pandas: drop duplicates in groupby 'date'

Question

In the dataframe below, I would like to eliminate the duplicate cid values so the output from df.groupby(\'date\').cid.size() matches the output fr

Answer 1

You don't need groupby to drop duplicates based on a few columns, you can specify a subset instead:

df2 = df.drop_duplicates(["date", "cid"])
df2.groupby('date').cid.size()
Out[99]: 
date
2005      3
2006     10
2007    227
2008     52
2009    142
2010     57
2011    219
2012     99
2013    238
2014    146
dtype: int64