In article <50a911ec$0$29978$c3e8da3$54964...@news.astraweb.com>,
Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> wrote:
> Oh I'm sorry, did something I say suggest that the couple of examples I
> gave are the *only* acceptable uses? My apologies for not giving an
> exhaustive list of every possible use of lists and tuples, I'll be sure
> to punish myself severely for the lapse.
Hmmm. I didn't mean any offense. I was just pointing out that what's
true in theory and what's true in practice aren't always the same.
> Under what sort of circumstances would somebody want to take a mutable
> list of data, say a list of email addresses, freeze it into a known
> state, and use that frozen state as a key in a dict?
I've got a script which trolls our log files looking for python stack
dumps. For each dump it finds, it computes a signature (basically, a
call sequence which led to the exception) and uses this signature as a
dictionary key. Here's the relevant code (abstracted slightly for
readability):
def main(args):
crashes = {}
[...]
for line in open(log_file):
if does_not_look_like_a_stack_dump(line):
continue
lines = traceback_helper.unfold(line)
header, stack = traceback_helper.extract_stack(lines)
signature = tuple(stack)
if signature in crashes:
count, header = crashes[signature]
crashes[signature] = (count + 1, header)
else:
crashes[signature] = (1, header)
You can find traceback_helper at
https://bitbucket.org/roysmith/python-tools/src/4f8118d175ed/logs/traceba
ck_helper.py
The stack that's returned is a list. It's inherently a list, per the
classic definition:
* It's variable length. Different stacks have different depths.
* It's homogeneous. There's nothing particularly significant about each
entry other than it's the next one in the stack.
* It's mutable. I can build it up one item at a time as I discover them.
* It's ordered. f1(f2()) is not the same as f2(f1()).
But, to use it as a dictionary key, I need to make it into a tuple,
since keys have to be immutable.
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