title: "Value is Required" on arcpy.CalculateField
body: <p>I'm pretty new to Python, and am trying to write a script for use in ArcGIS 10.1 (arcpy); the basic idea is to add a new field (francis), check the values in several other fields, if any are null (-99) then output 0 into Francis, otherwise run a simple computation.However, I'm getting this error and am having trouble moving beyond it:</p>
  <blockquote>
  <p>Traceback (most recent call last):</p>
  <p>File &quot;C:\gislab2\Python\take_home\part1\prelim_if2.py&quot;, line 28, in   arcpy.CalculateField_management(Output_Feature_Class, &quot;Francis&quot;, &quot;&quot;, &quot;PYTHON_9.3&quot;, &quot;&quot;)</p>
  <p>File &quot;C:\Program Files\ArcGIS\Desktop10.1\arcpy\arcpy\management.py&quot;, line 3128, in CalculateField raise e</p>
  <p>ExecuteError: Failed to execute. Parameters are not valid. ERROR 000735: Expression: Value is required Failed to execute (CalculateField).</p>
  </blockquote>
  <p>Here's the code</p>
  <pre><code># Import arcpy module
  import arcpy
  
  print &quot;start your engines&quot;
  # Script arguments
  Shapefile = &quot;C:\\gislab2\\Python\\take_home\\USCancer2000.shp&quot;
  
  Field_Name = Francis
  
  Output_Feature_Class = &quot;C:\\gislab2\\Python\\take_home\\USCancer2000.shp&quot;
  
  # Local variables:
  USCancer2000__2_ = Output_Feature_Class
  
  # Process: Add Field
  arcpy.AddField_management(Shapefile, &quot;Francis&quot;, &quot;LONG&quot;, &quot;&quot;, &quot;&quot;, &quot;&quot;, &quot;&quot;, &quot;NULLABLE&quot;, &quot;NON_REQUIRED&quot;, &quot;&quot;)
  
  # Process: Calculate Field
  arcpy.CalculateField_management(Output_Feature_Class, &quot;Francis&quot;, &quot;&quot;, &quot;PYTHON_9.3&quot;, &quot;&quot;)
  ##
  ### Process: If-then check for missing values
  ##
  if &quot;Cnt1&quot;==-99:
      Field_name=7
  elif &quot;Cnt2&quot;==-99:
      Field_name=7
  elif &quot;Cnt3&quot;==-99:
      Field_name=7
  elif &quot;Pop1&quot;==-99:
      Field_name==7
  elif &quot;Pop2&quot;==-99:
      Field_name=7
  elif &quot;Pop3&quot;==-99:
      Field_name=7
  else:
          Field_name=(&quot;Cnt1&quot;+&quot;Cnt2&quot;+&quot;Cnt3&quot;)/(&quot;Pop1&quot;+&quot;Pop2&quot;+&quot;Pop3&quot;)
  print &quot;done&quot;
  </code></pre>
  <p>Many thanks in advance!
  david</p>