title: Update dataframe based on match in another dataframe if condition is met
body: <p>I have two dataframes and I want to update the first one.
  df1 contains different Markets (M1, M2, etc.) and some Codes for each of them (numbers or Dummy)</p>
  
  <pre><code>import pandas as pd
  labels = ["Market","Code"]
  values = [["M1","1234"],["M1","Dummy"],["M1","1234"],["M2","Dummy"],["M1","1234"]]
  df = pd.DataFrame.from_records(values,columns=labels)
  print(df)
   Market   Code
  0     M1   1234
  1     M1  Dummy
  2     M1   1234
  3     M2  Dummy
  4     M1   1234
  </code></pre>
  
  <p>If Code == Dummy, then I want to update the Code in df based on the value in df2 for this specific market. So each Market should receive a different new Code.</p>
  
  <pre><code>labels = ["Market","Code(New)"]
  values = [["M1","4567"],["M2","5678"]]
  df2 = pd.DataFrame.from_records(values,columns=labels)
  print(df2)
   Market Code(New)
  0     M1      4567
  1     M2      5678
  </code></pre>
  
  <p>in the end I should get </p>
  
  <pre><code>labels = ["Market","Code"]
  values = [["M1","1234"],["M1","4567"],["M1","1234"],["M2","5678"],["M1","1234"]]
  df_clean = pd.DataFrame.from_records(values,columns=labels)
  print(df_clean)
    Market  Code
  0     M1  1234
  1     M1  4567
  2     M1  1234
  3     M2  5678
  4     M1  1234
  </code></pre>