gh-107369: optimize textwrap.indent() (#107374)

3 files changed, 17 insertions, 8 deletions

diff --git a/Doc/whatsnew/3.13.rst b/Doc/whatsnew/3.13.rst
index 7cad5d1..e20832a 100644
--- a/Doc/whatsnew/3.13.rst
+++ b/Doc/whatsnew/3.13.rst
@@ -150,7 +150,8 @@ typing
 Optimizations
 =============
 
-
+* :func:`textwrap.indent` is now ~30% faster than before for large input.
+  (Contributed by Inada Naoki in :gh:`107369`.)
 
 
 Deprecated
diff --git a/Lib/textwrap.py b/Lib/textwrap.py
index 98bedd2..7ca393d 100644
--- a/Lib/textwrap.py
+++ b/Lib/textwrap.py
@@ -476,13 +476,19 @@ def indent(text, prefix, predicate=None):
     consist solely of whitespace characters.
     """
     if predicate is None:
-        def predicate(line):
-            return line.strip()
-
-    def prefixed_lines():
-        for line in text.splitlines(True):
-            yield (prefix + line if predicate(line) else line)
-    return ''.join(prefixed_lines())
+        # 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)
+
+    return ''.join(prefixed_lines)
 
 
 if __name__ == "__main__":
diff --git a/Misc/NEWS.d/next/Library/2023-07-28-14-56-35.gh-issue-107369.bvTq8F.rst b/Misc/NEWS.d/next/Library/2023-07-28-14-56-35.gh-issue-107369.bvTq8F.rst
new file mode 100644
index 0000000..76aeab6
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-07-28-14-56-35.gh-issue-107369.bvTq8F.rst
@@ -0,0 +1,2 @@
+Optimize :func:`textwrap.indent`. It is ~30% faster for large input. Patch
+by Inada Naoki.