title: Loop on Union process [ArcGis10/Python]
body: <p>I work on ArcGIS 10 and I'd like to realize multiple unions between layers. 
  But that union task is a little bit particular. Indeed as the beginning of my code shows, I have seven layers (these layers have a specific classification).</p>
  
  <pre><code>import arcpy
  from arcpy import env
  
  workspace_2 = "V:/DONNEES/1_referentiels/mnt/1_france/1_dep_17/ign/litto3d/Litto3D_livraison_2011_10_24/traitements_lienss/tests/Workspace_chev/"
  M0 = workspace_2 + "0.shp"
  M1 = workspace_2 + "1.shp"
  M2 = workspace_2 + "2.shp"
  M3 = workspace_2 + "3.shp"
  M4 = workspace_2 + "4.shp"
  M5 = workspace_2 + "5.shp"
  M6 = workspace_2 + "6.shp"
  M7 = workspace_2 + "7.shp"
  </code></pre>
  
  <p>And my goal is to make an union between : M0 - M4, M1 - M5, M2 - M6 and so on. That is to say the union have to made <strong>every four layers</strong>. So I decided to make a loop but I must admit I'm completely lost. I tried that but it didn't work :</p>
  
  <pre><code>Union_1 = {"M0":M0, "M1":M1, "M2":M2, "M3":M3}
  Union_2 = {"M4":M4, "M5":M5, "M6":M6, "M7":M7}
  
  for i in Union_1:
      for j in Union_2:
          InFeatures = [Union_1[i], Union_2[j]]
          arcpy.Union_analysis (InFeatures, i + "_" + j + ".shp", "NO_FID", 0.0003, "")
          print "ok"
  </code></pre>
  
  <p>Could you help me to loop that union process ? </p>