Re: Non-deterministic set ordering
On 2022-05-16 04:20, Rob Cliffe via Python-list wrote:
On 16/05/2022 04:13, Dan Stromberg wrote:
On Sun, May 15, 2022 at 8:01 PM Rob Cliffe via Python-list
wrote:
I was shocked to discover that when repeatedly running the following
program (condensed from a "real" program) under Python 3.8.3
for p in { ('x','y'), ('y','x') }:
print(p)
the output was sometimes
('y', 'x')
('x', 'y')
and sometimes
('x', 'y')
('y', 'x')
Can anyone explain why running identical code should result in
traversing a set in a different order?
Sets are defined as unordered so that they can be hashed internally to
give O(1) operations for many tasks.
It wouldn't be unreasonable for sets to use a fixed-by-arbitrary
ordering for a given group of set operations, but being unpredictable
deters developers from mistakenly assuming they are ordered.
If you need order, you should use a tuple, list, or something like
https://grantjenks.com/docs/sortedcontainers/sortedset.html
Thanks, I can work round this behaviour.
But I'm curious: where does the variability come from? Is it deliberate
(as your answer seems to imply)? AFAIK the same code within the *same
run* of a program does produce identical results.
Basically, Python uses hash randomisation in order to protect it against
denial-of-service attacks. (Search for "PYTHONHASHSEED" in the docs.)
It also applied to dicts (the code for sets was based on that for
dicts), but dicts now remember their insertion order.
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Re: Non-deterministic set ordering
Thanks, Paul. Question answered!
Rob Cliffe
On 16/05/2022 04:36, Paul Bryan wrote:
This may explain it:
https://stackoverflow.com/questions/27522626/hash-function-in-python-3-3-returns-different-results-between-sessions
On Mon, 2022-05-16 at 04:20 +0100, Rob Cliffe via Python-list wrote:
On 16/05/2022 04:13, Dan Stromberg wrote:
On Sun, May 15, 2022 at 8:01 PM Rob Cliffe via Python-list
wrote:
I was shocked to discover that when repeatedly running the following
program (condensed from a "real" program) under Python 3.8.3
for p in { ('x','y'), ('y','x') }:
print(p)
the output was sometimes
('y', 'x')
('x', 'y')
and sometimes
('x', 'y')
('y', 'x')
Can anyone explain why running identical code should result in
traversing a set in a different order?
Sets are defined as unordered so that they can be hashed internally to
give O(1) operations for many tasks.
It wouldn't be unreasonable for sets to use a fixed-by-arbitrary
ordering for a given group of set operations, but being unpredictable
deters developers from mistakenly assuming they are ordered.
If you need order, you should use a tuple, list, or something like
https://grantjenks.com/docs/sortedcontainers/sortedset.html
Thanks, I can work round this behaviour.
But I'm curious: where does the variability come from? Is it deliberate
(as your answer seems to imply)? AFAIK the same code within the *same
run* of a program does produce identical results.
Best wishes
Rob Cliffe
--
https://mail.python.org/mailman/listinfo/python-list
Re: Non-deterministic set ordering
This may explain it:
https://stackoverflow.com/questions/27522626/hash-function-in-python-3-3-returns-different-results-between-sessions
On Mon, 2022-05-16 at 04:20 +0100, Rob Cliffe via Python-list wrote:
>
>
> On 16/05/2022 04:13, Dan Stromberg wrote:
> >
> > On Sun, May 15, 2022 at 8:01 PM Rob Cliffe via Python-list
> > wrote:
> >
> > I was shocked to discover that when repeatedly running the
> > following
> > program (condensed from a "real" program) under Python 3.8.3
> >
> > for p in { ('x','y'), ('y','x') }:
> > print(p)
> >
> > the output was sometimes
> >
> > ('y', 'x')
> > ('x', 'y')
> >
> > and sometimes
> >
> > ('x', 'y')
> > ('y', 'x')
> >
> > Can anyone explain why running identical code should result in
> > traversing a set in a different order?
> >
> >
> > Sets are defined as unordered so that they can be hashed internally
> > to
> > give O(1) operations for many tasks.
> >
> > It wouldn't be unreasonable for sets to use a fixed-by-arbitrary
> > ordering for a given group of set operations, but being
> > unpredictable
> > deters developers from mistakenly assuming they are ordered.
> >
> > If you need order, you should use a tuple, list, or something like
> > https://grantjenks.com/docs/sortedcontainers/sortedset.html
> Thanks, I can work round this behaviour.
> But I'm curious: where does the variability come from? Is it
> deliberate
> (as your answer seems to imply)? AFAIK the same code within the
> *same
> run* of a program does produce identical results.
> Best wishes
> Rob Cliffe
--
https://mail.python.org/mailman/listinfo/python-list
Re: Non-deterministic set ordering
On 16/05/2022 04:13, Dan Stromberg wrote:
On Sun, May 15, 2022 at 8:01 PM Rob Cliffe via Python-list
wrote:
I was shocked to discover that when repeatedly running the following
program (condensed from a "real" program) under Python 3.8.3
for p in { ('x','y'), ('y','x') }:
print(p)
the output was sometimes
('y', 'x')
('x', 'y')
and sometimes
('x', 'y')
('y', 'x')
Can anyone explain why running identical code should result in
traversing a set in a different order?
Sets are defined as unordered so that they can be hashed internally to
give O(1) operations for many tasks.
It wouldn't be unreasonable for sets to use a fixed-by-arbitrary
ordering for a given group of set operations, but being unpredictable
deters developers from mistakenly assuming they are ordered.
If you need order, you should use a tuple, list, or something like
https://grantjenks.com/docs/sortedcontainers/sortedset.html
Thanks, I can work round this behaviour.
But I'm curious: where does the variability come from? Is it deliberate
(as your answer seems to imply)? AFAIK the same code within the *same
run* of a program does produce identical results.
Best wishes
Rob Cliffe
--
https://mail.python.org/mailman/listinfo/python-list
Re: Non-deterministic set ordering
On Sun, May 15, 2022 at 8:01 PM Rob Cliffe via Python-list <
python-list@python.org> wrote:
> I was shocked to discover that when repeatedly running the following
> program (condensed from a "real" program) under Python 3.8.3
>
> for p in { ('x','y'), ('y','x') }:
> print(p)
>
> the output was sometimes
>
> ('y', 'x')
> ('x', 'y')
>
> and sometimes
>
> ('x', 'y')
> ('y', 'x')
>
> Can anyone explain why running identical code should result in
> traversing a set in a different order?
>
Sets are defined as unordered so that they can be hashed internally to give
O(1) operations for many tasks.
It wouldn't be unreasonable for sets to use a fixed-by-arbitrary ordering
for a given group of set operations, but being unpredictable deters
developers from mistakenly assuming they are ordered.
If you need order, you should use a tuple, list, or something like
https://grantjenks.com/docs/sortedcontainers/sortedset.html
--
https://mail.python.org/mailman/listinfo/python-list
Non-deterministic set ordering
I was shocked to discover that when repeatedly running the following
program (condensed from a "real" program) under Python 3.8.3
for p in { ('x','y'), ('y','x') }:
print(p)
the output was sometimes
('y', 'x')
('x', 'y')
and sometimes
('x', 'y')
('y', 'x')
Can anyone explain why running identical code should result in
traversing a set in a different order?
Thanks
Rob Cliffe
--
https://mail.python.org/mailman/listinfo/python-list
