Python creating a calculator

Question

I am fairly new to python.

I have been asked to create a calculator using only string commands, conversions between int/string/float etc.(if needed), and using funct

Answer 1

If you're making just a toy calculator, eval() accepts local and global variables, so you could use something like this:

def calculate(x=0, y=0, z=0):
    expression = raw_input('Enter an expression: ')

    return eval(expression, None, locals())

Here's a sample console session:

>>> calculate()
Enter an expression: x + 5 - y
5

Note that eval() is not secure. If you want to make something serious, you will have to parse the expression.

Also, since your expressions are simple, you could use a regex to validate the input before evaling it:

def validate(expression):
    operator = r'\s*[+\-/*]\s*'

    return bool(re.match(r'^\s*(?:x{o}y|x{o}y{o}z)$'.format(o=operator), expression))