[Python-Dev] Simple Switch statement (original) (raw)

Georg Brandl g.brandl at gmx.net
Mon Jun 26 01:12:30 CEST 2006

Raymond Hettinger wrote:

No thanks. That is its own can of worms. The obvious solutions (like const declarations, macros, or a syntax to force compile-time expression evaluation) are unlikely to sit well because they run afoul Python's deeply ingrained dynamism.

I think perhaps you haven't been paying close attention to Fredrik's proposal. Yes, I have been. That is one of the three options I listed above. Each has its own issues. The static() keyword works like Forth's brackets for forcing compile-time evaluation. The issue for us is that unlike other Python expressions, there are inconvenient limitiations on what can be expressed inside: five = 5 eight = [8] def f(x, six=6): seven = 7 a = static(five + 4) # this is legal b = static(six + 4) # this is illegal c = static(seven + 4) # this is illegal d = static(eight + [4]) # this is illegal

Why would the last line be illegal?

That will be a perpetual maintenance trap and conundrum for newbies.

In contrary to other "newbie traps" such as mutable default arguments, if this would give a clear exception message like "function-local name cannot be used in static expression", I can't imagine it would be a bigger problem than e.g. "why is my floating point result incorrect".

Georg

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