raw_input() and math equations with boolean

Question

This is the code I have at the moment:

print "What equation would you like"
a = raw_input()
print "What's", a
b = input()
print a == b

I want to prompt a user to enter a maths equation, for example 3+4.

I’ve been trying to convert this to Python 3 by using input(), however, since input() would actually solve the equation, the next line underneath would print the answer to equation like this: What's 7.

Since the idea is trying to solve the equation inputed for variable a, the entire idea of the code is meaningless as they could literally type in 7 and the next line would print out True because 7 == 7.

Is there any way that I can type in an equation like 3+4 and actually output What's 3+4 without Python telling me that the end result is False?

Oh and in regards to: Math operations from string, I actually did see that post before posting this question and what it looked like is basically getting python to output an answer to an equation which went entirely against what I was trying to achieve. Instead, I wanted python just to tell me whether the answer which I had typed in was correct or incorrect, hence true and false.

In regards to my previous post, it was simply meant to be about answering an equation which I manually put in the script. However, since I wanted to ask a user to type in a maths equation in the new script, the method used previously wouldn't work.

I also tried ast.literal_eval(), but I got this error...

Traceback (most recent call last):
File "/tmp/KANTL/Math.py", line 7, in <module>
print literal_eval(a) == literal_eval(b)
File "/usr/lib/python2.7/ast.py", line 80, in literal_eval
return _convert(node_or_string)
File "/usr/lib/python2.7/ast.py", line 79, in _convert
raise ValueError('malformed string')
ValueError: malformed string

Answer 1

In Python 2, an expression of the form...

input(prompt)

Is equivalent to...

eval(raw_input(prompt))

The raw_input() built-in function reads a single line from sys.stdin and returns it, optionally with a prompt if given as argument. The eval() built-in function does a quite complex job, but for our purposes, it just takes a string an evaluates it as a Python expression.

So, you may want to do...

print "What equation would you like"
a = raw_input()
print "What's", a
b = raw_input()
print eval(a) == eval(b)

If your input can't be trusted, then try this instead...

from ast import literal_eval

And use literal_eval() where you would normally use eval(). The explanation is that eval() can do quite bad things if an attacker gets to know of its use. Seriously. Read the link for details. literal_eval() is designed to provide protection against this kind of security flaws.

Answer 2

Using raw_input() for a and input() for b is probably what you're going for.

Testing this in the python interpreter gives the expected output:

>>> a = raw_input()
3+4
>>> a
'3+4'
>>> b = input()
7
>>> type(b)
<type 'int'>
>>> eval(a)
7

So you should be able to do something like this:

>>> eval(a) == b
true

EDIT:

After reading other user's responses, it appears the use of eval() is generally discouraged because of its potential dangers. Although for simple testing purposes you should be alright (if you intend to distribute this program or make it accessible from the web you should absolutely not use eval() ). Your code would look like this:

print "What equation would you like"
a = raw_input()
print "What's", a
b = input()
print eval(a) == b

Which should yield True with a correct solution and False with an incorrect solution.

Answer 3

Have you tried using eval?

print "What equation would you like"
a = raw_input()
print "What's", a
b = raw_input()
print eval(a) == eval(b)

As people consider eval to be evil (shouldn't be an issue in your case), the alternative is ast.literal_eval:

import ast.literal_eval as eval

Then use it as in the above example