How to use the exec statement

[Alex Martelli]

JB wrote:
        ...
>> can avoid the worst problems: build an artificial
>> namespace (i.e., a suitably prepared dictionary) in which
>> these statements are to execute -- populate it with
>> whatever set of names you think the user will need (module
>> r_exec is also a possibility).
> 
> How can I do this?

namespace = {}
# populate dict namespace with whatever you want, e.g.:
import math
namespace.update(math.__dict__)
# etc etc


Now, when it comes time to exec the user-given statement
X,
    exec X in namespace

This way the namespace will be the same from one such
statement to the next.  If you DON'T want that, e.g.:

    new_namespace = namespace.copy()
    exec X in new_namespace


Now, if needed, you can examine namespace or new_namespace
for any significant changes (settings/un-settings) that you want
to take into account.


BTW, you wouldn't have to write a parser anyway -- there are
some you can use already in the Python standard libraries, and
others you can download.  But exec isn't too bad if [a] Python is
the right language for the user to be using and [b] you use exec
in a well-controlled manner.  You'll probably want to have a
try/except around the exec statement, for more precise error
messages than just a program-crash, and if you need to exec
the same statement repeatedly, look into built-in "compile"
(you can compile the statement once, getting a code-object,
then exec the code-object repeatedly).


Alex