exec and locals

[Peter Otten]

Peter Otten wrote:

> 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

Some more bytcode fun, because it just occured to me that you can optimize 
away the code that triggered the modification:

>>> def spam():
...     return x
...     if 0: exec ""
... 
>>> dis.dis(spam)
  2           0 LOAD_NAME                0 (x)
              3 RETURN_VALUE