title: ModuleNotFoundError: No module named 'pythonds'
body: <p><strong>I am trying to simply add inputs to these codes but because of package <code>pythonds</code> i keep getting this error.</strong></p>
  <blockquote>
  <p>ModuleNotFoundError: No module named 'pythonds'.</p>
  </blockquote>
  <p><strong>This error is keeping me from finishing and I am unsure how I can get past this error. Please help and thank you in advance.</strong></p>
  <pre><code>from pythonds.basic.stack import Stack
  
  def infixToPostfix(infixexpr):
      prec = {}
      prec[&quot;^&quot;] = 4
      prec[&quot;%&quot;] = 3
      prec[&quot;*&quot;] = 3
      prec[&quot;/&quot;] = 3
      prec[&quot;+&quot;] = 2
      prec[&quot;-&quot;] = 2
      prec[&quot;(&quot;] = 1
      opStack = Stack()
      postfixList = []
      tokenList = infixexpr.split()
  
      for token in tokenList:
          if token in &quot;ABCDEFGHIJKLMNOPQRSTUVWXYZ&quot; or token in &quot;0123456789&quot;:
              postfixList.append(token)
          elif token == '(':
              opStack.push(token)
          elif token == ')':
              topToken = opStack.pop()
              while topToken != '(':
                  postfixList.append(topToken)
                  topToken = opStack.pop()
          else:
              while (not opStack.isEmpty()) and \
                 (prec[opStack.peek()] &gt;= prec[token]):
                    postfixList.append(opStack.pop())
              opStack.push(token)
  
      while not opStack.isEmpty():
          postfixList.append(opStack.pop())
      return &quot; &quot;.join(postfixList)
  
  def postfixEval(postfixExpr):
      operandStack = Stack()
      tokenList = postfixExpr.split()
  
      for token in tokenList:
          if token in &quot;0123456789&quot;:
              operandStack.push(int(token))
          else:
              operand2 = operandStack.pop()
              operand1 = operandStack.pop()
              result = doMath(token,operand1,operand2)
              operandStack.push(result)
      return operandStack.pop()
  
  def doMath(op, op1, op2):
      if op == &quot;*&quot;:
          return op1 * op2
      elif op == &quot;/&quot;:
          return op1 / op2
      elif op == &quot;+&quot;:
          return op1 + op2
      elif op == &quot;^&quot;:
          return op1 ** op2
      elif op == &quot;%&quot;:
          return op1 % op2
      else:
          return op1 - op2
  
  string = input(&quot;Enter a string: &quot;)
  
  print(infixToPostfix(string))
  </code></pre>