--- a/cutils/util/__init__.py	Fri Jan 10 11:38:31 2025 +0100
+++ b/cutils/util/__init__.py	Fri Jan 10 12:46:44 2025 +0100
@@ -8,6 +8,7 @@
 """
 
 __all__ = ["PY2",
+           "PY35",
            "normalize_filename",
            "argv2algo",
            "algotag2algotype",
@@ -26,6 +27,7 @@
 
 
 PY2 = sys.version_info[0] < 3
+PY35 = sys.version_info[:2] >= (3, 5)
 
 
 def default_algotag():
@@ -191,3 +193,33 @@
     if isinstance(what, bytes):
         return what
     return os.fsencode(what)
+
+
+def interpolate_bytes(formatstr, *values):
+    """Interpolate byte strings also on Python 3.4.
+
+    :param bytes formatstr:
+    :param values: params for interpolation: may *not* contain Unicode strings
+    :rvalue: the formatted octet
+    :rtype: bytes
+
+    """
+    assert isinstance(formatstr, bytes)
+    # Python 3.5+ or Python2 know how to interpolate byte strings
+    if PY35 or PY2:
+        return formatstr % values
+    # Workaround with a Latin-1 dance
+    tformatstr = formatstr.decode("latin1")
+    tvalues = []
+    for v in values:
+        if PY2:
+            if isinstance(v, unicode):  # noqa: F821  undefined name 'unicode'
+                assert False
+        else:
+            if isinstance(v, str):
+                assert False
+        if isinstance(v, bytes):
+            tvalues.append(v.decode("latin1"))
+        else:
+            tvalues.append(v)
+    return (tformatstr % tuple(tvalues)).encode("latin1")