Drawing a fractal tree in Python, not sure how to proceed

Question

I have this so far in python

import turtle
import math
t = turtle.Turtle()
t.shape("turtle")
t.lt(90)

lv = 11
l  = 100
s  = 17

t.penup()
t.bk(l)
t.pendown()
t.fd(l)

def draw_tree(l, level):
    l = 3.0/4.0*l
    t.lt(s)
    t.fd(l)
    level +=1
    if level<lv:
        draw_tree(l, level)

    t.bk(l)
    t.rt(2*s)
    t.fd(l)
    if level<=lv:
        draw_tree(l, level)
    t.bk(l)
    t.lt(s)
    level -=1

t.speed(100)        
draw_tree(l, 2)

But I'm kind of stuck on how to proges, because I need to reach for building this tree. This is what I'm trying to produce:

Can any one tell me what I am doing wrong?

Answer 1

I really liked @cdlane answer, so I played with the code for a while. The tree now looks much better, and the code is much more readable, so I think it is worth sharing.

The code:

import turtle

WIDTH = 15
BRANCH_LENGTH = 120
ROTATION_LENGTH = 27


class Tree_Fractal(turtle.Turtle):
    def __init__(self, level):
        super(Tree_Fractal, self).__init__()
        self.level = level
        self.hideturtle()
        self.speed('fastest')
        self.left(90)
        self.width(WIDTH)
        self.penup()
        self.back(BRANCH_LENGTH * 1.5)
        self.pendown()
        self.forward(BRANCH_LENGTH)
        self.draw_tree(BRANCH_LENGTH, level)

    def draw_tree(self, branch_length, level):
        width = self.width()
        self.width(width * 3. / 4.)
        branch_length *= 3. / 4.
        self.left(ROTATION_LENGTH)
        self.forward(branch_length)

        if level > 0:
            self.draw_tree(branch_length, level - 1)
        self.back(branch_length)
        self.right(2 * ROTATION_LENGTH)
        self.forward(branch_length)

        if level > 0:
            self.draw_tree(branch_length, level - 1)
        self.back(branch_length)
        self.left(ROTATION_LENGTH)

        self.width(width)


if __name__ == '__main__':
    tree_level = 11  # choose
    tree = Tree_Fractal(tree_level)
    turtle.done()