1 files changed, 22 insertions, 0 deletions
diff --git a/gbp/format.py b/gbp/format.py
index 2edf4ebd..541ab7b0 100644
--- a/gbp/format.py
+++ b/gbp/format.py
@@ -42,3 +42,25 @@ def format_str(msg, args):
     except KeyError as e:
         raise GbpError("Failed to format %s: Missing value %s in %s" %
                        (msg, e, args))
+
+
+def format_b(fmtstr, *args):
+    """String-like interpolation for bytes objects.
+
+    NOTE: This is a compatibility wrapper for older versions (<3.5) of Python 3
+    which do not support the percent operator ('%') for bytes objects. This
+    function should be removed (and replaced by simple '%') when Python 3.5
+    has gained wide enough adoption.
+
+    >>> format_b(b'%s %d', b'foo', 123)
+    b'foo 123'
+    >>> format_b(b'foo 123')
+    b'foo 123'
+    >>> format_b('%s %d', b'foo', 123)
+    Traceback (most recent call last):
+    ...
+    AttributeError: 'str' object has no attribute 'decode'
+    """
+    fmtstr = fmtstr.decode()
+    strargs = tuple([(a.decode() if isinstance(a, bytes) else a) for a in args])
+    return (fmtstr % strargs).encode()