1 files changed, 7 insertions, 1 deletions
diff --git a/bits.py b/bits.py
index b3409da..b8cf441 100644
--- a/bits.py
+++ b/bits.py
@@ -1,3 +1,5 @@
+empty = (0, 0)
+
 def u(num, bits):
     """ parse an unsigned integer into a bitfield.
 
@@ -83,7 +85,7 @@ def concat(*parts):
         size += psize
     return (val, size)
 
-def slice(bits, hi, lo):
+def slice(bits, hi, lo, allow_empty=False):
     """ slice a bitfield given hi and lo bit indices.
 
     >>> slice((0x7f, 8), 7, 4)
@@ -105,6 +107,8 @@ def slice(bits, hi, lo):
     Traceback (most recent call last):
       ...
     ValueError: cant slice reverse range
+    >>> slice((0xf, 4), 2, 3, allow_empty=True)
+    (0, 0)
     >>> slice((0xf, 4), 3, -1)
     Traceback (most recent call last):
       ...
@@ -113,6 +117,8 @@ def slice(bits, hi, lo):
     (val, size) = bits
 
     if hi < lo:
+        if allow_empty:
+            return empty
         raise ValueError("cant slice reverse range")
     elif lo < 0 or hi >= size:
         raise ValueError("slice [{}:{}] out of range ({} bit value)".format(hi, lo, size))