[CentralOH] Python Parsing Puzzle

import ast
import math
import re


def get_coeffs(eqn_str):
    """returns a dict of coefficient: value using the python ast"""
    ret = {}
    # the operations we are interested in are leaves of the parse tree (except
    # the dangling constant)
    for node in ast.walk(ast.parse(eqn_str)):
        if isinstance(node, ast.BinOp):
            if not isinstance(node.left, ast.Num):
                continue

            # the dangling constant does not involve a name for the BinOp, we
            # set the coefficient to 1
            if isinstance(node.right, ast.Name):
                ret[node.right.id] = node.left.n
            else:
                ret[1] = node.left.n
    return ret


def eval_formula(eqn_str, constants_dict):
    """
    evaluates a formula given a constants dict, e.g: {'A': 1, 'B': 2}
    """
    ret = 0
    for name, value in get_coeffs(eqn_str).iteritems():
        # handle dangling constant
        if name == 1:
            ret += value
            continue

        # handle the other terms, 0 if unspecified
        ret += constants_dict.get(name, 0) * value
    return math.tanh(ret)


def read_formulas(filename):
    """reads the formulas and returns a list of formulas"""
    FORMULA_RE = re.compile('^H\d+:$', re.MULTILINE)
    with open(filename) as fp:
        return [m.replace("\n", "") for m in FORMULA_RE.split(fp.read())]


if __name__ == "__main__":
    formulas = read_formulas("formulas.txt")
    # assume we have some data here!
    CONSTANTS = {}

    # first formula is the master formula
    master_formula = get_coeffs(formulas[0])

    final = 0
    for idx, formula in enumerate(formulas[1:]):
        # match the equation to the master
        eqn_name = "H%d" % (idx + 1)
        multiplier = master_formula.get(eqn_name, 0)

        # compute the term for the relevant formula
        final += multiplier * eval_formula(formula, CONSTANTS)

    # lastly, add in value for the dangling constant, 0 if unspecified
    final += master_formula.get(1, 0)
    print final