Fix ValueError: glog(0) when encoding zero-heavy data (Fixes #330)

SmileyChris · SmileyChris · commit 67904561e2d4 · 2026-03-20T20:29:15.000+13:00
Handle zero polynomial coefficients in Polynomial.__mul__ and __mod__
to avoid calling glog(0), which is undefined in GF(256) arithmetic.
diff --git a/qrcode/base.py b/qrcode/base.py
@@ -268,6 +268,8 @@ def __mul__(self, other):
 
         for i, item in enumerate(self):
             for j, other_item in enumerate(other):
+                if item == 0 or other_item == 0:
+                    continue
                 num[i + j] ^= gexp(glog(item) + glog(other_item))
 
         return Polynomial(num, 0)
@@ -277,6 +279,12 @@ def __mod__(self, other):
         if difference < 0:
             return self
 
+        if self[0] == 0:
+            num = list(self[1:])
+            if difference:
+                num.append(0)
+            return Polynomial(num, 0) % other
+
         ratio = glog(self[0]) - glog(other[0])
 
         num = [
diff --git a/qrcode/tests/test_qrcode.py b/qrcode/tests/test_qrcode.py
@@ -23,6 +23,19 @@ def test_large():
     qr.make(fit=False)
 
 
+def test_glog_zero_data_with_leading_zeros():
+    """Regression test for issue #330: glog(0) ValueError with zero-heavy data."""
+    qr = qrcode.QRCode(
+        version=6,
+        error_correction=qrcode.constants.ERROR_CORRECT_L,
+        box_size=10,
+        border=4,
+        mask_pattern=0,
+    )
+    qr.add_data("http://test.com/abc" + "000" * 90)
+    qr.make()
+
+
 def test_invalid_version():
     with pytest.raises(ValueError):
         qrcode.QRCode(version=42)
