title: How to replace values in pandas dataframe with another dataframe
body: <p>I have a dataframe (df1) that I want to replace the values in the <code>symtom_1</code>, <code>symptom_2</code>... with the <code>weight</code> values from the df2 dataframe.</p>
  <p>The first dataframe has 4000 rows and 17 columns</p>
  <pre><code>df1
  
     Disease            Symptom_1    Symptom_2    Symptom_3
  0  Fungal infection   itching      itching      NaN
  1  Fungal infection   skin_rash    itching      NaN
  2  Fungal infection   itching      itching      NaN
  3  Fungal infection   itching      itching      skin_rash
  4  vertigo            itching      skin_rash    skin_rash
  5  vertigo            vomiting     skin_rash    vomiting
  6  vertigo            vomiting     skin_rash    vomiting
  7  vertigo            vomiting     vomiting     skin_rash
  8  Fungal infection   vomiting     vomiting     vomiting
  9  Fungal infection   skin_rash    skin_rash    vomiting
  10 Fungal infection   skin_rash    vomiting     itching
  </code></pre>
  <p>The second dataframe has 133 rows</p>
  <pre><code>df2
  
      Symptom              weight
  0   itching                 1
  1   skin_rash               3
  2   nodal_skin_eruptions    4
  3   continuous_sneezing     4
  4   shivering               5
  
  </code></pre>