Re: [Tutor] eval and exec

[Brian van den Broek]

Brian van den Broek said unto the world upon 2004-12-04 04:28:
Marilyn Davis said unto the world upon 2004-12-04 01:37:
Hello Tutors,
I'm having trouble understanding the difference between eval and exec.
Can anyone explain it to me please?
Marilyn Davis
Hi Marilyn,
does this help?
<SNIP>
Darn. I left a few things that might help:
exec("a = 2 + 40")
exec("print a")
42
>>> eval('a')
42
>>>
As before, exec("a = 2 + 40") runs the code "a = 2 + 40", making 'a' 
point to 42.
Thus, exec("print a") is synonymous with:
>>> print a

*in the interpreter* "eval('a')" also gives 42. This is because, *in the 
interpreter*
>>> a
42

But, run this script:
exec('a=42')
exec('print a')
exec("print eval('a') == eval('21 * 2')")
eval('a')
a
OUTPUT:
>>> =========================== RESTART ===========================
>>>
42
True
>>>
*In a script*,
a
doesn't produce any output at all. This script does print 'True' because 
of the third line. It reads:

Run the sting "print eval('a') == eval('21 * 2')" as code.
So, print the expression you get by putting an '==' between the results 
of evaluating the expressions "a" and "21 * 2". Thus, print an 
expression equivalent to
42 == 42.

And its almost 5am and I've begun to worry I'm muddying the waters, 
rather than helping. It is to be hoped that someone will clean up any 
messes I have made. (Ken? Danny? . . . .)

Best,
brian vdB
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