title: A col in a Pandas DF and a list of data with 2 cols and if colA of the 2nd data list matches with the colA of first DF , assign the second colB value
body: <p>A col in a Pandas DF and a list of data with 2 cols and if colA of the 2nd data list matches with the colA of first DF, assign the second colB value:</p>
  <p>DF:</p>
  <pre><code>columnA
  123
  456
  789
  102
  103
  104
  105
  </code></pre>
  <p>Lookup file:</p>
  <pre><code>ColumnA, ColumnB
  123, 001
  456, 002
  789, 003
  102, 004
  103, 005
  104, 006
  105, 007
  </code></pre>
  <p>If DF columnA finds a key match in the lookup file with columnA then assign the value from columnB. I used nested np.where but it throws a memory error.</p>