How to pass an operator to a python function?

Use the operator module. It contains all the standard operators that you can use in python. Then use the operator as a functions:

import operator

def get_truth(inp, op, cut):
    return op(inp, cut):

get_truth(1.0, operator.gt, 0.0)

If you really want to use strings as operators, then create a dictionary mapping from string to operator function as @alecxe suggested.

Have a look at the operator module:

import operator
get_truth(1.0, operator.gt, 0.0)

...

def get_truth(inp, relate, cut):    
    return relate(inp, cut)
    # you don't actually need an if statement here

Make a mapping of strings and operator functions. Also, you don't need if/else condition:

import operator


def get_truth(inp, relate, cut):
    ops = {'>': operator.gt,
           '<': operator.lt,
           '>=': operator.ge,
           '<=': operator.le,
           '==': operator.eq}
    return ops[relate](inp, cut)


print(get_truth(1.0, '>', 0.0)) # prints True
print(get_truth(1.0, '<', 0.0)) # prints False
print(get_truth(1.0, '>=', 0.0)) # prints True
print(get_truth(1.0, '<=', 0.0)) # prints False
print(get_truth(1.0, '==', 0.0)) # prints False

FYI, eval() is evil: Why is using 'eval' a bad practice?