Importing variables non-deterministic?
Dave Angel
davea at davea.name
Mon Aug 19 08:33:32 EDT 2013
Antoon Pardon wrote:
> Op 19-08-13 11:18, Chris Angelico schreef:
<snip>
>> The issue
>> was regarding imports, and it's perfectly safe to import a constant,
>> even if the interpreter doesn't protect you from then being a total
>> idiot and changing it.
>
> Python doesn't have constants, so you statement about importing a
> constant doesn't make sense. The point is that python doesn't provide
> the mechanism for protecting names against reassignments. So you
> don't know whether the variable you think of as a constant is so
> in reality. And this from a pure language definition point of view.
> That you can use tools that make the interpreter no longer behave
> as the language should, doesn't negate that.
>
Who cares what the language "protects?" I don't know any language
whose protections can't be at least partially bypassed by clever
foot-shooters. In any case, we all know that Python doesn't protect
constants, so we're free to use the word in a friendlier way.
A Python constant is what I use as a constant. I follow Pep-8 and make
it all caps. So in any library I write struct_global.y would be a bug
or a design flaw. (And with a single character name like that, it
wouldn't be global in any case. Single character names are reserved
for play code and for short loops)
And if I subsequently change it in my calling code, it isn't a
constant any more. If I rebind the name, it's not even the same
variable any more.
That's a bug, not an exception to the rule "don't use global variables."
I also accept as a constant those values which are initialized
sufficiently early in the code that most places will only ever see the
final value. Those may be global without worry.
--
DaveA
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