Why Pythons eval() mistakes?

Question

I need to solve some mathematical equations, something like below (but each time different formula):

formula="(2/10^8*x^2)+0.0131*x-1017.3-30"

where x is an integer.

I used eval() function to solve the problem. Function gave me an exception of:

TypeError: unsupported operand type(s) for ^: 'float' and 'int'

I solved it like this:

formula=formula.replace('^','**')

Now, I encountered with another problem.

eval("2/10")

returns 0 instead I need 0.2, as a result I get a wrong outcome. I appreciate for any answer.

btw you do know ^ is for bitwise XOR

jamylak
– jamylak

2013-04-30 12:04:58 +00:00
Commented Apr 30, 2013 at 12:04 — jamylak
– jamylak, Commented Apr 30, 2013 at 12:04

Zaur Nasibov · Accepted Answer · 2013-04-30 11:59:49Z

3

Well, in Python 2.x int / int always returns an integer. Use Python 3 or explicitly write one of the arguments as a float, e.g. 2.0 / 10, or (as Marjin Pieters reminds us), import the explicit Py3-like division behaviour as: from __future__ import division.

answered Apr 30, 2013 at 11:59

Zaur Nasibov

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3 Comments

Martijn Pieters Over a year ago

Or add from __future__ import division.

Zaur Nasibov Over a year ago

@MartijnPieters Indeed, thanks for mentioning, I always tend to forget that :)

jamylak Over a year ago

@BasicWolf nvm I didn't see the second example. and he said x was an integer, but he must've lied. eg. >>> x=5 >>> formula="(2/10^8*x^2)+0.0131*x-1017.3-30" >>> eval(formula) -1005.2344999999999 In Python 3 that doesn't work, anyway that's what I thought, when I saw the second example I realised OP lied

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