From 51e0f2b06c042b8742145ce094323a34b61f02aa Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Date: Mon, 31 Mar 2025 18:31:39 +0100
Subject: gh-107369: Optimise ``textwrap.indent()`` (#131923)
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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
---
 Lib/textwrap.py                                    | 25 +++++++++++-----------
 .../2025-03-31-02-06-57.gh-issue-107369.8M-HVz.rst |  2 ++
 2 files changed, 15 insertions(+), 12 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-03-31-02-06-57.gh-issue-107369.8M-HVz.rst

diff --git a/Lib/textwrap.py b/Lib/textwrap.py
index bb6a118..bac98c9 100644
--- a/Lib/textwrap.py
+++ b/Lib/textwrap.py
@@ -451,19 +451,20 @@ def indent(text, prefix, predicate=None):
     it will default to adding 'prefix' to all non-empty lines that do not
     consist solely of whitespace characters.
     """
-    if predicate is None:
-        # str.splitlines(True) doesn't produce empty string.
-        #  ''.splitlines(True) => []
-        #  'foo\n'.splitlines(True) => ['foo\n']
-        # So we can use just `not s.isspace()` here.
-        predicate = lambda s: not s.isspace()
-
     prefixed_lines = []
-    for line in text.splitlines(True):
-        if predicate(line):
-            prefixed_lines.append(prefix)
-        prefixed_lines.append(line)
-
+    if predicate is None:
+        # str.splitlines(keepends=True) doesn't produce the empty string,
+        # so we need to use `str.isspace()` rather than a truth test.
+        # Inlining the predicate leads to a ~30% performance improvement.
+        for line in text.splitlines(True):
+            if not line.isspace():
+                prefixed_lines.append(prefix)
+            prefixed_lines.append(line)
+    else:
+        for line in text.splitlines(True):
+            if predicate(line):
+                prefixed_lines.append(prefix)
+            prefixed_lines.append(line)
     return ''.join(prefixed_lines)
 
 
diff --git a/Misc/NEWS.d/next/Library/2025-03-31-02-06-57.gh-issue-107369.8M-HVz.rst b/Misc/NEWS.d/next/Library/2025-03-31-02-06-57.gh-issue-107369.8M-HVz.rst
new file mode 100644
index 0000000..9b1bd9e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-03-31-02-06-57.gh-issue-107369.8M-HVz.rst
@@ -0,0 +1,2 @@
+Improved performance of :func:`textwrap.dedent` by an average of ~1.3x.
+Patch by Adam Turner.
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