How to calculate mathematical term packed in a string (python)

Question

I'm having a problem which I had for ages without finding a solution.

The code is this:

    Df = range(100)
    function = '3*x+4'
    for i in Df:
        listoffunction = list(function) #Will return ['3', '*', ...]
        for obj in listoffunction:
            if obj == 'x' :
                obj = i
        resultfori = ''
        for part in listoffunction:
            resultfori.join(part) #until here everything is fine.
                                  #resultfori is the following: '3*i+4'
                                  #with i the number of Df

Here is my problem: How can I calculate this term? I've tried it with the exec command but it always returns None. Why? I want it for example if i is 3 to return 13. Can you help me?

Thanks a lot!

Why 12? `3*3+4` could reasonable be evaluated to 13 or 21, but I don't see a 12 coming out of that. — zondo, Mar 12 '16 at 13:37
Have you heard of SymPy? And you may want to try `eval` instead of `exec` — OneCricketeer, Mar 12 '16 at 13:37
Related: "`exec` ignores the return value and always returns `None`" http://stackoverflow.com/a/29456463/2308683. — OneCricketeer, Mar 12 '16 at 13:41

score 0 · Accepted Answer · answered Mar 12 '16 at 13:41

0

try something like this:

x = input()
s = '3*x + 4'
print eval(s)

answered Mar 12 '16 at 13:41

Sanindra

70
7

I had to adapt the code a bit but the system behind the code worked perfectly fine! thx! – h_e_u_r_e_k_a Mar 12 '16 at 15:20

How to calculate mathematical term packed in a string (python)

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