[Someone]

It could, but detecting whether it has to “drop out” has a performance impact. Engineering such a feature without horrendously impacting performance is complex.

Also, users would not like to see performance drop permanently, only because they redefined some function (e.g. to log it’s arguments in a debugging session), so they’ll expect the system to (eventually) re-optimize code using the new state.

Languages such as JavaScript and Java do this kind of thing (Java not because programs can redefine what len means, but because the JITter makes assumptions such as “there’s only one implementation of interface Foo” or “the Object passed to this function always is an integer”), but I think both have it easier to detect the points in the code where they need to change their assumptions.

I also guess both have had at least an order of magnitude more development effort poured into them.

[nomel]

> It could, but detecting whether it has to “drop out” has a performance impact.

I would be ok with a flag that raised an exception/halted if the restricted dynamicism was encountered, if it meant appreciable performance gains.

For every project I've worked on, and every project I've really become familiar with, the "magic" that requires these crazy levels of dynamicism can relatively easily be avoided, with more "standard" interfaces, and possibly a very slight increase in complexity presented to the library user.

I used to play with python magic frequently, but eventually realized my motivation was just some meta code-flex game I was playing, and it's almost certainly never worth it, if other developers are involved.