title: How to populate value of a column in a pandas df from another df?
body: <p>I have a dataframe:</p>
  <pre><code>dict1 = {'Name': ['abc', 'def', 'ghi' , 'jkl'], 'Group': ['Group1', 'Group2', np.nan, np.nan], 'Class' : ['ClassA', 'ClassB' , np.nan, np.nan]}
  df1 = pd.DataFrame(dict1)
  df1:
      Class   Group Name
  0  ClassA  Group1  abc
  1  ClassB  Group2  def
  2     NaN     NaN  ghi
  3     NaN     NaN  jkl
  </code></pre>
  <p>Here I want to replace the NaN values by looking up the name and its corresponding group and class values from another df:</p>
  <pre><code>dict2 = {'Name': ['ghi', 'jkl'], 'Group': ['Group3', 'Group4'], 'Class':['ClassC', 'ClassD']}
  df2 = pd.DataFrame(dict2)
  df2:
      Class   Group Name
  0  ClassC  Group3  ghi
  1  ClassD  Group4  jkl
  </code></pre>
  <p>So basically I want to replace the df1 values by looking at the mapping in df2. So my df1 should look like:</p>
  <pre><code>df1:
      Class   Group Name
  0  ClassA  Group1  abc
  1  ClassB  Group2  def
  2  ClassC  Group3  ghi
  3  ClassD  Group4  jkl
  </code></pre>
  <p>I tried doing df.loc but I am not sure how to make it work in this case.</p>