Automatically Type Cast Parameters In Python

Question

Background:
I mostly run python scripts from the command line in pipelines and so my arguments are always strings that need to be type casted to the app

Answer 1

You could just use plain eval to input string if you trust the source:

>>> eval("3.2", {}, {})
3.2
>>> eval("True", {}, {})
True

But if you don't trust the source, you could use literal_eval from ast module.

>>> ast.literal_eval("'hi'")
'hi'
>>> ast.literal_eval("(5, 3, ['a', 'b'])")
(5, 3, ['a', 'b'])

Edit: As Ned Batchelder's comment, it won't accept non-quoted strings, so I added a workaround, also an example about autocaste decorator with keyword arguments.

import ast

def my_eval(s):
    try:
        return ast.literal_eval(s)
    except ValueError: #maybe it's a string, eval failed, return anyway
        return s       #thanks gnibbler

def autocaste(func):
    def wrapped(*c, **d):
        cp = [my_eval(x) for x in c]
        dp = {i: my_eval(j) for i,j in d.items()} #for Python 2.6+
        #you can use dict((i, my_eval(j)) for i,j in d.items()) for older versions
        return func(*cp, **dp)

    return wrapped

@autocaste
def f(a, b):
    return a + b

print(f("3.4", "1")) # 4.4
print(f("s", "sd"))  # ssd
print(my_eval("True")) # True
print(my_eval("None")) # None
print(my_eval("[1, 2, (3, 4)]")) # [1, 2, (3, 4)]