https://github.com/Rosuav/cpython/tree/pep-671 So uhh... anyone who knows about the internals of CPython and wants to join me on this, I would *really* appreciate coauthors! The implementation ended up a lot more invasive than I originally planned. Some of that is inherent to the problem, but other parts might be able to be done more cleanly. The way I've done it: * Argument defaults (either in __defaults__ or __kwdefaults__) are now tuples of (desc, value) or (desc,) for early-bound and late-bound respectively * Early-bound defaults get mapped as normal. Late-bound defaults are left unbound at time of function call. * For each late-bound default as of the 'def' statement, a check is coded: if the local is unbound, set it based on the given expression. This means that it's possible to replace an early-bound default with a late-bound, but instead of actually evaluating the expression, it just leaves it unbound:

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def f(x=1): print(x) ... f.__defaults__ ((None, 1),) f.__defaults__ = ((None,),) f() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 1, in f UnboundLocalError: cannot access local variable 'x' where it is not associated with a value

I'm not entirely happy with this, but I also don't want to have too much of a performance impact in the normal case. So far unimplemented is the description of the argument default. My plan is for early-bound defaults to have None there (as they currently do), but late-bound ones get the source code. (In theory, it may be of value to retain the source code for earlies too, which would allow hex or octal integer literals to show up in help() as such, rather than showing the (decimal) repr of the resulting value.) Anyone got good pointers on how to do this, or is that likely to be impractical? Feel free to criticize my code. As you'll see from the commit messages in that branch, I have no idea what I'm doing here :) ChrisA