I'm not sure if this is really a bug, but it is unexpected behavior. When you call "bind" on a Python 3.3 signature object, if you omit an optional argument, the default is not provided in the arguments dict. Similarly, if there is a "var positional" or "var keyword" parameter but there are no extra arguments, it will not be included. To emulate the effect on the namespace of an actual function, I would expect these to be included, as an empty tuple/dict in the case of variable arguments. I realize the current behavior may be useful in some cases, but if so, then another method could be added: bind_full, which would include all parameters of the signature.