title: Use variable in Pandas query
body: <p>I'm trying to query a Pandas dataframe like this:</p>
  
  <pre><code>        inv = pd.read_csv(infile)
          inv.columns = ['County','Site','Role','Hostname'] 
          clist = inv.County.unique() # Get list of counties
          for county in clist: # for each county
              csub=inv.query('County == county') # create a county subset
              ... do stuff on subset
  </code></pre>
  
  <p>But I get an error:</p>
  
  <pre><code>pandas.core.computation.ops.UndefinedVariableError: name 'county' is not defined
  </code></pre>
  
  <p>I'm sure it's a trivial error, but I can't figure it out.  How do I pass a variable to the query method?</p>