在Python 3中，如何在单独的.txt文件中搜索字符串？

I am trying to write a code in Python 3 that will allow the user to enter the name of a .txt file. Then the user enters a keyword or phrase and Python searches that .txt file and returns what line the phrase is on. Also, the program will write some things to a .html file, but I can take care of that once I get this part figured out. Here is my code so far:I am trying to write a code in Python 3 that wi