Thoughts about Python

[Mike C. Fletcher]

Marco Aschwanden wrote:
...

>'(3, 4, 20.010100000000001)'  # 8o)
>What is not unique about this string?
>Okay, I can think of cases where this approach is problematic -
>foremost when sorting is involved.
>  
>
Actually, this is the classic problem of data-escaping.  It's *possible* 
to guarantee unique values when encoding as a string, but it's a *lot* 
harder than just doing str( data ).  You need to make sure that 
different things are going to show up with different values:

    d = {}
    myWorkingList = (3,4,20.010100000000001)
    somePieceOfDataFromSomewhere = '(3, 4, 20.010100000000001)'   
    d[ myWorkingList ] = 32
    d[ somePieceOfDataFromSomewhere ] = 34

That is, you'd need to encode into your string information regarding the 
*type* and internal structure of the object to avoid collisions where 
something just happens to look the same as another object, so something 
like:

    '(I3\nI4\nF20.010100000000001\nt.' # for the tuple and
    "S'(3,4,20.010100000000001)'\np1\n." # for the string

Thing is, creating those static string representations is a major PITA 
(in case you're wondering, the above are pickles of the values), and 
would waste a lot of memory and processing cycles (to build the safe 
string representation, (a task which is somewhat involved)) compared to 
just using a pointer to the object and the object's built-in __hash__ 
and comparison methods.

>It is also a bit complicated to retrieve the values from the string
>(it has to be "split"ted or "eval"ed back to a tuple or list).
>If this pattern would be seldom used... but it seems, that everybody
>except me uses this dicts as keys.
>  
>
Round-trip-eval-capable objects are considered good style, but are not 
by any means universal even among the built-in objects:

     >>> x = object()
     >>> x
    <object object at 0x00C7D3B8>

pickle/cPickle is closer to the idea of a robust mechanism for coercion 
to/from string values, but even it has its limits, particularly with 
respect to older built-in types that assumed they'd never be pickled.

wrt sorting, it's not really a huge issue, dictionaries are not ordered, 
so to produce a sorted set you'd need to produce the keys as a 
list/sequence of reconstituted objects anyway.

Have fun, and enjoy yourself,
Mike

_______________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://members.rogers.com/mcfletch/