title: Pandas replace not working on known values in a Dataframe
body: <p>have a quick question about pandas replace.</p>
  <pre><code>    import pandas as pd
      import numpy as np
  
  
      infile = pd.read_csv('sum_prog.csv')
      df = pd.DataFrame(infile)
      df_no_na = df.replace({0: np.nan})
      df_no_na = df_no_na.dropna()
  
     print(df_no_na.head())
     print(df.head())
  </code></pre>
  <p>This code will return:</p>
  <pre><code>   Cell ID  Duration  ...  Overall Angle Median  Overall Euclidean Median
     0   372003       148  ...                   0.0        1.9535615635898635
     1   372005       536  ...     45.16432169606084         37.85959470668756
     2   372006       840  ...                   0.0        1.0821891332154392
     3   372010       840  ...                   0.0        1.4200380286464513
     4   372011       840  ...                   0.0        1.0594536197046835
  
    [5 rows x 20 columns]
    Cell ID  Duration  ...  Overall Angle Median  Overall Euclidean Median
    0   372003       148  ...                   0.0        1.9535615635898635
    1   372005       536  ...     45.16432169606084         37.85959470668756
    2   372006       840  ...                   0.0        1.0821891332154392
    3   372010       840  ...                   0.0        1.4200380286464513
    4   372011       840  ...                   0.0        1.0594536197046835
  </code></pre>
  <p>I have done this exact same thing and it has worked before I have no idea why it won't now, any help would be awesome, thanks!</p>