title: Why isn't Pandas .fillna() filling values in DataFrame?
body: <p>I'm running Pandas in Python 3 and I noticed that the following:</p>
  <pre><code>import pandas as pd
  import numpy as np
  from pandas import DataFrame
  from numpy import nan
  
  df = DataFrame([[1, nan], [nan, 4], [5, 6]])
  
  print(df)
  
  df2 = df
  df2.fillna(0)
  
  print(df2)
  </code></pre>
  <p>Returns the following:</p>
  <pre><code> 0   1
  0   1 NaN
  1 NaN   4
  2   5   6
      0   1
  0   1 NaN
  1 NaN   4
  2   5   6
  </code></pre>
  <p>While the following:</p>
  <pre><code>import pandas as pd
  import numpy as np
  from pandas import Series
  from numpy import nan
  
  sr1 = Series([1,2,3,nan,5,6,7])
  
  sr1.fillna(0)
  </code></pre>
  <p>Returns the following:</p>
  <pre><code>0    1
  1    2
  2    3
  3    0
  4    5
  5    6
  6    7
  dtype: float64
  </code></pre>
  <p>So it's filling in Series values but not DataFrame values with 0 when I use .fillna(). What am I missing here to get 0s in place of null values in DataFrames?</p>