how to compare geometries (contains, within) in arcpy?

Question

I am trying to use 2 geometry objects to check if a line is within a (rather complex) polygon. The geometry check always fails, even if the line is clearly within the polygon. I use the following code:

#get the polygon
sC = arcpy.SearchCursor(polygon)
for row in sC:
    polygonshape = row.Shape
del sC

#creation of the polyline(s):

feature_info = [[[p1.x, p1.y], [x, y]], [[p2.x, p2.y], [x, y]], [[p3.x, p3.y], [x, y]]]
features = []
for feature in feature_info:
    array = arcpy.Array()
    for point in feature:
        P = arcpy.Point()
        P.X = point[0]
        P.Y = point[1]
        array.add(P)
    features.append(arcpy.Polyline(array))

#the actual check:
for item in features:
    in_poly = polygonshape.contains(item) #this is always false

Any ideas why this fails? I considered, that one of the layers (polygon) has a coordinate system, while the other doesn't but according to another answer here (which I can't find right now) coordinate systems are supposedly ignored for this kind of checks. Could the size/complexity of the Polygon be an issue?

If I create a Feature Class of the line and do a spatial selection (within) afterwards, it works as expected. Also: this behaviour returns unexpected results for contains as well as within.

Answer 1

My methodology to accomplish this task has always been using ArcGIS's selection tools on feature layers.

Something like this:

arcpy.MakeFeatureLayer_management (polygon, "lyr")

#The check
for item in features:
    arcpy.SelectLayerByLocation_management ("lyr", "CONTAINS", item)
    #Check for selection, i.e. item is contained within polygon
    if arcpy.Describe ("lyr").FIDSet:

item might not be a valid input for a SelectLayerByLocation select_feature, but if not conversion to feature class will do the trick.