Question about math.pi is mutable

[BartC]

On 08/11/2015 18:01, Chris Angelico wrote:
> On Mon, Nov 9, 2015 at 4:54 AM, BartC <bc at freeuk.com> wrote:
>> That would be true for locals as well. But as far as I can tell from the
>> CPython source code, byte-codes uses an index to represent a local, which
>> represents an entry into a linear table.
>>
>> I can't quite see why that can't be done for global names and for attributes
>> too (ie. the names that follow a ".").
>
> At compilation time, the set of local names is locked in permanently.
> That's not true of any other namespace (except nonlocals, which are
> still locals, just not yours). Attributes and global names (which are
> attributes of the current module) can be added and removed
> dynamically, so they can't be assigned to slots; attributes can even
> be simulated via __getattr__ and __getattribute__.

Yes, that's yet another way that the language design makes things 
difficult (for those who want to streamline its implementation).

For most purposes, the set of attribute names that can follow a dot are 
limited and fixed, by the occurrences of such names in a source file. So 
they can be assigned a slot.

But then, you say that additional attributes, potentially millions of 
different ones, can be invented at runtime. Although I don't see how it 
can remove names that are part of the source code: if "A.B" is in the 
file, then surely "A" and "B" always have to be present in some table or 
other.

-- 
Bartc