title: Pandas won't fillna() inplace
body: <p>I'm trying to fill NAs  with "" on 4 specific columns in a data frame that are string/object types. I can assign these columns to a new variable as I fillna(), but when I fillna() inplace the underlying data doesn't change.</p>
  
  <pre><code>a_n6 = a_n6[["PROV LAST", "PROV FIRST", "PROV MID", "SPEC NM"]].fillna("")
  a_n6
  </code></pre>
  
  <p>gives me:</p>
  
  <pre><code>&lt;class 'pandas.core.frame.DataFrame'&gt;
  Int64Index: 1542 entries, 0 to 3611
  Data columns (total 4 columns):
  PROV LAST     1542  non-null values
  PROV FIRST    1542  non-null values
  PROV MID      1542  non-null values
  SPEC NM       1542  non-null values
  dtypes: object(4)
  </code></pre>
  
  <p>but</p>
  
  <pre><code>a_n6[["PROV LAST", "PROV FIRST", "PROV MID", "SPEC NM"]].fillna("", inplace=True)
  a_n6
  </code></pre>
  
  <p>gives me:</p>
  
  <pre><code>&lt;class 'pandas.core.frame.DataFrame'&gt;
  Int64Index: 1542 entries, 0 to 3611
  Data columns (total 7 columns):
  NPI           1103  non-null values
  PIN           1542  non-null values
  PROV FIRST    1541  non-null values
  PROV LAST     1542  non-null values
  PROV MID      1316  non-null values
  SPEC NM       1541  non-null values
  flag          439  non-null values
  dtypes: float64(2), int64(1), object(4)
  </code></pre>
  
  <p>It's just one row, but still frustrating. What am I doing wrong?</p>