodes #158861

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gh-158860: Make minidom Node.normalize() linear in adjacent text nodes #158861

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15 changes: 15 additions & 0 deletions Lib/test/test_minidom.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1605,6 +1605,21 @@ def testNormalizeDeleteAndCombine(self):
, "testNormalizeDeleteAndCombine -- result")
doc.unlink()

def testNormalizeManyTextNodes(self):

@aisk aisk Oct 5, 2026

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This test doesn't assert anything, so it doesn't really verify the change. I think it's hard to write a test for this, so it may be fine to just remove it.

I've run the benchmark locally (debug build, one element with n text nodes of 48 chars each):

n before after
8,000 2.8 s 18 ms
16,000 11.3 s 37 ms
32,000 45.6 s 75 ms
64,000 181 s 148 ms

# Ensure that normalize() is fast with many adjacent text nodes.
N = 100_000
doc = parseString("<doc/>")
root = doc.documentElement
for i in range(N):
root.appendChild(doc.createTextNode("x" * 16))
if i % 3 == 0:
root.appendChild(doc.createTextNode(""))
doc.normalize()
self.assertEqual(len(root.childNodes), 1)
self.assertEqual(root.firstChild.data, "x" * 16 * N)
self.assertIsNone(root.firstChild.nextSibling)
doc.unlink()

def testNormalizeRecursion(self):
doc = parseString("<doc>"
"<o>"
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13 changes: 12 additions & 1 deletion Lib/xml/dom/minidom.py
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Expand Up @@ -196,6 +196,9 @@ def removeChild(self, oldChild):

def normalize(self):
L = []
# (text node, [data, ...]) for each text node that absorbs others.
# Join each run once at the end; concatenating as we go is quadratic.
runs = None

@aisk aisk Oct 5, 2026

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Maybe we can initialize it here, then the None guards at L214 and L230 can be removed:

Suggested change
runs = None
runs = []

for child in self.childNodes:
if child.nodeType == Node.TEXT_NODE:
if not child.data:
Expand All @@ -208,7 +211,12 @@ def normalize(self):
elif L and L[-1].nodeType == child.nodeType:
# collapse text node
node = L[-1]
node.data = node.data + child.data
if runs is None:
runs = []
if runs and runs[-1][0] is node:
runs[-1][1].append(child.data)
else:
runs.append((node, [node.data, child.data]))
node.nextSibling = child.nextSibling
if child.nextSibling:
child.nextSibling.previousSibling = node
Expand All @@ -219,6 +227,9 @@ def normalize(self):
L.append(child)
if child.nodeType == Node.ELEMENT_NODE:
child.normalize()
if runs is not None:
for node, data in runs:
node.data = ''.join(data)
self.childNodes[:] = L

def cloneNode(self, deep):
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-10-05-16-40-00.gh-issue-158860.Mn7dXk.rst
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@@ -0,0 +1,2 @@
:meth:`xml.dom.minidom.Node.normalize` now merges runs of adjacent text
nodes in linear time instead of quadratic time.
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