Evaluating symbolic expression, when some variables are finally fixed

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Thanks to other answers, which concentrated on particular example, I wanted to explain better what I'm after (i.e. the general solution), and in that process I've got the answer:

I = eval(preparse(str(I)))

where

1. str(I) converts the 'I' expression to string
2. preparse(...) preparses the code of expression in the sage style ('^' as power, and so on)
3. eval(...) finally evaluates expression (in particular example: taking into account that R is already set to 3)

The code thus would be:

V = var('V')
R = var('R')
I = V/R
R = 3  # this is one line representation of possible several lines of code
       # to compute value in general case. Let's also note that the value can 
       # even be provided externally (read from file, or so) 
I = eval(preparse(str(I)))
plot(I)

and having in mind even more general solution with evaluate() definition:

def evaluate(expression):
    return eval(preparse(str(expression)))

V = var('V')
R = var('R')
I = V/R
R = 3  # this is one line representation of possible several lines of code
       # to compute value in general case. Let's also note that the value can 
       # even be provided externally (read from file, or so) 
I = evaluate(I)
plot(I)

Few days later:
There is a small improvement possible - by adding simplify(), and so making the whole thing
I = simplify(eval(preparse(str(I))))

Please let know if the code could be more elegant (pythonic or sageonic) and if there are some disadvantages or threads in such solution