Easy questions from a python beginner

[Alf P. Steinbach /Usenet]

* MRAB, on 12.07.2010 00:37:
> Alf P. Steinbach /Usenet wrote:
>> * Stephen Hansen, on 11.07.2010 21:00:
>>> On 7/11/10 11:45 AM, wheres pythonmonks wrote:
>>>> Follow-up:
>>>> Is there a way to define compile-time constants in python and have the
>>>> bytecode compiler optimize away expressions like:
>>>>
>>>> if is_my_extra_debugging_on: print ...
>>>>
>>>> when "is_my_extra_debugging" is set to false? I'd like to pay no
>>>> run-time penalty for such code when extra_debugging is disabled.
>>>
>>> Any code wrapped in a __debug__ guard is utterly ommitted if you run
>>> Python with the -O option. That, and asserts go away.
>>>
>>>> On #2: My point regarding the impossibility of writing the swap
>>>> function for ints is to explicitly understand that this isn't
>>>> possible, so as not to look for solutions along those lines when
>>>> trying to write python code.
>>>
>>> Its impossible because Python's calling and namespace semantics simply
>>> don't work like that. There's no references in the traditional sense,
>>> because there's no variables-- boxes that you put values in. There's
>>> just concrete objects. Objects are passed into the function and given
>>> new names; that those objects have names in the enclosing scope is
>>> something you don't know, can't access, and can't manipulate.. even the
>>> objects don't know what names they happen to be called.
>>>
>>> Check out http://effbot.org/zone/call-by-object.htm
>>
>> Oh, I wouldn't give that advice. It's meaningless mumbo-jumbo. Python
>> works like Java in this respect, that's all; neither Java nor Python
>> support 'swap'.
>>
>> Of course there are variables, that's why the docs call them variables.
>>
> In Java a variable is declared and exists even before the first
> assignment to it. In Python a 'variable' isn't declared and won't exist
> until the first 'assignment' to it.

That is a misconception.

In Python a variable is declared by having an assignment to it, which for a 
local variable may be anywhere within a routine.

If such a variable is used before it's been assigned to, then you get an 
uninitialized variable exception. Clearly the variable must exist in order for 
the exception to refer to it (not to mention the exception occurring at all).

   def foo():
       print( blah )
       blah = "this is both an assignment and a declaration causing it to exist"

   foo()

Clearly when the exception is raised, referring to the variable, the variable 
exists.

Contrary to your statement that is before the assignment.

However, as stated up-thread, I do not expect facts, logic or general reasoning 
to have any effect whatsoever on such hard-core religious beliefs. And I do not 
care whether I convince you or not. But I *do not* want the religious subset of 
the community to succeed too much in propagating nonsense idiot beliefs to 
newbies  --  hence the concrete example that any newbie can try.


Cheers & hth.,

- Alf

-- 
blog at <url: http://alfps.wordpress.com>