python - Writing to a .txt file in Flask

Writing to a file has nothing to do with Flask. You can learn how to read/write to files here.
Keep in mind writing a large amount of data is slow, and will slow down the request.

if request.method == 'POST' and inputed_email:
    # open is a built-in function, w is for write-mode (default is read)
    with open('workfile', 'w') as f: 
         f.write(inputed_email)
    return render_template(....)