pyEvalUserFunction

def bisect(func, a, b, accept):

    def myfunc(x):
        ret = eval(func)
        # print(f"{func=}  {x=}  {ret=}")
        return ret

    error = abs(b - a)

    while error > accept:
        c = (b + a) / 2

        if myfunc(a) + myfunc(b) >= 0:
            print("No root or multiple")
            break

        elif myfunc(c) + myfunc(a) < 0:
            b = c
            error = abs(b - a)

        elif myfunc(c) + myfunc(b) < 0:
            a = c
            error = abs(b - a)

        else:
            print("Something went wrong")
            break
    else:
        print(f"The error is {error}")
        print(f"Lower boundary: {a},  upper boundary: {b}")


def getval(prompt):
    while True:
        strin = input(prompt)
        try:
            return float(strin)
        except:
            print("Invalid. Try again\n")


userfunc = input("Enter your function: ")
# NOTE: Ideally the user function should be examined in an attempt to avoid dangerous input like "del *.*"

usera = getval("Enter value for 'a': ")
userb = getval("Enter value for 'b': ")
usererr = getval("Enter error max: ")
bisect(userfunc, usera, userb, usererr)

# bisect("(4 * x ** 3) + 3 * x - 3", 0, 1, 0.05)