title: Find key from value for Pandas Series
body: <p>I have a dictionary whose values are in a pandas series. I want to make a new series that will look up a value in a series and return a new series with associated key. Example:</p>
  
  <pre><code>import pandas as pd
  
  df = pd.DataFrame({'season' : ['Nor 2014', 'Nor 2013', 'Nor 2013', 'Norv 2013',
                             'Swe 2014', 'Swe 2014',  'Swe 2013',
                             'Swe 2013', 'Sven 2013', 'Sven 2013', 'Norv 2014']})
  
  nmdict = {'Norway' : [s for s in list(set(df.season)) if 'No' in s],
                    'Sweden' : [s for s in list(set(df.season)) if 'S' in s]}
  </code></pre>
  
  <p>Desired result with <code>df['country']</code> as the new column name:</p>
  
  <pre><code>       season country
  0    Nor 2014  Norway
  1    Nor 2013  Norway
  2    Nor 2013  Norway
  3   Norv 2013  Norway
  4    Swe 2014  Sweden
  5    Swe 2014  Sweden
  6    Swe 2013  Sweden
  7    Swe 2013  Sweden
  8   Sven 2013  Sweden
  9   Sven 2013  Sweden
  10  Norv 2014  Norway
  </code></pre>
  
  <p>Due to nature of my data I must manually make the <code>nmdict</code> as shown. I've tried <a href="https://stackoverflow.com/questions/30312566/python-how-to-get-values-from-a-dictionary-from-pandas-series">this</a> but couldn't reverse my <code>nmdict</code> as arrays are not same length. </p>
  
  <p>More importantly, I think my approach may be wrong. I'm coming from Excel and thinking of a vlookup solution, but according to <a href="https://stackoverflow.com/questions/8023306/get-key-by-value-in-dictionary">this answer</a>, I shouldn't be using the dictionary in this way. </p>
  
  <p>Any answers appreciated.</p>