title: Insert colA into DF1 with vals from DF2['colB'] by matching colC in both DFs
body: <p>I have two CSV files, CSV_A and CSV_B.csv. I must insert the column (Category) from CSV_B into CSV_A.</p>
  <p>The two CSVs share a common column: StockID, and I must add the correct category onto each row by matching the StockID columns.</p>
  <p>This can be done using merge, like this:</p>
  <pre><code>dfa.merge(dfb, how='left', on='StockID')
  </code></pre>
  <p>but I only want to add the one column, not join the two dataframes.</p>
  <p>CSV_A (indexed on StockID):</p>
  <pre><code>StockID,Brand,ToolName,Price
  ABC123,Maxwell,ToolA,1.25
  BCD234,Charton,ToolB,2.22
  CDE345,Bingley,ToolC,3.33
  DEF789,Charton,ToolD,1.44
  </code></pre>
  <p>CSV_B:</p>
  <pre><code>PurchDate,Supplier,StockID,Category
  20201005,Sigmat,BCD234,CatShop
  20210219,Zorbak,AAA111,CatWares
  20210307,Phillips
  20210417,Tandey,CDE345,CatThings
  20210422,Stapek,BBB222,CatElse
  20210502,Zorbak,ABC123,CatThis
  20210512,Zorbak,CCC999,CatThings
  20210717,Phillips,DEF789,CatShop
  </code></pre>
  <p>My task is to insert a <code>Cat</code> field into CSV_A, matching each inserted Category with its correct StockID.</p>
  <p><strong>Note1:</strong> CSV_A is indexed on the StockID column. CSV_B has the default indexing.<br>
  <strong>Note2:</strong> There are some columns in CSV_B (e.g. row 3) that do not have complete information.
  <strong>Note3:</strong> Adding column &quot;Category&quot; from CSV_B into CSV_A but calling it &quot;Cat&quot; in CSV_A</p>