import turtle


t = turtle
t.tracer(0,0)
axiom = "F-G-G"
angle = 120
rules = [["F","F-G+F+G-F"],["G","GG"]]
steps = 4
distance = 10
t.speed(10)


def generate(axiom):
    sentence = ""
    for rule in rules:
        for letter in axiom:
            if letter == rule[0]:
                letter = rule[1]
            sentence+=letter
    return sentence


for i in range(steps):
    axiom = generate(axiom)


def drawsentence(sentence,angle,distance):
    t.mode("logo")
    t.penup()
    t.setpos(0,-300)
    t.pendown()
    for letter in sentence:
        if letter == "F" or letter == "G":
            t.forward(distance)
        if letter == "+":
            t.left(angle)
        if letter == "-":
            t.right(angle)

drawsentence(axiom,angle,distance)

t.update()
t.mainloop()