exec with partial globals

[Helmut Jarausch]

On Tue, 30 Oct 2012 08:33:38 -0400, Dave Angel wrote:

> On 10/30/2012 08:00 AM, Helmut Jarausch wrote:
>> Hi,
>>
>> I'd like to give the user the ability to enter code which may only rebind
>> a given set of names but not all ones.
>> This does NOT work
>> A=1
>> B=2
>> Code=compile('A=7','','exec')
>> exec(Code,{'A':0})
>> print("I've got A={}".format(A)) # prints 1
>>
>>
>> How can 'filter' the gobal namespace such that modifying 'A' is allowed
>> but any attempt to modify 'B' should give an exception.
>>
>>
>> Many thanks for a hint,
>> Helmut.
> 
> A=1
> B=2
> Code=compile('A=7','','exec')
> vars = {'A':A}
> exec(Code, vars)
> A = vars["A"]
> print("I've got A={}".format(A)) # prints 1
> 
> That now prints "I've got A=7"
> 
> More generally, you could write a loop, copying globals into vars, and
> another one, copying them back.
> 
> No idea what you're really after;  this is one of the more dangerous
> things to try.
> 
> Although you can constrain the globals seen by the code, that code can
> still use builtins, do imports, delete files, etc.
> 
> Further, if your user is clever enough, he can do:
> 
> Code=compile('A=7; print("howdy"); import __main__;
> __main__.B=42','','exec')
> 
> What are you really trying to permit him to do?  Initialize some
> variables for you?  How about an .ini file ?

Many thanks Chris, many thanks to Dave.

First, in my application only trusted people will use it.

Here my example. I'd like to update a spreadsheet by data given by another
spreadsheet. I like to allow to modify only certain columns of the first
spreadsheet. The update formula and possible conditions are entered at 
run time and the available fields are only known once the spreadsheets 
have been read.

Given spreadsheet  S (Source) and D (Destination) as objects (wrapping a 
dictionary) a possible (legal) input would be

D.price= D.price-S.discount

No other fields of 'D' should be modifiable.

Again, I don't need to take actions against a malicious user,
just take care about a typo or mistake

Thanks again,
Helmut.