what is the difference between "eval" and "int"

Question

Earlier I heard that eval(input(a)) will convert a string automatically to int, but if i code

age = eval(input("enter age"))

during input i type in 01 it is an error, but when i code

age = int(input("enter age"))

01 as input works perfectly. Why is it?

Answer 1

eval evaluates the python expression. In python 3, numbers starting by 0 aren't allowed (except for 0000, see Why does 000 evaluate to 0 in Python 3?). In python 2, those are interpreted as octal (base 8) numbers. Not better... (python 3 base 8 now uses exclusively Oo prefix)

int performs a string to integer conversion, so it cannot evaluate a complex expression (that you don't need), but isn't subjected to this leading zero syntax.

Another nice feature is that you can check if the entered expression is an integer by using a simple and qualified try/except block:

while True:
   try:
       age = int(input("enter age"))
       break
   except ValueError:
       print("Retry!")

(with eval you would have to protect against all exceptions)

Advice: use int, because it's safer, doesn't have security issues (eval can evaluate any expression, including system calls and file deletion), and suits your purpose perfectly.

Note: the above code is still unsafe with python 2: input acts like eval. You could protect your code against this with the simple code at the start of your module:

try:
    input = raw_input
except NameError:
    pass

so python 2 input is not unreachable anymore and calls raw_input instead. Python 3 ignores that code.