import os
parent_folder_path = 'somepath/parent_folder'
for eachFile in os.listdir(parent_folder_path):
if eachFile.endswith('.xml'):
newfilePath = parent_folder_path+'/'+eachFile
file = open(newfilePath, 'r')
xml = file.read()
file.close()
xml = xml.replace('thing to replace', 'with content')
file = open(newfilePath, 'w')
file.write(str(xml))
file.close()
Надеюсь, это то, что вы ищете.