From be59df8359083b9dce08ab4eb8b83a4e77799f33 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sun, 30 Mar 2025 20:27:55 +0100
Subject: [PATCH 01/11] Add whitespace tests

---
 Lib/test/test_textwrap.py | 40 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/Lib/test/test_textwrap.py b/Lib/test/test_textwrap.py
index dfbc2b93dfc0d6..6478c75dd854e4 100644
--- a/Lib/test/test_textwrap.py
+++ b/Lib/test/test_textwrap.py
@@ -769,6 +769,46 @@ def assertUnchanged(self, text):
         """assert that dedent() has no effect on 'text'"""
         self.assertEqual(text, dedent(text))
 
+    def test_dedent_only_whitespace(self):
+        # The empty string.
+        text = ""
+        self.assertUnchanged(text)
+
+        # Only spaces.
+        text = "    "
+        expect = ""
+        self.assertEqual(expect, dedent(text))
+
+        # Only tabs.
+        text = "\t\t\t\t"
+        expect = ""
+        self.assertEqual(expect, dedent(text))
+
+        # A mixture.
+        text = " \t  \t\t  \t "
+        expect = ""
+        self.assertEqual(expect, dedent(text))
+
+        # ASCII whitespace.
+        text = "\f\n\r\t\v "
+        expect = "\f\n\r\t\v "
+        self.assertEqual(expect, dedent(text))
+
+        # One newline.
+        text = "\n"
+        expect = "\n"
+        self.assertEqual(expect, dedent(text))
+
+        # Windows-style newlines.
+        text = "\r\n"
+        expect = "\r\n"
+        self.assertEqual(expect, dedent(text))
+
+        # Whitespace mixture.
+        text = "    \n\t\n  \n\t\t\n\n\n       "
+        expect = "\n\n\n\n\n\n"
+        self.assertEqual(expect, dedent(text))
+
     def test_dedent_nomargin(self):
         # No lines indented.
         text = "Hello there.\nHow are you?\nOh good, I'm glad."

From fec6717d1d6546e4b5df76211c9add57b8e2110e Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sun, 30 Mar 2025 19:51:40 +0100
Subject: [PATCH 02/11] Optimise ``textwrap.dedent()``

Co-authored-by: Marius Juston <marius.juston@hotmail.fr>
---
 Lib/test/test_textwrap.py                     |  4 +-
 Lib/textwrap.py                               | 55 ++++++-------------
 ...-03-30-19-55-10.gh-issue-131792.NNjzFA.rst |  2 +
 3 files changed, 22 insertions(+), 39 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst

diff --git a/Lib/test/test_textwrap.py b/Lib/test/test_textwrap.py
index 6478c75dd854e4..7715d14fd12b5e 100644
--- a/Lib/test/test_textwrap.py
+++ b/Lib/test/test_textwrap.py
@@ -791,7 +791,7 @@ def test_dedent_only_whitespace(self):
 
         # ASCII whitespace.
         text = "\f\n\r\t\v "
-        expect = "\f\n\r\t\v "
+        expect = "\n"
         self.assertEqual(expect, dedent(text))
 
         # One newline.
@@ -801,7 +801,7 @@ def test_dedent_only_whitespace(self):
 
         # Windows-style newlines.
         text = "\r\n"
-        expect = "\r\n"
+        expect = "\n"
         self.assertEqual(expect, dedent(text))
 
         # Whitespace mixture.
diff --git a/Lib/textwrap.py b/Lib/textwrap.py
index 1bf07aa46cad99..99950278a690cf 100644
--- a/Lib/textwrap.py
+++ b/Lib/textwrap.py
@@ -413,9 +413,6 @@ def shorten(text, width, **kwargs):
 
 # -- Loosely related functionality -------------------------------------
 
-_whitespace_only_re = re.compile('^[ \t]+$', re.MULTILINE)
-_leading_whitespace_re = re.compile('(^[ \t]*)(?:[^ \t\n])', re.MULTILINE)
-
 def dedent(text):
     """Remove any common leading whitespace from every line in `text`.
 
@@ -429,42 +426,26 @@ def dedent(text):
 
     Entirely blank lines are normalized to a newline character.
     """
-    # Look for the longest leading string of spaces and tabs common to
-    # all lines.
-    margin = None
-    text = _whitespace_only_re.sub('', text)
-    indents = _leading_whitespace_re.findall(text)
-    for indent in indents:
-        if margin is None:
-            margin = indent
-
-        # Current line more deeply indented than previous winner:
-        # no change (previous winner is still on top).
-        elif indent.startswith(margin):
-            pass
-
-        # Current line consistent with and no deeper than previous winner:
-        # it's the new winner.
-        elif margin.startswith(indent):
-            margin = indent
-
-        # Find the largest common whitespace between current line and previous
-        # winner.
-        else:
-            for i, (x, y) in enumerate(zip(margin, indent)):
-                if x != y:
-                    margin = margin[:i]
-                    break
+    if not text:
+        return text
+
+    lines = text.split('\n')
+
+    non_blank_lines = [l for l in lines if l and not l.isspace()]
+    if not non_blank_lines:
+        return '\n'.join([l if l and not l.isspace() else '' for l in lines])
 
-    # sanity check (testing/debugging only)
-    if 0 and margin:
-        for line in text.split("\n"):
-            assert not line or line.startswith(margin), \
-                   "line = %r, margin = %r" % (line, margin)
+    # Get length of leading whitespace, inspired by ``os.path.commonprefix()``
+    l1 = min(non_blank_lines)
+    l2 = max(non_blank_lines)
+    margin = 0
+    for i, c in enumerate(l1):
+        if c != l2[i] or c not in ' \t':
+            break
+        margin += 1
 
-    if margin:
-        text = re.sub(r'(?m)^' + margin, '', text)
-    return text
+    return '\n'.join([l[margin:] if l and not l.isspace() else ''
+                      for l in lines])
 
 
 def indent(text, prefix, predicate=None):
diff --git a/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst b/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
new file mode 100644
index 00000000000000..d1ae12461bcf9c
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
@@ -0,0 +1,2 @@
+Improved performance of :func: `textwrap.dedent` by ~2.4x.
+Patch by Adam Turner and Marius Juston.

From 19e76d3efde3527eb2b38fa6cea65b17be22d4d7 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sun, 30 Mar 2025 20:33:19 +0100
Subject: [PATCH 03/11] Further micro-optimisation for entirely blank input

---
 Lib/textwrap.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/Lib/textwrap.py b/Lib/textwrap.py
index 99950278a690cf..6d87e3b1207cd6 100644
--- a/Lib/textwrap.py
+++ b/Lib/textwrap.py
@@ -429,13 +429,14 @@ def dedent(text):
     if not text:
         return text
 
-    lines = text.split('\n')
+    # If the input is entirely whitespace, return normalized lines
+    if text.isspace():
+        return '\n' * text.count('\n')
 
-    non_blank_lines = [l for l in lines if l and not l.isspace()]
-    if not non_blank_lines:
-        return '\n'.join([l if l and not l.isspace() else '' for l in lines])
+    lines = text.split('\n')
 
     # Get length of leading whitespace, inspired by ``os.path.commonprefix()``
+    non_blank_lines = [l for l in lines if l and not l.isspace()]
     l1 = min(non_blank_lines)
     l2 = max(non_blank_lines)
     margin = 0

From 850c07ab83f5c5b140b58162259d887fca6eab59 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sun, 30 Mar 2025 21:26:43 +0100
Subject: [PATCH 04/11] whitespace

---
 .../next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst b/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
index d1ae12461bcf9c..934d0afd05efcc 100644
--- a/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
+++ b/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
@@ -1,2 +1,2 @@
-Improved performance of :func: `textwrap.dedent` by ~2.4x.
+Improved performance of :func:`textwrap.dedent` by ~2.4x.
 Patch by Adam Turner and Marius Juston.

From 796fb3eefceb8d68c129c912eeae20ce9e1de2b1 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sun, 30 Mar 2025 22:09:42 +0100
Subject: [PATCH 05/11] Use margin as the loop index variable

---
 Lib/textwrap.py | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/Lib/textwrap.py b/Lib/textwrap.py
index 6d87e3b1207cd6..a5da476355faea 100644
--- a/Lib/textwrap.py
+++ b/Lib/textwrap.py
@@ -439,11 +439,9 @@ def dedent(text):
     non_blank_lines = [l for l in lines if l and not l.isspace()]
     l1 = min(non_blank_lines)
     l2 = max(non_blank_lines)
-    margin = 0
-    for i, c in enumerate(l1):
-        if c != l2[i] or c not in ' \t':
+    for margin, c in enumerate(l1):
+        if c != l2[margin] or c not in ' \t':
             break
-        margin += 1
 
     return '\n'.join([l[margin:] if l and not l.isspace() else ''
                       for l in lines])

From ecc9cfcff34c5e7e23fce7bd115f44e7eaa1280d Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sun, 30 Mar 2025 22:10:22 +0100
Subject: [PATCH 06/11] Add full stops

---
 Lib/textwrap.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Lib/textwrap.py b/Lib/textwrap.py
index a5da476355faea..573ab49be1de18 100644
--- a/Lib/textwrap.py
+++ b/Lib/textwrap.py
@@ -429,13 +429,13 @@ def dedent(text):
     if not text:
         return text
 
-    # If the input is entirely whitespace, return normalized lines
+    # If the input is entirely whitespace, return normalized lines.
     if text.isspace():
         return '\n' * text.count('\n')
 
     lines = text.split('\n')
 
-    # Get length of leading whitespace, inspired by ``os.path.commonprefix()``
+    # Get length of leading whitespace, inspired by ``os.path.commonprefix()``.
     non_blank_lines = [l for l in lines if l and not l.isspace()]
     l1 = min(non_blank_lines)
     l2 = max(non_blank_lines)

From 311ac87656935d32ae49defc4b51606964977550 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sun, 30 Mar 2025 22:43:53 +0100
Subject: [PATCH 07/11] Further micro-optimisation suggested by Pieter Eendebak

---
 Lib/textwrap.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/Lib/textwrap.py b/Lib/textwrap.py
index 573ab49be1de18..cdda2a6811decd 100644
--- a/Lib/textwrap.py
+++ b/Lib/textwrap.py
@@ -443,8 +443,7 @@ def dedent(text):
         if c != l2[margin] or c not in ' \t':
             break
 
-    return '\n'.join([l[margin:] if l and not l.isspace() else ''
-                      for l in lines])
+    return '\n'.join([l[margin:] if not l.isspace() else '' for l in lines])
 
 
 def indent(text, prefix, predicate=None):

From f2050aef1128021cb6fa5e29b60cb19741d3be2f Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sun, 30 Mar 2025 23:05:16 +0100
Subject: [PATCH 08/11] Remove early-exit check for whitespace-only input, as
 suggested by Pieter Eendebak

---
 Lib/textwrap.py | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/Lib/textwrap.py b/Lib/textwrap.py
index cdda2a6811decd..bb6a1186316275 100644
--- a/Lib/textwrap.py
+++ b/Lib/textwrap.py
@@ -429,16 +429,13 @@ def dedent(text):
     if not text:
         return text
 
-    # If the input is entirely whitespace, return normalized lines.
-    if text.isspace():
-        return '\n' * text.count('\n')
-
     lines = text.split('\n')
 
     # Get length of leading whitespace, inspired by ``os.path.commonprefix()``.
     non_blank_lines = [l for l in lines if l and not l.isspace()]
-    l1 = min(non_blank_lines)
-    l2 = max(non_blank_lines)
+    l1 = min(non_blank_lines, default='')
+    l2 = max(non_blank_lines, default='')
+    margin = 0
     for margin, c in enumerate(l1):
         if c != l2[margin] or c not in ' \t':
             break

From 1f73bcbc04ac844dd63df32f6cbbdafa68cefa5f Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Mon, 31 Mar 2025 01:02:57 +0100
Subject: [PATCH 09/11] More test updates

---
 Lib/test/test_textwrap.py | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/Lib/test/test_textwrap.py b/Lib/test/test_textwrap.py
index 7715d14fd12b5e..77366988b57fa7 100644
--- a/Lib/test/test_textwrap.py
+++ b/Lib/test/test_textwrap.py
@@ -769,7 +769,7 @@ def assertUnchanged(self, text):
         """assert that dedent() has no effect on 'text'"""
         self.assertEqual(text, dedent(text))
 
-    def test_dedent_only_whitespace(self):
+    def test_dedent_whitespace(self):
         # The empty string.
         text = ""
         self.assertUnchanged(text)
@@ -800,8 +800,8 @@ def test_dedent_only_whitespace(self):
         self.assertEqual(expect, dedent(text))
 
         # Windows-style newlines.
-        text = "\r\n"
-        expect = "\n"
+        text = "\r\n" * 5
+        expect = "\n" * 5
         self.assertEqual(expect, dedent(text))
 
         # Whitespace mixture.
@@ -809,6 +809,16 @@ def test_dedent_only_whitespace(self):
         expect = "\n\n\n\n\n\n"
         self.assertEqual(expect, dedent(text))
 
+        # Lines consisting only of whitespace are always normalised
+        text = "a\n \n\t\n"
+        expect = "a\n\n\n"
+        self.assertEqual(expect, dedent(text))
+
+        # Whitespace characters on non-empty lines are retained
+        text = "a\r\n\r\n\r\n"
+        expect = "a\r\n\n\n"
+        self.assertEqual(expect, dedent(text))
+
     def test_dedent_nomargin(self):
         # No lines indented.
         text = "Hello there.\nHow are you?\nOh good, I'm glad."

From cc67165718b620e86c06a7a1aa516234d89f4dad Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Mon, 31 Mar 2025 01:05:17 +0100
Subject: [PATCH 10/11] Update NEWS

---
 .../Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst    | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst b/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
index 934d0afd05efcc..6c97638dc84e89 100644
--- a/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
+++ b/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
@@ -1,2 +1,4 @@
-Improved performance of :func:`textwrap.dedent` by ~2.4x.
+Improved performance of :func:`textwrap.dedent` by ~2.4x,
+and fixed a bug where blank lines with whitespace characters other than space
+or horizontal tab were not normalised to the newline.
 Patch by Adam Turner and Marius Juston.

From bf58dfa27118c0f724eb8395fc9db66df98fccc1 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Mon, 31 Mar 2025 01:11:07 +0100
Subject: [PATCH 11/11] Update NEWS

---
 .../Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst   | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst b/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
index 6c97638dc84e89..62b619c0d80f4d 100644
--- a/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
+++ b/Misc/NEWS.d/next/Library/2025-03-30-19-55-10.gh-issue-131792.NNjzFA.rst
@@ -1,4 +1,5 @@
-Improved performance of :func:`textwrap.dedent` by ~2.4x,
+Improved performance of :func:`textwrap.dedent` by an average of ~2.4x,
+(with improvements of up to 4x for large inputs),
 and fixed a bug where blank lines with whitespace characters other than space
 or horizontal tab were not normalised to the newline.
-Patch by Adam Turner and Marius Juston.
+Patch by Adam Turner, Marius Juston, and Pieter Eendebak.