title: Opening files to read in Python
body: <p>I'm trying to make a cash machine simulation.
  This is my code so far:</p>
  
  <pre><code>print ("Cash Machine\n")
  input("Press Enter to begin...")
  card_number = int(input("Enter your card number... "))
  
  
  f = open((card_number + ".txt"),"r")
  lines = f.readlines()
  
  x = lines[1]
  print(x)
  </code></pre>
  
  <p>Currently I have a text file called 123 in my folder. Inside the file is how much money the bank account 123 has, but I am having trouble trying to open the file.
  Specifically with the line 6. I get an error saying 
  "No such file or directory: 'card_number.txt'"
  How can I make it work?
  Thanks</p>