Question about math.pi is mutable

[Steven D'Aprano]

On Sun, 8 Nov 2015 11:28 pm, Chris Angelico wrote:

> On Sun, Nov 8, 2015 at 10:19 PM, BartC <bc at freeuk.com> wrote:
>> I've never understood why this seems to be necessary in Python. Why do
>> names have to be looked up? (I'm assuming this is searching by name in
>> some sort of table.)
> 
> Yes, if by "searching" you include hash-table lookup. A CPython
> dictionary is a *highly* optimized data structure, specifically
> because it's used virtually everywhere (I understand Lua's "table"
> type has similar optimizations for the same reason). In the common
> case, where your names come from literal text in the module, the
> strings used for the lookup will be interned constants, and their
> hashes will have been precalculated and stored, so the lookup is
> pretty easy. So it's a constant-time operation, and while that
> constant may be larger than a simple offset-and-fetch, it's still
> pretty fast in the overall scheme of things.

*Usually*.

There are pathological cases -- if you're unlucky enough, or malicious
enough, to have a whole lot of names with the same hash value, the look-up
degrades to a linear search.

More importantly, consider the case of a function referring to a variable
named "spam". In principle, at least, it may need to:

- search the local namespace;
- search the namespace of one or more enclosing functions;
- search the global namespace;
- search the built-in namespace;

before locating the variable's value. In practice, most Python
implementations will provide at least one optimization: local variables are
recognised and searched using an offset-and-fetch model.

Similarly for method calls: spam.eggs needs to:

- search the instance dict and slots;
- search the class dict;
- search the dict for each additional superclass;

where searching involves more than just a hash table lookup. It also
involves checking the classes for __getattribute__ and __getattr__ methods,
checking for descriptors, and more. The whole lookup process is quite
complicated, and little of it can be done at compile time.



-- 
Steven