Tuples and immutability

Chris Angelico rosuav at gmail.com
Sat Mar 1 02:39:05 CET 2014


On Sat, Mar 1, 2014 at 11:22 AM, Mark H. Harris <harrismh777 at gmail.com> wrote:
> lists within a tuple should be converted to tuples.    If you want a tuple to hold a list,  make it a list in the first place.  Tuples should not be changed... and as you point out... half changing a tuple is not a good condition if there is an exception...
>

A tuple is perfectly fine containing a list. If you want a tuple to be
"recursively immutable", then you're talking about hashability, and
*then* yes, you need to convert everything into tuples - but a tuple
is not just an immutable list. The two are quite different in pupose.

> I really think this is a bug; honestly.  IMHO it should be an error to use  +=  with an immutable type and that means not at all.  In other words,  the list should not even be considered, because we're talking about changing a tuple... which should not be changed (nor should its members be changed).
>

Definitely not! Incrementing an integer with += is a perfectly normal
thing to do:

x = 5
x += 1

It's just a matter of knowing the language and understanding what's going on.

ChrisA



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