title: subtracting two dataframes
body: <p>df1: </p>
  
  <pre><code>City, 2015-12-31, 2016-01-31, ...
  YYZ  562.14, -701.18, ...
  DFW  562.14, -701.18, ...
  YYC  562.14, -701.18, ...
  </code></pre>
  
  <p>df2: </p>
  
  <pre><code>City, 2015-12-31, 2016-01-31, ...
  SFO  562.14, -701.18, ...
  PDX  562.14, -701.18, ...
  LAX  562.14, -701.18, ...
  </code></pre>
  
  <p>I want to subtract df1 from df2. i.e. subtract values in respective date columns.</p>
  
  <p>I tried the following:</p>
  
  <pre><code>df2.subtract(df1, fill_value=0)
  </code></pre>
  
  <p>But I receive the following error:</p>
  
  <pre><code>TypeError: unsupported operand type(s) for -: 'str' and 'float'
  </code></pre>
  
  <p>I think the error is because the operation cannot understand how to subtract strings in the City column, which obviously makes sense since subtracting the Cities is nonsensical. </p>
  
  <p>The accepted answer in this post <a href="https://stackoverflow.com/questions/38906297/operation-with-non-existing-dataframes/38907008#38907008">[link]</a> seems to suggest this is possible. I am the author of that question but can't seem to get it to work now. </p>