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copied!<p>The implementation of your formula is flawed. It looks ahead to values in your x, y lists that haven't been set yet with <code>(x[i]*y[i+1] - x[i+1]*y[i])</code></p> <p>If you put a print statement inside your try-except block you will see that you are simply multiplying by zero and getting zero area:</p> <pre><code>try: x[i], y[i] = polygon[i] area += (x[i]*y[i+1] - x[i+1]*y[i]) print x[i], y[i+1], x[i+1], y[i] except IndexError, e: return area/2 #1 0 0 2 #2 0 0 5 </code></pre> <p>Also, you are not returning the results of your recursive call to areaofpolygon, so you will never get that <code>area/2</code>. You want: <code>return areaofpolygon(polygon, i+1)</code>. And make sure you actually divide by 2.0 so that you get float precision since your points are ints.</p> <p>Try just using the formula you found or that was suggested in another question.</p> <p><strong>Update</strong></p> <p>Here is a fixed version of your code:</p> <pre><code>#!/usr/bin/env python from random import randint from shapely.geometry import Polygon area = 0 def areaofpolygon(polygon, i): global area if i == 0: area = 0 try: x1, y1 = polygon[i] x2, y2 = polygon[i+1] area += (x1*y2) - (x2*y1) except IndexError, e: x1, y1 = polygon[0] x2, y2 = polygon[-1] area += (x2*y1) - (x1*y2) return abs(area/2.0) return areaofpolygon(polygon, i+1) def main(): mypolygon = [(randint(0, 100), randint(0, 100)) for _ in xrange(10)] print mypolygon area = areaofpolygon(mypolygon, 0) assert area == Polygon(mypolygon).area print "Test passed." return 0 if __name__ == '__main__': main() ### Output ### $ ./test.py [(29, 76), (85, 49), (27, 80), (94, 98), (19, 1), (75, 6), (55, 38), (74, 62), (0, 25), (93, 94)] Test passed. $ ./test.py [(13, 37), (98, 74), (42, 58), (32, 64), (95, 97), (34, 62), (34, 59), (21, 76), (55, 32), (76, 31)] Test passed. $ ./test.py [(38, 67), (66, 59), (16, 71), (53, 100), (64, 52), (69, 31), (45, 23), (52, 37), (27, 21), (42, 74)] Test passed. </code></pre> <p>Notice that you didn't need global x,y lists. And you also missed the last part of the equation where you use the last point and the first point.</p>

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