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Date: Tue, 12 Feb 2013 15:15:14 +0000
From: MRAB <python@mrabarnett.plus.com>
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To: python-list@python.org
Subject: Re: Python3 exec locals - this must be a FAQ
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On 2013-02-12 13:27, 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.
>
>> 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.
>
> 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.
>
The docs for locals() warns """The contents of this dictionary should
not be modified; changes may not affect the values of local and free
variables used by the interpreter."""

That's because local variables in a function are implemented (at least
in CPython) using 'slots' for efficiency reasons.

 >>> def example():
     a = 1
     print("a is {}, locals() is {}".format(a, locals()))
     print("Changing a to 2 directly")
     a = 2
     print("a is {}, locals() is {}".format(a, locals()))
     print("Changing a to 3 via locals()")
     locals()["a"] = 3
     print("a is {}, locals() is {}".format(a, locals()))
     locals()["b"] = 0
     print("locals() is {}".format(locals()))
     print("b is {}".format(b))

 >>> example()
a is 1, locals() is {'a': 1}
Changing a to 2 directly
a is 2, locals() is {'a': 2}
Changing a to 3 via locals()
a is 2, locals() is {'a': 2}
locals() is {'b': 0, 'a': 2}
Traceback (most recent call last):
   File "<pyshell#39>", line 1, in <module>
     example()
   File "<pyshell#38>", line 12, in example
     print("b is {}".format(b))
NameError: global name 'b' is not defined

Note how attempting to change variable "a" in locals() is ignored, and
how adding variable "b" to locals() doesn't actually make it a local
variable (it raises a NameError).