Create column of value_counts in Pandas dataframe

by 
df['Counts'] = df.groupby(['Color'])['Value'].transform('count')

For example,

In [102]: df = pd.DataFrame({'Color': 'Red Red Blue'.split(), 'Value': [100, 150, 50]})

In [103]: df
Out[103]: 
  Color  Value
0   Red    100
1   Red    150
2  Blue     50

In [104]: df['Counts'] = df.groupby(['Color'])['Value'].transform('count')

In [105]: df
Out[105]: 
  Color  Value  Counts
0   Red    100       2
1   Red    150       2
2  Blue     50       1

Note that transform('count') ignores NaNs. If you want to count NaNs, use transform(len).

To the anonymous editor: If you are getting an error while using transform('count') it may be due to your version of Pandas being too old. The above works with pandas version 0.15 or newer.

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