square.py

I put documentation in my code, explaining each line.
I hadn't written code in 12 months, so I had to refresh my previous learning.
You have to think logically, unlike computers, which don't think at all!
For larger programs, draw a flowchart!

square.py

@@ -1,17 +1,21 @@
 from turtle import *
 
-def square(side_length):
-    forward(side_length)
-    right(90)
-    forward(side_length)
-    right(90)
-    forward(side_length)
-    right(90)
-    forward(side_length)
-    right(90)
 
-sizes = [20, 40, 60, 80, 100]
-for size in sizes:
-    square(size)
-input()
+sizes = [20, 40, 60, 80, 100] # list
+for size in sizes: # outer loop, different square sizes from list
+    def square(): # function definition
+        for x in range(0,4): # inner loop creating a square
+            forward(size) # size doesn't change until the outer loop is executed
+            right(90) # right turn 90 degrees
+    
+    # this code block for the outer loop (below) has to be 
+    # precisely indented in line with the def square(): function definition,
+    # otherwise it won't run properly
+    pendown() # same starting point for each square
+    color('blue')
+    square() # first calling and drawing of a square side from the inner loop,
+    # then drawing a different square size from the "for loop" outer loop
+    penup()
+
+input() # pause the turtle display screen to see the drawing