This is not a bug.  The behaviour you're seeing is described here:

http://docs.python.org/reference/executionmodel.html#naming-and-binding

"If a name binding operation occurs anywhere within a code block, all
uses of the name within the block are treated as references to the
current block. This can lead to errors when a name is used within a
block before it is bound. This rule is subtle. Python lacks declarations
and allows name binding operations to occur anywhere within a code
block. The local variables of a code block can be determined by scanning
the entire text of the block for name binding operations."

In the failing example, the registerdecorator function contains an
assignment to in_std, so by the rules above in_std is local to the
function.  The 'if in_std' line therefore tries to lookup 'in_std' in
the local namespace;  it doesn't exist (yet), so an UnboundLocalError
exception occurs.