How to find all .txt files in a directory

import os
root = "H:\\ab123"
def findtxt(path, ret):
    """Finding the *.txt file in specify path"""
    filelist = os.listdir(path)
    for filename in filelist:
        de_path = os.path.join(path, filename)
        if os.path.isfile(de_path):
            if de_path.endswith(".txt"): #Specify to find the txt file.
                ret.append(de_path)
        else:
            findtxt(de_path, ret)
 
ret = []
findtxt(root, ret)
for path in ret:
    print(path)