vector - Loading a Shapefile in standalone PyQGIS script

I am writing a script to validate a shapefile.

Here my code:

#-*- coding: UTF-8 -*-


import os
from osgeo import ogr
from PyQt4.QtGui import *
from qgis.core import *
import sys

app = QApplication([])
QgsApplication.setPrefixPath("/usr/", True) # Adjust prefix path according to your installation (see note below)
QgsApplication.initQgis()


class validacaoVetorialMCV(QgsVectorLayer):
    """docstring for Arquivo"""
    def __init__(self, arquivo_shp):
        self.arquivo_shp = arquivo_shp

    def campo_not_null(self, arquivo_shp):

        #Fields to check
        for value in ('geometriaA', 'regime', 'tipoTrecho', 'tipoMassaD', 'tipoLocali', 'jurisdicao', 'administra', 'nivel_1', 'nivel_2', 'nivel_3'):
            idx = arquivo_shp.fieldNameIndex(value);

            values = arquivo_shp.uniqueValues(idx);

            if all(x for x in values) ==  False:
                return 'Registros nulos em %s do: %s' (value, %arquivo_shp.name());


#rootdir = raw_input('Caminho da pasta:')
rootdir = '/home/infra/PycharmProjects/untitled/projeto_desenvolvimento_script/folha_mi_1584_2'
extensions = ('.shp')


for subdir, dirs, files in os.walk(rootdir):
    for file in files:
        ext = os.path.splitext(file)[-1].lower()
        #Print all files which have .shp extension
        if ext in extensions:
            #print os.path.join(subdir, file)

            layer = QgsVectorLayer(os.path.join(rootdir, file), file, "ogr")

            #Check if layer was setting

            print layer.name()

            if not layer.isValid():
                print "Layer failed to load!"

            print validacaoVetorialMCV(layer).validar_normalizacao_campo(layer)

Why does my method "campo_not_null" not work? Why did my layer failed to load? Why were other extensions that are not .shp (.gdb, timestamp) read?

Answer in python console:

mi_1584_2_cerrado.shp Layer failed to load! []

mi_1584_2_localidade.shp Layer failed to load! [] 
mi_1584_2_massa_dagua.shp Layer failed to load! [] 
mi_1584_2_trecho_massa_dagua.shp Layer failed to load! [] 
mi_1584_2_trecho_drenagem.shp Layer failed to load! [] 
mi_1584_2_trecho_rodoviario.shp Layer failed to load! [] 
timestamps Layer failed to load! [] 
gdb Layer failed to load! []

Answer

I thought to learn more about Qgis API.

Why not use directly the Python console ?

for subdir, dirs, files in os.walk(rootdir):
    for file in files:
        if file.endswith(".shp"):
           layer = QgsVectorLayer(os.path.join(rootdir, file), os.path.splitext(file)[0], "ogr")
           layer.isValid()

Works, but if you have subdirectories in rootdir, the layers in the subdirectories are not valid, because of the path os.path.join(rootdir, file)

The solution is to use os.path.join(subdir, file)

class validacaoVetorialMCV(QgsVectorLayer):
    """docstring for Arquivo"""
    def __init__(self, arquivo_shp):
        self.arquivo_shp = arquivo_shp
    def campo_not_null(self):
        for value in ('IDENT','NUM'):
            idx = self.arquivo_shp.fieldNameIndex(value)
            values = self.arquivo_shp.uniqueValues(idx)
            if all(x for x in values) ==  False:
                return (value,layer.name())


for subdir, dirs, files in os.walk(rootdir):
    for file in files:
        if file.endswith(".shp"):
            layer = QgsVectorLayer( os.path.join(subdir, file), os.path.splitext(file)[0], "ogr")
            if layer.isValid(): 
               print  validacaoVetorialMCV(layer).campo_not_null()