title: Pandas concat with different indices
body: <p>I have three data frames that I want to concatenate, but they all have different indices. All three indices have the same length. My first df look like this:</p>
  
  <pre><code>Index    Time_start    Time_end    duration    value
  0        5             10          5           1.0
  1        10            16          6           NaN
  ...
  39       50            53          3           NaN
  </code></pre>
  
  <p>The second df looks like this:</p>
  
  <pre><code>Index    Time_start    Time_end    duration    value
  40        5             10         5           2.0
  42        10            16         6           NaN
  ...
  79        50            53         3           NaN
  </code></pre>
  
  <p>And the third looks exactly the same but with Index = [80..119]
  But time_start, Time_end and duration are exactly the same. Value differs.</p>
  
  <p>I want to concatenate the value column so that it looks like this</p>
  
  <pre><code>Index    Time_start    Time_end    duration    value1    value2 value3
  1        5             10          5           1.0       2      3
  2        10            16          6           NaN       NaN    NaN
  ...
  39       50            53          3           NaN       NaN    NaN
  </code></pre>
  
  <p>So far I tried this </p>
  
  <pre><code>pd.concat([df1, df2.value, ms3.value], axis=1, join_axes = [df1.index])
  </code></pre>
  
  <p>but indices are not the same, so it doesn't work. I know I can try first with </p>
  
  <pre><code>df2.reset_index(drop=True)
  </code></pre>
  
  <p>and then do the concat, which works, but I'm sure there's a better way.</p>