Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Chris Angelico <rosuav@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: How well do you know Python?
Date: Tue, 5 Jul 2016 19:21:47 +1000
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On Tue, Jul 5, 2016 at 6:36 PM, Peter Otten <__peter__@web.de> wrote:
> What will
>
> $ cat foo.py
> import foo
> class A: pass
> print(isinstance(foo.A(), A))
> $ python -c 'import foo'
> ...
> $ python foo.py
> ...
>
> print?

I refuse to play around with isinstance and old-style classes.
Particularly when circular imports are involved. Run this under Python
3 and/or explicitly subclass object, and then I'd consider it. :)

> It looks like
>
> $ python3 -c 'print({1, 2})'
> {1, 2}
> $ python3 -c 'print({2, 1})'
> {1, 2}
>
> will always print the same output. Can you construct a set from two small
> integers where this is not the case? What's the difference?

Given that the display (iteration) order of sets is arbitrary, I'm not
sure what the significance would ever be, but my guess is that the
display order would be the same for any given set, if constructed this
way. But it sounds as if you know of a set that behaves differently.

> What happens if you replace the ints with strings? Why?

Then hash randomization kicks in, and you can run the exact same line
of code multiple times and get different results. It's a coin toss.

rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'1', '2'}
rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'1', '2'}
rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'1', '2'}
rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'2', '1'}
rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'2', '1'}
rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'1', '2'}
rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'2', '1'}
rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'1', '2'}
rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'2', '1'}
rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'1', '2'}
rosuav@sikorsky:~$ python3 -c 'print({"1", "2"})'
{'1', '2'}

ChrisA