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Sat Oct 1 08:46:09 EDT 2016 · https://mail.python.org/pipermail/python-dev/2016-October/146576.html

Ah, interesting!

Thanks for the clarification. So it is really possible to write
code with an implicit future statement in it, or to switch the behavior
off.
Good to know. I will probably not use it, since I can't decide on a good
default, but getting rid of print_statement is tempting...

 > https://docs.python.org/2/reference/simple_stmts.html#the-exec-statement
 >
 > [2] When strings are executed, __future__ directives active in the
 > surrounding context will be active for the compiled code also. If this
 > is not desired, see the compile() function's dont_inherit parameter.
 >
 > Would that clarify?

Yes please, that would be a good place to document it. For some reason
I did not look up __future__.

Thanks -- Chris

On 01/10/16 14:17, Chris Angelico wrote:
> On Sat, Oct 1, 2016 at 9:39 PM, Christian Tismer <tismer at stackless.com> wrote:
>> The exec() script inherited the __future__ statement!
>> It behaved like the future statement were implicitly there.
>>
>> Is that a bug or a feature?
>
> It's documented, but not very noisily.
>
> https://docs.python.org/2/reference/simple_stmts.html#future-statements
>
> So if you want to isolate the execution environments, you can use
> compile() explicitly. Without isolation:
>
> Python 2.7.12+ (default, Sep  1 2016, 20:27:38)
> [GCC 6.2.0 20160822] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>> exec(compile("print('hello','world')", "-", "exec"))
> ('hello', 'world')
>>>> exec(compile("from __future__ import print_function; print('hello','world')", "-", "exec"))
> hello world
>>>> from __future__ import print_function
>>>> exec(compile("print('hello','world')", "-", "exec"))
> hello world
>>>> exec(compile("from __future__ import print_function; print('hello','world')", "-", "exec"))
> hello world
>
> With isolation:
>
>>>> exec(compile("print('hello','world')", "-", "exec", 0, 1))
> ('hello', 'world')
>>>> exec(compile("from __future__ import print_function; print('hello','world')", "-", "exec", 0, 1))
> hello world
>
> So I'd call it a feature, but possibly one that warrants a mention in
> the exec and eval docs. Maybe something like:
>
> https://docs.python.org/2/reference/simple_stmts.html#the-exec-statement
>
> [2] When strings are executed, __future__ directives active in the
> surrounding context will be active for the compiled code also. If this
> is not desired, see the compile() function's dont_inherit parameter.
>
> Would that clarify?
>
> ChrisA
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