Easy questions from a python beginner

[Rami Chowdhury]

On Jul 12, 2010, at 15:55 , Alf P. Steinbach /Usenet wrote:
> * Rami Chowdhury, on 13.07.2010 00:14:
>> Perhaps I'm misunderstanding, but ...
>> 
>> On Jul 12, 2010, at 13:57 , Alf P. Steinbach /Usenet wrote:
>>> 
>>> Existence of a variable means, among other things, that
>>> 
>>>  * You can use the value, with guaranteed effect (either unassigned exception
>>>    or you get a proper value)
>> 
>> Surely by that definition any variable in any Python program "exists" -- you
>> are guaranteed to get one of NameError, UnboundLocalError, or a value. That
>> seems to argue away the meaning of the word entirely, and renders it not
>> particularly useful.
> 
> No, you're conflating non-existence (NameError) with accessing the value of an existing but unassigned variable (UnboundLocalError). It is like arguing that vacuum is the same as cement because sticking your head into it for ten minutes or so yields an effect  --  you're dead  --  that in many ways is just about the same.

Right -- but you're playing with the definition of "existence" there, and since you're not using it quite consistently, I wasn't sure what you meant. Your discussion of the language reference below helps clear it up.

> 
>>> How the Python implementation implements that is an implementation detail.
>>> 
>>> In short, how CPython does things is completely irrelevant to the language's
>> semantics, so you're conflating things here.
>>> 
>> 
>> As I'd understood the previous discussion, it is the CPython implementation
>> that reserves local names and produces UnboundLocalErrors. The language
>> semantics don't call for it, and another implementation might choose to handle
>> function locals the same way as globals, through a namespace dictionary -- in
>> which case the variable *wouldn't* exist in any way, shape, or form until it
>> was assigned to.
>> 
>> What am I getting wrong here?
> 
> The bit about the language semantics not specifying the effect.
> 
> From the 3.1.1 language reference §4.1:
> 
> "When a name is not found at all, a NameError exception is raised. If the name refers to a local variable that has not been bound, a UnboundLocalError exception is raised. UnboundLocalError is a subclass of NameError."
> 
> And it goes on to elaborate on that, a little later:
> 
> "If a name binding operation occurs anywhere within a code block, all uses of the name within the block are treated as references to the current block. This can lead to errors when a name is used within a block before it is bound. This rule is subtle. Python lacks declarations and allows name binding operations to occur anywhere within a code block. The local variables of a code block can be determined by scanning the entire text of the block for name binding operations."

Ah, interesting. Thank you for pointing that out -- I didn't know that. I notice the same paragraph is in the 2.7 reference, so it presumably holds for 2.x implementations as well.

> This is the usual reaction of the religious when exposed to reality.
> 
> Cheers & hth.

It does, thank you. Although the repeated references to the "religious" and their "refusal to accept reality" are less than helpful.

> 
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