title: TypeError: 'str' does not support item assignment in python
body: <p>I'm working on data analysis as I want to navigate and show the time, I'm using codeskulptor(python) and I used this code to navigate:</p>
  
  <pre><code>def keydown(key):
      global season, year, navtime
      if key == 37:
          navtime += 1
          season[2] = str(int(season[2]) - 3) # error
          if int(season[0] - 3) &lt;= 0:
              year = str(int(year) - 1)
              season = '10-12' 
          else:
              season[0] = str(int(season[0] - 3))
      if key == 39:
          navtime -= 1
          season[2] = str(int(season[2]) + 3) # error
          if int(season[0] + 3) &gt;= 12:
              year = str(int(year) + 1)
              season = '1-3'
          else:
              season[0] = str(int(season[0] + 3))
  </code></pre>
  
  <p>I already previously defined all the variables and I came up with the error: <code>TypeError: 'str' does not support item assignment</code>in python. How do I fix this? </p>
  
  <p>I'm using the simplegui module for this project.</p>