[Python-Dev] __future__ and eval()

[Christian Tismer]

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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