exec and locals

Alister alister.ware at ntlworld.com
Wed Feb 26 15:00:59 CET 2014


On Wed, 26 Feb 2014 13:15:25 +0000, Steven D'Aprano wrote:

> I have to dynamically generate some code inside a function using exec,
> but I'm not sure if it is working by accident or if I can rely on it.
> 
> Here is a trivial example:
> 
> 
> py> def spam():
> ...     exec( """x = 23""" )
> ...     return x ...
> py> spam()
> 23
> 
> 
> (My real example is more complex than this.)
> 
> According to the documentation of exec, I don't think this should
> actually work, and yet it appears to. The documentation says:
> 
>     The default locals act as described for function locals() below:
>     modifications to the default locals dictionary should not be
>     attempted. Pass an explicit locals dictionary if you need to see
>     effects of the code on locals after function exec() returns.
> 
> http://docs.python.org/3.4/library/functions.html#exec
> 
> 
> I *think* this means that if I want to guarantee that a local variable x
> is created by exec, I need to do this instead:
> 
> py> def eggs():
> ...     mylocals = {}
> ...     exec( """x = 23""", globals(), mylocals)
> ...     x = mylocals['x']
> ...     return x ...
> py> eggs()
> 23
> 
> The fact that it works in spam() above is perhaps an accident of
> implementation? Yes no maybe?

I have no idea but as exec is generally considered to be a bad idea are 
you absolutely sure this is the correct way to achieve your end goal?

perhaps if you detailed your requirement someone may be able to suggest a 
safer solution.


-- 
"Regardless of the legal speed limit, your Buick must be operated at
speeds faster than 85 MPH (140kph)."
-- 1987 Buick Grand National owners manual.



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