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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: exec and locals
Date: Wed, 26 Feb 2014 14:46:39 +0100
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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?

eggs() should work in Python 2 and 3,
spam() should work in Python 2, but not in Python 3.

Fun fact: Python 2 tweaks the bytecode (LOAD_NAME instead of LOAD_GLOBAL) to 
make spam() work:

>>> def spam():
...     return x
... 
>>> dis.dis(spam)
  2           0 LOAD_GLOBAL              0 (x)
              3 RETURN_VALUE        
>>> def spam():
...     exec ""
...     return x
... 
>>> dis.dis(spam)
  2           0 LOAD_CONST               1 ('')
              3 LOAD_CONST               0 (None)
              6 DUP_TOP             
              7 EXEC_STMT           

  3           8 LOAD_NAME                0 (x)
             11 RETURN_VALUE