On 24/05/2017 15:31, Steve D'Aprano wrote:
Same as if Python supported constants (names that can only be assigned to
once).
That's not quite the same as my notion of 'named constant' that I expand
below. This suggests that the actual literal bound to the name may not
be known until runtime.
Again, the pain of building a custom "constant-like" solution is
greater than the benefit, so I've done without.
But if Python did support these things, maybe the compiler could make a few
optimizations, or maybe we could write a few less unit tests, or catch a
few bugs a little earlier... or not.
It's just a different way at looking at minimalism. Guido says, the language
should be as small as possible.
A funny way of making things minimal when people are then obliged to
create dozens of different add-on libraries to try and add the missing
functionality. It doesn't make the language any faster either when the
solutions involve extra modules, classes, attribute lookups and type
dispatching.
Anyway, my Python 3.4 comprises 5000 files in around 90MB. Minimalism
hasn't really worked.
If you don't want to assign to a name, then
just don't assign to it. No need to build support for constants into the
language when you can trust the developer to Don't Do That.
Yet the language will stop you assigning to 672, or "abc" (Had to do a
quick that that was actually the case! Python is full of surprises.)
Maybe the language could have been made even smaller if that was not
checked; after all the developer ought to be trusted not to assign to them!
Well, named constants are just such literals, but with a name. The
design of Python however means that that immediately causes a difficult
because then we have to deal with different scopes, name lookups, and
named constants defined inside a module or class so requiring an
attribute lookup.
Ideally a named constant would be instantly mapped to the equivalent
literal, and done during compilation. Then a range of optimisations also
become possible (and enables a fast 'switch' statement).
--
bartc
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