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).
Intersect—Help | ArcGIS Desktop
keep the attributes of both and you can query the shapelength field of the new file for those values greater than your threshold distance.