Fix thread safety issue in bisect_left usage (Fixes #421)

Author: SmileyChris

qrcode/main.py

@@ -219,8 +219,9 @@ def best_fit(self, start=None):
             data.write(buffer)
 
         needed_bits = len(buffer)
+        # Create a thread-local copy of the table to avoid concurrent access issues (Python 3.13+)
         self.version = bisect_left(
-            util.BIT_LIMIT_TABLE[self.error_correction], needed_bits, start
+            util.BIT_LIMIT_TABLE[self.error_correction][:], needed_bits, start
         )
         if self.version == 41:
             raise exceptions.DataOverflowError

qrcode/tests/test_thread_safe.py

@@ -0,0 +1,35 @@
+from unittest import mock
+import qrcode
+from qrcode import util
+
+
+def test_best_fit_passes_copy_to_bisect():
+    """
+    Verify that best_fit passes a copy of BIT_LIMIT_TABLE to bisect_left
+    to ensure thread safety in Python 3.13+.
+    """
+    qr = qrcode.QRCode()
+    qr.add_data("test data")
+
+    # Identify the global table being used
+    original_table = util.BIT_LIMIT_TABLE[qr.error_correction]
+
+    with mock.patch("qrcode.main.bisect_left") as mock_bisect:
+        # Mock return value to be a valid version number to avoid side effects
+        mock_bisect.return_value = 1
+
+        qr.best_fit()
+
+        assert mock_bisect.called
+
+        # Check arguments: bisect_left(a, x, lo=0, hi=len(a), *, key=None)
+        # We are interested in 'a' (the list)
+        args, _ = mock_bisect.call_args
+        passed_table = args[0]
+
+        # Verify content matches
+        assert passed_table == original_table
+
+        # Verify it is a NEW object (copy), not the original global list
+        # This confirms the thread-safety fix
+        assert passed_table is not original_table