Exec statement python

Question

How do I execute a statement dynamically in Python?

For ex: assume the value x contains the following expression, (a+b)/2,

a = 1
b = 3
x = (a+b)/2

The value for x will be from a table

Answer 1

Probably you want eval

#!/usr/bin/env python
a = 1
b = 3
x = "(a+b)/2"
print eval(x)

But it's usually considered a bad practice (click here for a more elaborate and funnier explanation)

Answer 2

You can do:

a = 1
b = 3
x = '(a+b)/2'
print eval(x)

Note that the value for x is enclosed in quotes as eval requires a string or code object.

Also, perhaps read this to make sure you use it safely (as that can often be a concern and I'm not going to pretend to be an expert in its flaws :) ).

• Ned Batchelder: There is no way to use eval safely, that article is wrong. Read Eval Really is Dangerous for details.
• RocketDonkey: Haha, yeah, I have actually completely avoided it solely due to reading others' comments on this site (and will continue to do so :) ).

Answer 3

Although python has both "exec()" and "eval()", I believe you wan to use the latter in this case:

>>> a = 1
>>> b = 3
>>> x = "(a + b)/2"
>>> eval(x)
2

Answer 4

You can use eval, as in

eval(x)

Actually you can use

x=eval('(a+b)/2')

to get the result (eval will return the result of the computation in this case).