[Tutor] global variables/constants versus volatile variables/constants

[Albert-Jan Roskam]

----- Original Message -----

> From: Steven D'Aprano <steve at pearwood.info>
> To: tutor at python.org
> Cc: 
> Sent: Friday, June 13, 2014 1:45 PM
> Subject: Re: [Tutor] global variables/constants versus volatile	variables/constants
> 
> On Fri, Jun 13, 2014 at 12:51:25PM +0530, diliup gabadamudalige wrote:
>>  Hi All!
>>  Hope everyone is well.
>> 
>>  In my code there are many dictionaries and lists which are used in various
>>  functions. Is it better/pythonic/efficient to have these inside the
>>  function itself or declared at the beginning of the program in which case
>>  they will be global? They are all read only. I understand that global
>>  constants and variable have memory allocated to them but when declared
>>  inside a function are created on the fly, used and discarded. Please
>>  enlighten me further on this and correct me if i'm wrong.
> 
> A good question.
> 
> Depending on the size of these dictionaries and lists, worrying about 
> efficiency here may be premature optimization. As they say:
> 
> 
>    "We should forget about small efficiencies, say about 97% of 
>     the time: premature optimization is the root of all evil."
>     -- Donald Knuth

The other day I used collections.namedtuple and I re-initialized Record (see below) with every function*) call. Bad idea!
It looks nicer because I did not need a global (and globals are baaad, mkay?), but it was *much* slower. I processed a log of a few million lines, I think.

# bad --> time-consuming
import collections

def do_something_with(raw_record):
   Record = collections.namedtuple("_", " ".join("v%%03d" % i for i in range(100)))
   return Record(*raw_record.split())

# better --> even though it uses a global variable
import collections

Record = collections.namedtuple("_", " ".join("v%%03d" % i for i in range(100)))

def do_something_with(raw_record):
   return Record(*raw_record.split())



*) the function contained more code, of course! 

Albert-Jan