The equals operation in ShapelySun 08 July 2018
Shapely provides two ways of testing the equivalence of geometries:
- Using the == operator, e.g. a == b
- Using the .equals method, e.g. a.equals(b)
The result of the two methods are not identical, although it may appear that way at first.
For example, take two points:
>>> A = Point([1, 2]) >>> B = Point([1, 2]) >>> A == B True >>> A.equals(B) True
So far so good, but what about a more complex example?
>>> A = LineString([(1, 2), (3, 4)]) >>> B = LineString([(3, 4), (1, 2)]) >>> A == B False >>> A.equals(B) True
The difference is that the == operator does a comparison of the coordinate sequences of the geometries (in addition to checking their type), while the .equals method is a test of geometric equivalence. Another way to think of this is as a test for when the symmetric difference of two geometries is empty, i.e. neither geometry has a part that the other doesn't.
Another example of this is two geometries that have different coordinate sequences but represent the same geometry.
>>> A = LineString([(0, 0), (5, 0)]) >>> B = LineString([(0, 0), (2, 0), (5, 0)]) >>> A == B False >>> A.equals(B) True
This also means that two geometries are considered equivalent by .equals even if they have different types, so long as they are geometrically equivalent.
>>> A = Point(1, 2) >>> B = MultiPoint([(1, 2)]) >>> A == B False >>> A.equals(B) True
Shapely uses GEOS's GEOSEquals method internally. There is also a GEOSEqualsExact method, exposed as .equals_exact, which allows a tolerance in the comparison.
>>> A = Point([1, 2]) >>> B = Point([1, 2.5]) >>> A.equals_exact(B, tolerance=0.0) False >>> A.equals_exact(B, tolerance=1.0) True
So which method should you use? This depends on what your data represents. Both methods are useful in different situations.