eval() woes

[rdrink]

Ok, maybe now I can make some more sense of this, with an example of
real code (sorry if it's a bit dense):
This is the basic function...

def equate(parts,new_eq):

	oL = int(parts[0])
	iL = int(parts[1])
	iR = int(parts[2])
	oR = int(parts[3])
	oLoL =  int(str(oL)+str(oL))
	oLiL =  int(str(oL)+str(iL))
	oLiR =  int(str(oL)+str(iR))
	oLoR =  int(str(oL)+str(oR))
	iLoL =  int(str(iL)+str(oL))
	iLiL =  int(str(iL)+str(iL))
	iLiR =  int(str(iL)+str(iR))
	iLoR =  int(str(iL)+str(oR))
	iRoL =  int(str(iR)+str(oL))
	iRiL =  int(str(iR)+str(iL))
	iRiR =  int(str(iR)+str(iR))
	iRoR =  int(str(iR)+str(oR))
	oRoL =  int(str(oR)+str(oL))
	oRiL =  int(str(oR)+str(iL))
	oRiR =  int(str(oR)+str(iR))
	oRoR =  int(str(oR)+str(oR))

	new_seed = eval(new_eq)
	return new_seed

... into which is passed two items:
- 'parts' , which is a list e.g ['12','34','56','78'] (of strings)
- and 'new_eq',which is also a string read from a text file, e.g.
"pow(oLiL,2)"

And so...for the first 9 entries (of 480) in the text file where...
pow(oLiL,2)
pow(oLiL,2) - oL
pow(oLiL,2) - iL
pow(oLiL,2) - iR
pow(oLiL,2) - oR
pow(oLiL,2) + oL
pow(oLiL,2) + iL
pow(oLiL,2) + iR
pow(oLiL,2) + oR
pow(oLiL,2)
pow(oL - iL,2)
... eval() works fine.
But on the 10th...
pow(oL - iL,2) - oL
... it bombs with the error:
pow(oL - iL,2) - oL
Traceback (most recent call last):
  File "the_farmer2.py", line 264, in ?
    seed = equate(parts,equation)
  File "the_farmer2.py", line 112, in equate
    iL = int(parts[1])
ValueError: invalid literal for int(): -

And what is interseting/odd is:
- For the third, '- iL' evaluates fine...
- as well as the 9th, [where it is nested, oL - iL, inside pow()] ...
- but in the 10th, where a subtraction is called twice, it doesn't.
Which leads me to believe that the problem is either inside eval() it's
self, or in the way these variables are being cast... but I can't tell
which.

(BTW, as a footnote: For each of the above 'equations' the function
equate() was called 500 times... in some cases with the list 'parts'
equaling things like ['0',2','3','0'], so I have no reason to believe
that the problem is with the way the list is being passed in... but I
could be wrong)
 
Can anyone see something I can't?

Robb