title: Outer merging two data frames in place in pandas
body: <p>How can I outer merge two data frames in place in pandas?</p>
  
  <p>For example, assume we have these two data frames:</p>
  
  <pre><code>import pandas as pd
  
  s1 = pd.DataFrame({
      'time':[1234567000,1234567005,1234567009],
      'X1':[96.32,96.01,96.05]
  },columns=['time','X1'])  # to keep columns order
  
  s2 = pd.DataFrame({
      'time':[1234567001,1234567005],
      'X2':[23.88,23.96]
  },columns=['time','X2'])  # to keep columns order
  </code></pre>
  
  <p>They could be merged with <a href="https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.merge.html" rel="noreferrer">pandas.DataFrame.merge</a> (<code>s3 = pd.merge(s1,s2,how='outer')</code>) or with  <a href="https://pandas.pydata.org/pandas-docs/stable/generated/pandas.merge.html?highlight=merge#pandas.merge" rel="noreferrer">pandas.merge</a> (<code>s3=s1.merge(s2,how='outer')</code>), but it isn't in place. Instead, I'd like the merged data frame to replace s1 in memory.</p>