simple python-like expressions

gistfile1.py

import ast
import operator


ops = {
        ast.Add: operator.add,
        ast.Mult: operator.mul,
        ast.Sub: operator.sub,
        ast.Div: operator.truediv,
        ast.FloorDiv: operator.floordiv,
        ast.Mod: operator.mod,

        # unary
        ast.USub: operator.neg,
        ast.UAdd: operator.pos,
        }

fns = {
        'min': min,
        'max': max,
        }


def eval_in(env):
    return lambda expr: eval(expr, env)


def eval(expr, env):
    if isinstance(expr, ast.Expr):
        return eval(expr.value, env)
    elif isinstance(expr, ast.Name):
        return env[expr.id]
    elif isinstance(expr, ast.Num):
        return expr.n
    elif isinstance(expr, ast.UnaryOp):
        op = ops[type(expr.op)]
        return op(eval(expr.operand, env))
    elif isinstance(expr, ast.BinOp):
        op = ops[type(expr.op)]
        return op(eval(expr.left, env), eval(expr.right, env))
    elif isinstance(expr, ast.Call):
        fn = fns[expr.func.id]
        if expr.kwargs:
            raise Exception('kwargs not supported')
        return fn(*map(eval_in(env), expr.args))
    else:
        raise Exception("I don't understand %s" % expr)


def eval_expr(str, env={}):
    module = ast.parse(str)
    if not isinstance(module, ast.Module):
        raise Exception("I don't understand")
    body = module.body
    if len(body) != 1:
        raise Exception("One and only one expression is supported")
    expr = body[0]
    if not isinstance(expr, ast.Expr):
        raise Exception("Only expressions are supported")

    return eval(expr, env)