Using eval with substitutions

[Anthra Norell]

Abhishek,

      I hesitate to propose this idea, because there's got to be a better (more conventional) way of doing this. Anyway consider
this:

>>> x = "a+b"; y = "x*a; z = "x+y"   # Your definitions
>>> import SE
>>> def  f  (x, y, z):
         substitutions = 'z=(%s) | y=(%s) | x=(%s)' % (z, y, x)
         print 'substitutions:', substitutions
         Formula_Expander = SE.SE (substitutions)
         formula = 'x + y + z'
         expanded_formula = Formula_Expander (formula)
         print 'expanded_formula:', expanded_formula
         a = 3; b = 4
         print 'eval (expanded_formula):', eval (expanded_formula)
         w = eval (Formula_Expander ('y * b'))
         print 'w', w

>>> f (x, y, z)
substitutions: z=(x+y) | y=(x*a) | x=(a+b)
expanded_formula: (a+b) + ((a+b)*a) + ((a+b)+((a+b)*a))
eval (expanded_formula): 56
w 84

Well--- it's kind of neat the way it expands the formula. But why not just write functions? As you say, the expansion has to be done
in exactly the right order to complete.
      If you want to run this example you'll find SE here: http://cheeseshop.python.org/pypi/SE/2.2%20beta

Regards

Frederic



----- Original Message -----
From: <abhishek at ocf.berkeley.edu>
Newsgroups: comp.lang.python
To: <python-list at python.org>
Sent: Thursday, August 31, 2006 1:15 PM
Subject: Using eval with substitutions


> >>> a,b=3,4
> >>> x="a+b"
> >>> eval(x)
> 7
> >>> y="x+a"
>
> Now I want to evaluate y by substituting for the evaluated value of x.
> eval(y) will try to add "a+b" to 3 and return an error. I could do
> this,
> >>> eval(y.replace("x",str(eval(x))))
> 10
>
> but this becomes unwieldy if I have
> >>> w="y*b"
> and so on, because the replacements have to be done in exactly the
> right order. Is there a better way?
>
> Thanks,
> Abhishek
>
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> http://mail.python.org/mailman/listinfo/python-list