Input command taking input as an int instead of a str in Python

Question

I am a beginner programing with Python 2.79. I am writing a program that "tokenizes" a mathematical formula, basically turning each number and operator into an item in a list.

My problem is currently (because it is a semantic error i haven't been able to test the rest of the code yet) in my input command. I am asking the user to enter a mathematical equation. Python is interpreting this as an int.

I tried making it into a string, and Python basically solved the formula, and ran the solution through my tokenize function

My code is as follows:

#Turn a math formula into tokens

def token(s):
    #Strip out the white space
    s.replace(' ', '')
    token_list = []
    i = 0
    #Create tokens
    while i < len(s):
        #tokenize the operators
        if s[i] in '*/\^':
            token_list.append(s[i])
        #Determine if operator of negation, and tokenize
        elif s[i] in '+-':
            if i > 0 and s[i - 1].isdigit() or s[i - 1] == ')':
                token_list.append(s[i])
            else:
                num = s[i]
                i += 1
                while i < len(s) and s[i].isdigit():
                    num += s[i]
                    i += 1
                token_list.append(num)
        elif s[i].isdigit():
            num = ''
            while i < len(s) and s[i].isdigit():
                num += s[i]
                i += 1
            token_list.append(num)
        else:
            return []
    return token_list

def main():
    s = str(input('Enter a math equation: '))
    result = token(s)
    print(result)

main()

Any help would be appreciated

I am looking to

Also, str.replace() doesn't work in-place. Do s = s.replace(...) to get what you want. — yati sagade
– yati sagade, Commented Oct 23, 2015 at 15:31
Thank You! I am learning on Python 3.4, but am using 2.79 today because of the Linux machine i am on. I guess they are different in that way. — DntMesArnd
– DntMesArnd, Commented Oct 23, 2015 at 15:31
@DntMesArnd FYI the reason input "sucks" on Py2 (since no one said) is because it is equivalent to eval(input(...)) on Py3 — Two-Bit Alchemist
– Two-Bit Alchemist, Commented Oct 23, 2015 at 15:33

Robert Lacher · Accepted Answer · 2015-10-23 15:34:07Z

1

The reason Python is interpreting the user's input as an integer is because of the line input('Enter a math equation: '). Python interprets that as eval(raw_input(prompt)). The raw_input function creates a string from the user input, and eval evaluates that input -- so an input of 5+2 is considered "5+2" by raw_input, and eval evaluates that to be 7.

Documentation

answered Oct 23, 2015 at 15:34

Robert Lacher

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