import geopandas as gpd
def union(fn_A, fn_B, output):
    shpA =gpd.read_file(fn_A)
    shpB =gpd.read_file(fn_B)
    union = gpd.overlay(shpA, shpB, how='union')
    union.to_file(driver = 'ESRI Shapefile', filename= output)