title: check for file existence in Python 3
body: <p>I would like to know how I could check to see if a file exist based on user input and if it does not, then i would like to create one.</p>
  
  <p>so I would have something like:</p>
  
  <pre><code>file_name = input('what is the file name? ')
  </code></pre>
  
  <p>then I would like to check to see if file name exist.</p>
  
  <p>If file name does exist, open file to  write or read, if file name does not exist, create the file based on user input.</p>
  
  <p>I know the very basic about files but not how to use user input to check or create a file.</p>