exec with partial globals

[Helmut Jarausch]

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.

 

[Chris Angelico]

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.

How can 'filter' the gobal namespace such that modifying 'A' is allowed
but any attempt to modify 'B' should give an exception.

I don't know of any way to do that _as such_, but you can simply
follow up the exec with direct retrieval from the globals.

a=1; b=2
code=compile("a=7",'','exec')
ns={'a':0}
exec(code,ns)
a=ns["a"]

(Incidentally, it's normal to use lower case for most names, reserving
the leading uppercase letter for types/classes.)

The exec'd code gets its own namespace (defined by the dictionary,
same as you were doing - note that the 'a' inside that namespace has
nothing to do with the 'a' outside it), and then you explicitly fetch
the values you want.

A slightly more sophisticated example might include a list of shared
variables, for example:

shared = ['a']
outer = globals()
ns = {vuter[v] for v in shared}
exec(code,ns);
for v in shared: outer[v]=ns[v]

Untested but should work. (Is there any way to directly apply filter()
to a dictionary? That's what I'm really looking for here.)

ChrisA

 

[Dave Angel]

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 ?

 

[Helmut Jarausch]

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.

 

[Chris Angelico]

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.

That's a bit harder. What you're describing, using exec with specific
globals, would not be able to do this. You'd need to build a proxy
object that decides whether or not to pass on the change.

ChrisA

 

[Dave Angel]

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.

If the user will only be modifying fields of specific class instances,
then generate those instances with property wrappers around the readonly
attributes.

If the class were statically defined, you'd do something like:


class MyClass(object):
def __init__(self, data1, data2, data3):
self._data1 = data1
self._data2 = data2
self.data3 = data3 #this one is writable, directly

@property
def data1(self): #this is readonly
return self._data1

@property
def data2(self): #this is readonly
return self._data2

mysrc = MyClass(1,2,3)
mydest = MyClass(4,5,6)

print( mydest.data1, mydest.data2, mydest.data3, "\n") #prints 4,5,6

vars = {"src": mysrc, "dest": mydest}

Code=compile('dest.data3=17','','exec')
exec(Code, vars)

print( mydest.data1, mydest.data2, mydest.data3, "\n") #prints 4,5,17


Now, once you see how to do it statically, you can try to do it dynamically, deciding which attributes need property wrappers, and which ones are plain data.