John Nagle wrote:
In the beginning, strings, tuples, and numbers were immutable, and
everything else was mutable. That was simple enough. But over time,
Python has acquired more immutable types - immutable sets and immutable
byte arrays. Each of these is a special case.
<snip>
Immutability is interesting for threaded programs, because
immutable objects can be shared without risk. Consider a programming
model where objects shared between threads must be either immutable or
"synchronized" in the sense that Java uses the term. Such programs
are free of most race conditions, without much programmer effort to
make them so.
Of course, tuples would still be a special case because they
may contain mutable objects. You need to check they're
immutable all the way down.
Nothing to do with threading, but it's also the cause of
this weirdness:
http://bytes.com/topic/python/answers/752154-list-tuple
>>a = ([1], 2)
>>a[0] += [3]
succeeds, but raises an error.
Graham
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