exec and locals

Steven D'Aprano steve+comp.lang.python at pearwood.info
Wed Feb 26 14:15:25 CET 2014


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?



-- 
Steven



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