gh-130167: Improve difflib.IS_LINE_JUNK performance by using string methods (#130170)

donbarbos · AA-Turner · picnixz · web-flow · commit bce45bcaf88c · 2025-05-01T04:11:36.000Z
Co-authored-by: Adam Turner &lt;9087854+AA-Turner@users.noreply.github.com&gt;
Co-authored-by: Bénédikt Tran &lt;10796600+picnixz@users.noreply.github.com&gt;
Co-authored-by: Hugo van Kemenade &lt;1324225+hugovk@users.noreply.github.com&gt;
Co-authored-by: Tim Peters &lt;tim.peters@gmail.com&gt;
diff --git a/Lib/difflib.py b/Lib/difflib.py
@@ -1038,11 +1038,9 @@ def _qformat(self, aline, bline, atags, btags):
 # remaining is that perhaps it was really the case that " volatile"
 # was inserted after "private".  I can live with that <wink>.
 
-import re
-
-def IS_LINE_JUNK(line, pat=re.compile(r"\s*(?:#\s*)?$").match):
+def IS_LINE_JUNK(line, pat=None):
     r"""
-    Return True for ignorable line: iff `line` is blank or contains a single '#'.
+    Return True for ignorable line: if `line` is blank or contains a single '#'.
 
     Examples:
 
@@ -1054,6 +1052,11 @@ def IS_LINE_JUNK(line, pat=re.compile(r"\s*(?:#\s*)?$").match):
     False
     """
 
+    if pat is None:
+        # Default: match '#' or the empty string
+        return line.strip() in '#'
+   # Previous versions used the undocumented parameter 'pat' as a
+   # match function. Retain this behaviour for compatibility.
     return pat(line) is not None
 
 def IS_CHARACTER_JUNK(ch, ws=" \t"):
@@ -2027,7 +2030,6 @@ def make_table(self,fromlines,tolines,fromdesc='',todesc='',context=False,
                      replace('\1','</span>'). \
                      replace('\t','&nbsp;')
 
-del re
 
 def restore(delta, which):
     r"""
diff --git a/Misc/NEWS.d/next/Library/2025-02-16-06-25-01.gh-issue-130167.kUg7Rc.rst b/Misc/NEWS.d/next/Library/2025-02-16-06-25-01.gh-issue-130167.kUg7Rc.rst
@@ -0,0 +1 @@
+Improve speed of :func:`difflib.IS_LINE_JUNK`. Patch by Semyon Moroz.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+Improve speed of :func:`difflib.IS_LINE_JUNK`. Patch by Semyon Moroz.