Pandas: DataFrame filtering using groupby and a function

Question

Using Python 3.3 and Pandas 0.10

I have a DataFrame that is built from concatenating multiple CSV files. First, I filter out all values in the Name column that conta

Answer 1

Instead of length len, I think you want to consider the number of unique values of Name in each group. Use nunique(), and check out this neat recipe for filtering groups.

df[df.groupby('ID').Name.transform(lambda x: x.nunique() == 1).astype('bool')]

If you upgrade to pandas 0.12, you can use the new filter method on groups, which makes this more succinct and straightforward.

df.groupby('ID').filter(lambda x: x.Name.nunique() == 1)

A general remark: Sometimes, of course, you do want to know the length of the group, but I find that size is a safer choice than len, which has been troublesome for me in some cases.