Joshua Stokes Joshua Stokes - 1 month ago 16
Python Question

Number recognition input in python

I'm trying to make a script which asks a maths equation, then the user has to type in what they think answer is and then python would output the answer

However, for some reason python doesn't like raw_input() in correlation with the eval statement.

For example, the code is:

print "What's 3+7"
a = raw_input()
print eval('a +4')


If I was to type in 10 in var 'a', I'd get this error message.

Traceback (most recent call last):
File "/tmp/W1SVH/Math.py", line 3, in <module>
print eval('a +4')
File "<string>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'int' objects


But if I were to replace raw_input with the number '5', the script looks like this.

print "What's 3+7"
a = 5
print eval('a +4')


Then it would return 9, hence 5+4=9.

Is there anyway to fix this, so that I can use raw_input() to prompt the user?

Update

Ah, I forgot to mention that the 'print "What's 3+7"' statement was when I was experimenting with the code and has nothing to do with the overall goal.

Also, thanks to everyone who told me about the dangers of eval. I certainly didn't know that and I will definitely keep that in mind.

Answer

The object returned by raw_input() is a string. You need to convert it to an integer before performing arithmetic with it. You can do this with the int built-in function.

print "What's 3+7"
a = int(raw_input())
print eval('a +4')

EDIT:

As ShadowRanger observes, using eval is redundant once a is an integer, so you can just let Python evaluate the result:

print "What's 3+7"
a = int(raw_input())
print(a + 4)

This is not only easier to read, but safer. Using eval on input received from a user is dangerous and should be avoided. A malicious user could enter something that, when evaluated, could cause data loss, or use up your computer's resources.