interactive execution

[Jive Dadson]

How does one execute arbitrary text as code within a module's context?

I've got some code that compiles some text and then executes it. When
the string is "print 'Hello'", it prints "Hello". I get no exception
when I compile and execute "foo = 555". If I then compile and exec
"print foo", I get a name error. The variable foo is undefined. My
assumption is that the "exec" command created a new namespace, put "foo"
in that namespace, and then threw the namespace away. Or something.

I know it must be possible to do this, because it's exactly what
programs like IDLE do.

Thankee.

 

[Jeff Shannon]

How does one execute arbitrary text as code within a module's context?

I've got some code that compiles some text and then executes it. When
the string is "print 'Hello'", it prints "Hello". I get no exception
when I compile and execute "foo = 555". If I then compile and exec
"print foo", I get a name error. The variable foo is undefined. My
assumption is that the "exec" command created a new namespace, put "foo"
in that namespace, and then threw the namespace away. Or something.

You can do

exec codestring in globaldict, localdict

(Or something like that, this is from unused memory and is untested.)
The net effect is that exec uses the subsequent dictionaries as its
globals and locals, reading from and writing to them as necessary.

(Note that this doesn't get you any real security, because malicious
code can still get to __builtins__ from almost any object...)

Jeff Shannon
Technician/Programmer
Credit International

 

[George Yoshida]

I've got some code that compiles some text and then executes it. When
the string is "print 'Hello'", it prints "Hello". I get no exception
when I compile and execute "foo = 555". If I then compile and exec
"print foo", I get a name error. The variable foo is undefined. My
assumption is that the "exec" command created a new namespace, put "foo"
in that namespace, and then threw the namespace away. Or something.

I know it must be possible to do this, because it's exactly what
programs like IDLE do.

exec statement in name_space
will do the trick.

>>> d = {}
>>> exec 'foo=555' in d
>>> d['foo'] 555
>>> exec "print foo" in d

555

- george

 

[Jive Dadson]

You can do

exec codestring in globaldict, localdict

(Or something like that, this is from unused memory and is untested.)
The net effect is that exec uses the subsequent dictionaries as its
globals and locals, reading from and writing to them as necessary.

(Note that this doesn't get you any real security, because malicious
code can still get to __builtins__ from almost any object...)

Jeff Shannon
Technician/Programmer
Credit International

Promising, but,

Traceback (most recent call last):
File "F:/C++ Projects/zardude/temp.py", line 9, in -toplevel-
exec "foo = 555" in globaldict, localdict
NameError: name 'globaldict' is not defined

 

[Jive Dadson]

Yeah. I got it.

exec "foo = 555" in globals(), locals() does the trick.

 

[Jeff Shannon]

Yeah. I got it.

exec "foo = 555" in globals(), locals() does the trick.

You can do it with your own dicts, too -- but they must already exist,
exec doesn't create them out of nowhere.

This gives you some control over what the exec'ed statement actually
sees, as well as what happens with the results. (But as I mentioned
before, there is no real security here if you're exec'ing arbitrary
code -- there's no sandboxing involved, and the exec'ed string *can*
use that __builtins__ reference (among other things) to do all sorts
of malicious stuff.)

Jeff Shannon
Technician/Programmer
Credit International