Python3 exec locals - this must be a FAQ

Helmut Jarausch jarausch at skynet.be
Tue Feb 12 15:29:08 CET 2013


On Tue, 12 Feb 2013 08:27:41 -0500, Dave Angel wrote:

> On 02/12/2013 06:46 AM, Helmut Jarausch wrote:
>> Hi,
>>
>> I've tried but didn't find an answer on the net.
>>
>> The exec function in Python modifies a copy of locals() only.
>> How can I transfer changes in that local copy to the locals of my
>> function ** without **  knowing the names of these variables.
>>
>> E.g.  I have a lot of local names.
>>
>> Doing
>>
>> _locals= locals()
> 
> This doesn't copy everything.  But perhaps you know that and you're just
> testing us.

No, I didn't know. And I'm bit surprised since this is recommend
several times, e.g. in "Python Essential Reference, 4th ed" by
David Beazley.

> 
>> expr=compile(input('statements assigning to some local variables '),
>>                                                   'user input','exec')
>> exec(expr,globals(),_locals)
>>
>> How can I "copy" the new values within _locals to my current locals.
>>
>> If I knew that  Var1  has changed I could say Var1 = _locals['Var1']
>> but what to do in general?
> 
> locals()["Var1"] = _locals["Var1"]  will set the same Var1 local.

Thanks for this hint which surprises me again since I thought
locals() by itself is a copy only.

> 
> So you might write a loop on _locals.
> 
> But beware if someone has deleted one of the "variables" it may not do
> what you'd like.  You cannot necessarily add back a local with the above
> syntax.

Does this mean that adding something completely new won't work?

Many thanks,
Helmut.




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