title: using or with .isin() on dataframe
body: <p>I have a dataframe and want to use .isin() on two columns to filter out certain rows.</p>
  <p>df =</p>
  <pre><code>amount    currency1    currency2
  50        GBP          nan
  47        nan          USD
  30        nan          GBP
  60        EUR          nan
  57        nan          EUR
  </code></pre>
  <p>If currency1 is populated with a value then currency2 will not be and vice versa. i want to use isin() to only pick out GBP and USD. i tried using:</p>
  <pre><code>df = df.loc[df['currency1'].isin(['GBP','USD'])]
  </code></pre>
  <p>but then the rows which have GBP or USD in currency2 are taken out of the dataframe so I'd like to do something where it looks for GBP and USD in both columns using maybe an or statement so the dataframe would look like:</p>
  <pre><code>amount    currency1     currency2
  50        GBP           nan
  47        nan           USD
  30        nan           GBP
  </code></pre>