save project

my_tree.py

@@ -13,6 +13,7 @@ pen.backward(150)    # 后退
 pen.down()
 
 def tree(n):
+    if n >= 0:
         if n <= 12:
             color_list = ['snow','lightcoral']
             color = random.choice(color_list)
@@ -32,8 +33,8 @@ def tree(n):
         pen.up()
         pen.backward(n)
         pen.down()   
-tree(100)
-turtle.done()
+    tree(100)
+    turtle.done()