How can I convert a string to either int or float with priority on int?

Question

I couldn't find another answer when I wanted this, so I thought I would post my own solution for anyone else and also get corrections if I've done something wrong.

I had to make an automatic config file parser and I preferred to make numbers int if possible and float if not. The usual try/except conversion doesn't work by itself since any float will just be coerced to an int.

To clarify as someone asked, the case is basically to convert numbers as the person writing the config file data intended them to be. So anything with decimals would probably be intended to be a float. Also, I believe that float can be dangerous for some operators (e.g. ==, <, >) due to the nature of floating point numbers, and int will be converted to float when necessary. Therefore I prefer numbers to stay in int when possible. Not a big thing, just as a kind of convention for myself.

Answer 1

eval(s)

eval() is a function that accepts a string and processes it as Python code. When given any kind of literal as a string, it will return a Python object containing that value.

However, from a security standpoint, this is a bad idea, as someone could potentially feed instructions that crash or tamper with your program. You could require it to be a certain set of characters before processing, i.e.:

eval(s) if not set(s).difference('0123456789.') else None

This still probably isn't worth the headache, though. That said, you can get fancy, and allow expressions with calculations as well... and that might be worth trying!

num = lambda s: eval(s) if not set(s).difference('0123456789. *+-/e') else None

>>> num('10')
10
>>> num('10.0')
10.0
>>> num('1e3')
1000.0
>>> num('4.8 * 10**34')
4.8e+34
>>> num('1+2/3')
1.6666666666666665