String to Int conversion in python

Question

If I have a string "2+3", is there anyway to convert it to an integer so it comes out as 5?

I tried this:

string = 2+3
answer = int(string)

But I get an error:

ValueError: invalid literal for int() with base 10: '2+3'

I'm trying to take a fully parenthesized equation and use stacks to answer it.

ex. Equation = ((2+3) - (4*1))

I tried taking the equation as an input, but python just solves it on its own. So to avoid that problem, I took the equation as a raw_input.

• Binayaka Chakraborty: ummm, converting to postfix then evaluating?...
• Ryan Saxe: eval is actually a function that does exactly this!
• David Chang: oh I didn't even know that existed, thank you! It works now.

Answer 1

There is one way, eval function..

>>> x = raw_input()
2 + 6
>>> x
'2 + 6'
>>> eval(x)
8

But be sure to verify that the input only has numbers,and symbols.

>>> def verify(x):
 for i in x:
 if i not in '1234567890.+-/*%( )':
 return False
 return True
>>> x = raw_input()
2 + 6
>>> x
'2 + 6'
>>> if verify(x):
 print eval(x)
8

ast.literal_eval doesn't work:

>>> ast.literal_eval('2+3')
Traceback (most recent call last):
 File "<pyshell#1>", line 1, in <module>
 ast.literal_eval('2+3')
 File "C:\Python2.7 For Chintoo\lib\ast.py", line 80, in literal_eval
 return _convert(node_or_string)
 File "C:\Python2.7 For Chintoo\lib\ast.py", line 79, in _convert
 raise ValueError('malformed string')
ValueError: malformed string

Answer 2

Use eval (and remember to sanitize your input. More on this if you read the docs):

>>> eval('2+3')
5

It even supports variables:

>>> x = 1
>>> eval('x+1')
2

• aaronman: If the input comes from the user you should not use eval
• aaronman: Everyone knows that eval is not safe to use if you get user input, because it is executing arbitrary code. He should at least sanitize the input if he uses it.
• aaronman: (@Keyser) I know your answer works, not arguing that, just don't want to teach someone who might be a beginner bad habits
• Paolo Moretti: Yeah, probably ast.literal_eval would be a safer choice, since it only considers a small subset of Python's syntax to be valid.
• SingleNegationElimination: whether you should use eval() with user input depends on if you trust the user. Often, you do; but still need to protect the user from accidentally executing code on their behalf from a third party your user does not trust.