当字符串被其他字符串搜索时，如何在python中修复找不到子串的错误？

target_string = "This is a sample string"
substring = "sample"
if substring in target_string:
    print("Substring found")
else:
    print("Substring not found")

target_string = "This is a sample string"
substring = "sample"
index = target_string.find(substring)
if index != -1:
    print("Substring found at index", index)
else:
    print("Substring not found")

import re
target_string = "This is a sample string"
substring = "sample"
matches = re.findall(substring, target_string)
if len(matches) > 0:
    print("Substring found", len(matches), "times")
else:
    print("Substring not found")