are the triangles in a shapefile valid?

aretrianglesvalid.py

import sys
import os
import fiona
from shapely.geometry import Polygon
from shapely.geometry import asShape
from shapely.geometry import mapping

#-- Global variables
INFILE = "Vlak_3D_LOD0/terreinVlak_3D_LOD0.shp"

    
def main():
  aretrianglesvalid(INFILE)
  print "done."


def aretrianglesvalid(infile):
    c = fiona.open(infile, 'r')
    #-- Find and print the number of geometries
    print "Number of features:", len(c)
    #-- store the geometries in a list
    lsPolys = []
    for each in c:
        lsPolys.append(asShape(each['geometry']))
    totaltr = 0
    totalinvalid = 0
    for mp in lsPolys:
        totaltr += len(mp)
        for p in mp:
            valid = True
            if len(list(p.exterior.coords)) != 4:
                print p.wkt
                valid = False
            if len(p.interiors) != 0:
                print p.wkt
                valid = False
            if valid == False:
                totalinvalid += 1


    print "total triangles", totaltr
    print "total invalid", totalinvalid    
      
if __name__ == "__main__":
    main()