From 3680e13bc0bbd0a68b69858623f738f1d8a6ead4 Mon Sep 17 00:00:00 2001
From: "S. M. Mohiuddin Khan Shiam"
 <147746955+mohiuddin-khan-shiam@users.noreply.github.com>
Date: Wed, 25 Jun 2025 01:19:22 +0600
Subject: [PATCH] hygiene: fix mutable default argument in
 `vttests/common.py::sgr_n` (#19028)
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sgr_n previously used a mutable list (`seq=[]`) as its default parameter.
In Python, default arguments are instantiated once at
function‐definition time and then shared across all calls.
If any caller mutated that list (e.g., `seq.append(...)`), later
invocations would inherit the mutated state, producing unpredictable or
corrupted VT-test sequences.
---
 src/tools/vttests/common.py | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/tools/vttests/common.py b/src/tools/vttests/common.py
index 2ecd854145..b08d23618b 100644
--- a/src/tools/vttests/common.py
+++ b/src/tools/vttests/common.py
@@ -59,7 +59,18 @@ def clear_all():
 def sgr(code=0):
     csi('{}m'.format(code))
 
-def sgr_n(seq=[]):
+def sgr_n(seq=None):
+    """Apply multiple SGR (Select Graphic Rendition) parameters.
+
+    Parameters
+    ----------
+    seq : Iterable of int | None
+        Sequence of numeric SGR codes to emit. If ``None`` (default) an empty
+        sequence is assumed. A mutable default argument must be avoided to
+        prevent unwanted state sharing between calls.
+    """
+    if seq is None:
+        seq = []
     csi('{}m'.format(';'.join(str(code) for code in seq)))
 
 def tbc():