save project

my_tree.py

@@ -13,27 +13,28 @@ pen.backward(150)    # 后退
 pen.down()
 
 def tree(n):
-    if n <= 12:
-        color_list = ['snow','lightcoral']
-        color = random.choice(color_list)
-        pen.color(color)
-        pen.pensize(n / 3)
-    else:
-        pen.color('sienna')
-        pen.pensize(n / 10)
-    pen.forward(n)
-    angle = random.random()
-    pen.right(30*angle)
-    length = 1.5*random.random()
-    tree(n-10*length)
-    pen.left(60*angle)
-    tree(n-10*length)
-    pen.right(30*angle)
-    pen.up()
-    pen.backward(n)
-    pen.down()   
-tree(100)
-turtle.done()
+    if n >= 0:
+        if n <= 12:
+            color_list = ['snow','lightcoral']
+            color = random.choice(color_list)
+            pen.color(color)
+            pen.pensize(n / 3)
+        else:
+            pen.color('sienna')
+            pen.pensize(n / 10)
+        pen.forward(n)
+        angle = random.random()
+        pen.right(30*angle)
+        length = 1.5*random.random()
+        tree(n-10*length)
+        pen.left(60*angle)
+        tree(n-10*length)
+        pen.right(30*angle)
+        pen.up()
+        pen.backward(n)
+        pen.down()   
+    tree(100)
+    turtle.done()