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

Guido van Rossum guido at python.org
Mon Jun 26 02:49:49 CEST 2006

On 6/25/06, Ka-Ping Yee <python-dev at zesty.ca> wrote:

On Sun, 25 Jun 2006, Guido van Rossum wrote: > What do you think of Nick C's 'once'?

It's a bit closer to the right meaning... but what about: def f(x): def g(y): return y + once x return g Does "once" mean not really once here, but "once for each new function object that's created for g"?

He specifically wants the latter semantics because it solves the problem of binding the value of a loop control variable in an outer scope:

def f(n): return [(lambda: once i) for i in range(n)]

should return n functions returning the values 0 through n-1. Without the once it returns n identical functions all returning n-1; this is due to outer-scope references referencing variables, not values. (In Scheme this is solved by making the for loop create a new variable for each iteration, but that's not Pythonic.)

> Right. But there are all sorts of objects that are compared by object > identity (e.g. classes, modules, even functions) which may contain > mutable components but are nevertheless "constant" for the purpose of > switch or optimization. Let's not confuse this concept of constness > with immutability.

That's a good point. We need a concept like "stable for equality" separate from "constant", since "constant" and "immutable" will mislead those who are used to the meanings of these words in other languages.

Anyone familiar with const in C++ will have a good grasp of the infinite shades of gray it can express. :-)

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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