Polygon does not close in Shapely

Question

When creating a polygon using the Shapely, I push 4 vertices in the the polygon function. The output should be a tuple with 5 elements (the first vertex is doubled, and described as the last one too).

It seems, however, that the order of the input vertices I pass to the function has impact the result: sometime the polygon is described with 5 vertices (as it should) and sometimes with 4 - meaning, it`s not a closed polygon (or in other words - it is not a polygon at all) It must be some bug.

In the following example, the only difference between poly1 and poly 2 is the order of the vertices I pass. The direction is exactly the same though:

from shapely.geometry import Polygon

print ('poly1 = ', Polygon([(620, 420, 500), (620, 420, 0), (620, 40, 0),(620, 40, 500)]))
print ('poly2 = ',Polygon([(620, 40, 500), (620, 420, 500), (620, 420, 0), (620, 40, 0)]))

However, the result is different - one is a closed polygon, the other is open. The type of both, btw, is still a shapely polygon.

poly1 =  POLYGON Z ((620 420 500, 620 420 0, 620 40 0, 620 40 500, 620 420 500))
poly2 =  POLYGON Z ((620 40 500, 620 420 500, 620 420 0, 620 40 0))

Any solution?

I don't see anything in the docs about the `__str__`/`__repr__` representation of a polygon. What is the output of `polygon.exterior.coords()`? — Trevor Merrifield, Sep 04 '16 at 18:31
It gives me a TypeError: 'CoordinateSequence' object is not callable — Yair, Sep 04 '16 at 22:27
I think it would be worth opening an issue for this on github: https://github.com/Toblerity/Shapely. Even if it's not a bug the documentation should clarify this imo. — Trevor Merrifield, Sep 05 '16 at 08:03

score 4 · Accepted Answer · edited Mar 22 '21 at 16:31

I think it is related with the third coordinate. In the documentation (shapely doc), it tells:

A third z coordinate value may be used when constructing instances, but has no effect on geometric analysis. All operations are performed in the x-y plane.

This means that shapely simply does not process the z coordinate. In your example, if you erase the z coordinate, you get:

[(620, 420), (620, 420), (620, 40), (620, 40)]
[(620, 40), (620, 420), (620, 420), (620, 40)]

When you pass a linear string to build a polygon, shapely Polygon constructor checks if the last point is equal to the first one. If not, the point is added to get a linear ring. In the second case, as far as shapely can see, the last coordinate is already repeated and there is no need to add any other point.

Polygon does not close in Shapely

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