import os

from PyPDF2 import PdfFileMerger

# création liste de numéros compatibles en tete classe + numéros

Liste = ['1GA'+x+y  for x in "012" for y in "123456789"]

directory_path = os.getcwd()

folder_name = os.path.basename(directory_path)

fileList  = []

#boucle dans la liste de numéro

for numero in Liste:

   for path, currentDirectory, files in os.walk(directory_path):

      for fileName in files:

         if fileName.startswith(numero):

            fileList.append(fileName)

for numero in Liste:

    merger = PdfFileMerger() # <===== to INSIDE

    for fileName in fileList:

        if numero not in fileName:

            continue

        merger.append( fileName)

 #   merger.write(directory_path +'/union/'+ numero + '.pdf')

    merger.write(directory_path +'/union/'+ numero + '.pdf')

    merger.close()