foo.py

;;;
;;; how to tokenize an arbitrary string and evaluate it in local context (quick and dirty)
;;; you can replace call to locals with a dictionary that specifies variables (as strings)
;;; and their values.
;;;

from hy.lex import tokenize
from hy.importer import hy_eval

a = 10
b = 5

; tokenize returns a list of HyExpressions, so we simply pick the first one to evaluate.
; In case you already have a HyExpression instead of string, skip the tokenize part

hy_eval(tokenize("(+ a b)")[0], locals(), "")

;;;
;;; How to import and call Hy code in python
;;; Lets assume we have function "my-function" in "example" package
;;;

import hy
from example import my_function

my_function("foo")