Creating a local variable scope.

[Alf P. Steinbach]

* Jean-Michel Pichavant:
> Steven D'Aprano wrote:
>> On Mon, 30 Nov 2009 02:11:12 +0100, Alf P. Steinbach wrote:
>>
>>  
>>> I think if one could somehow declare names as const (final, readonly,
>>> whatever) then that would cover the above plus much more.
>>>     
>>
>> Having real constants is one feature that I miss. Because Python 
>> doesn't have constants, I find I've lost the discipline to avoid 
>> "magic numbers" (and strings) in my code.
>>
>>
>>   
> I don't get you, this can be easily avoid with a strong naming 
> convention. I mean, despite they are not really constants, you can still 
> use variables to name your numbers and string, to give the reader a 
> usefull hint on the meaning of your code. Yet it is still a matter of 
> discipline, it is sometimes very tempting to put the string directly 
> (unlike numbers string can be meaningful sometimes)

It may be surprising how many things turn out to be readonly.

Consider the OP's code:


<code>
class ValueColumn(AbstractColumn):
     def __init__(self, name, header, domain_names):
         if type(domain_names) != tuple:
             raise ValueError('a tuple of domain names must be given')
         for name in domain_names:
             if type(name) != str:
                 raise ValueError('a tuple of domain names must be given')
         self.domain_names = domain_names
         super(ValueColumn, self).__init__(name, header)
</code>


Here the arguments should ideally be read only names, which would have cought 
the bug of reusing 'name' in the for loop.

But you really wouldn't code this as


<code>
class ValueColumn(AbstractColumn):
     def __init__(SELF, NAME, HEADER, DOMAIN_NAMES):
         if type(DOMAIN_NAMES) != tuple:
             raise ValueError('a tuple of domain names must be given')
         for name in DOMAIN_NAMES:
             # Argh, at this point 'name' is readonly, but cannot express that.
             if type(name) != str:
                 raise ValueError('a tuple of domain names must be given')
         SELF.domain_names = domain_names
         super(ValueColumn, SELF).__init__(NAME, HEADER)
</code>


Or, at least I wouldn't do that.

It's ugly.

And you have to /change the names/ as the readonly-ness changes.


Cheers,

- Alf