title: python replacing item in list
body: <p>Hi my question relates to a post a long time ago by some user here:</p>
  
  <p><a href="https://stackoverflow.com/questions/3136689/find-and-replace-string-values-in-python-list">Find and replace string values in Python list</a></p>
  
  <p>For my specific situation. I have a list like this:</p>
  
  <pre><code>appl =  ['1', 'a', 'a', '2', 'a']
  </code></pre>
  
  <p>I want to replace only the "a single unknown" instance of "a" with just an empty space(that holds the position of where "a" used to be). I am unsure of how to do it for only one character. Can anyone help? Thanks in advance.</p>
  
  <p>EDIT: i Should mention that I need to use an "index" function to first locate the "a" since it is a changing variable. And then I need code that will replace the character.</p>
  
  <p>EDIT2: A lot of people are assuming the index will be 2, but I want to point out that the index of the character is not known. Also "a" will be present in the list approximately ~20 times(will remain a fixed number) but their locations will be changing. I want to replace an "a" based on user input. This is the actual list I am working with:</p>
  
  <pre><code>track  [2] =   ["|","@","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"," ","|"]
  </code></pre>
  
  <p>@ is a charachter, | is a border and " " is an empty space.
  A user enters input to choose how much @ moves to the right and a new image is displayed that shows the new location of @ and the old location of @ is replaced with a space. The length of the list remains constant. This is the context of my question.</p>