Trigger python code from Google spreadsheets?

You should create a webservice in GAE which then can be called using Google Apps Script UrlFetch class. This is how I usually do to integrate a third party app with Apps Script App.

In a Spreadsheet container script you can create a code like

function myFunction(){
  //your code
  //Call the webservice
 var response = UrlFetchApp.fetch('my_webservice_url', {payload:'...', method:'POST'});
 Logger.log(response.getContentText());
 // your code based on response
}

Above code can be triggered by a time driven trigger in Apps Script based on some conditions