title: How to accumulate not null values of each row into separate column/series?
body: <p>I have a dataframe with a subset of columns with the form:</p>
  <pre><code>   col_A  col_B  col_C
  0    NaN    1.0    NaN
  1    NaN    NaN    NaN
  2    NaN    NaN    2.0
  3    3.0    NaN    4.0
  </code></pre>
  <p>I want to create a series of the notnull values for each row. For rows with multiple not null values, I want either the average or first value e.g.</p>
  <pre><code>   new_col
  0      1.0
  1      NaN
  2      2.0
  3      3.5
  </code></pre>
  <p>I'll eventually want to add this back to the original dataframe as a separate column so I need to persist rows with all NaN values by forward filling them e.g.</p>
  <pre><code>   new_col
  0      1.0
  1      1.0
  2      2.0
  3      3.5
  </code></pre>
  <p>I know how to determine if there is a not null value in the dataframe, but I don't know how to select for it:</p>
  <pre><code>df[['col_A', 'col_B', 'col_C']].count(axis=1) &gt;= 1
  </code></pre>