tuples, index method, Python's design
tjreedy at udel.edu
Thu Apr 12 22:00:44 CEST 2007
"Alan Isaac" <aisaac at american.edu> wrote in message
news:HNuTh.2516$ok6.3 at trnddc07...
| I doubt that *anyone* who programs in Python
| has not encountered the situation where they change
| a tuple to a list *solely* for the purpose of getting
| access to the index method. This suggests a missing
| method, does it not? Who has not done this?
| Name yourself!
Me. Not that I can remember, anyway.
But perhaps I have just not done enough of the right sort of programming.
| There is simply no conflict between the index method and immutability,
| but at the moment one is forced to choose. Why? Nobody has
| offered a real explanation of this.
Where 'real' == 'one you accept as valid'.
| I offered a simple use case. Consider a game,
| where the *fixed* set p of players have a *fixed* order.
So the natural internal identifiers of the players are 0, ..., p-1 or, if
you prefer and are willing to waste a tiny bit of space, 1, ..., p. If you
only access players via a sequential collection object, then you will
access them by id. If you pass around player instances separate from their
container, than give them an id attribute.
I take it that in your game, no player, human or ai, ever gets eliminated
or leaves or has a connection drop.
| A tuple is natural.
That is partly what is under debate. Certainly you do not need a tuple for
indexing a dict.
| Now for a player you want to construct the opponents.
opponents = (op for op in players if op != player)
To do it once for everyone:
for player in players:
player.opponents = tuple(op for op in players if op != player)
| If I had the index i it wd be p[:i]+p[i+1:], but how to get the index?
You either use the index to get the player or get it from the player (see
opponents = p*
for i in range(p):
opponents[i] = players[:i] + players[i+1:]
opponents = tuple(opponents) # if you really care
for player in players:
i = player.id
player.opponents = players[:i] + players[i+1:]
It would never occur to me to get a player id by the O(n) scanning process.
| Other use cases have also been offered on this thread.
| What is the basic response? "Do not worry about the
| loss of immutability, just use a list." What kind of a
| response is that?? This sounds to me like "I do not
| really see the point of immutability", which is no
| response at all.
The main point of immutability is hashability by value (rather than by id).
Terry Jan Reedy
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