title: Python: string concatenation with None to return None?
body: <p>I'm trying to do a conditional assignment based on the return value for environmental variables.</p>
  
  <pre><code>self._TBLFilePath = iTBLFilePath or os.environ.get("CDO_TBLPATH") + os.environ.get("CDO_INSTANCE_TYPE") + ".tbl" or os.environ.ge    t("CDO_ROOT") + "/cdo/tbl/" + os.environ.get("CDO_INSTANCE_TYPE") + ".tbl"
  </code></pre>
  
  <p>The problem is when CDO_TBLPATH is undefined I get a string with None concatenation error. I wonder if there is a way to force None + string to return a None as well, or is there any other way to overcome this?</p>
  
  <p>Thanks.</p>