Interpreting Strings as Other Data Types in Python

Question

I'm reading a file into python 2.4 that's structured like this:

field1: 7
field2: "Hello, world!"
field3: 6.2

The idea is to parse it into a dictionary that takes fieldfoo as the key and whatever comes after the colon as the value.

I want to convert whatever is after the colon to it's "actual" data type, that is, '7' should be converted to an int, "Hello, world!" to a string, etc. The only data types that need to be parsed are ints, floats and strings. Is there a function in the python standard library that would allow one to make this conversion easily?

The only things this should be used to parse were written by me, so (at least in this case) safety is not an issue.

Answer 1

First parse your input into a list of pairs like fieldN: some_string. You can do this easily with re module, or probably even simpler with slicing left and right of the index line.strip().find(': '). Then use a literal eval on the value some_string:

>>> import ast
>>> ast.literal_eval('6.2')
6.2
>>> type(_)
<type 'float'>
>>> ast.literal_eval('"Hello, world!"')
'Hello, world!'
>>> type(_)
<type 'str'>
>>> ast.literal_eval('7')
7
>>> type(_)
<type 'int'>

Answer 2

You can attempt to convert it to an int first using the built-in function int(). If the string cannot be interpreted as an int a ValueError exception is raised. You can then attempt to convert to a float using float(). If this fails also then just return the initial string

def interpret(val):
    try:
        return int(val)
    except ValueError:
        try:
            return float(val)
        except ValueError:
            return val