exec and locals
Peter Otten
__peter__ at web.de
Wed Feb 26 08:46:39 EST 2014
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
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