I have a Polygon and a Polyline layers, I want to join them spatially (using the spatial join tool) only if 1 mile or more of the line is inside the polygon
is there a spatial relationship can help me to define this join condition?
You want to do this with the ArcGIS api for python?
ArcMap? or ArcGIS Pro?
Why a spatial join when other options exist?
As a start, I would intersect the polyline featureclass with the polygon featureclass (returning polylines).
keep the attributes of both and you can query the shapelength field of the new file for those values greater than your threshold distance.