[Python-Dev] future and eval() (original) (raw)

Christian Tismer tismer at stackless.com
Sat Oct 1 08:46:09 EDT 2016

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/simplestmts.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 printfunction; print('hello','world')", "-", "exec")) hello world from future import printfunction exec(compile("print('hello','world')", "-", "exec")) hello world exec(compile("from future import printfunction; print('hello','world')", "-", "exec")) hello world With isolation:

exec(compile("print('hello','world')", "-", "exec", 0, 1)) ('hello', 'world') exec(compile("from future import printfunction; 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/simplestmts.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 dontinherit parameter. Would that clarify? ChrisA

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