Adding new column to pandas DataFrame results in N

问题:

I have a pandas DataFrame data with the following transaction data:

           A         date
0      M000833  2016-08-01
1      M000833  2016-08-01
2      M000833  2016-08-02
3      M000833  2016-08-02 
4      M000511  2016-08-05

I want a new column with the count of number of visits (multiple visits per day should be treated as 1) per consumer.

So I tried this:

import pandas as pd
data['noofvisits'] = data.groupby(['A'])['date'].nunique()

When I just run the statement without assigning it to the DataFrame, I get a pandas series with the desired output. However, the above statement result in:

           A         date       noofvisits
0      M000833  2016-08-01         NaN         
1      M000833  2016-08-01         NaN
2      M000833  2016-08-02         NaN
3      M000833  2016-08-02         NaN
4      M000511  2016-08-05         NaN

The expected output is:

           A         date       noofvisits
0      M000833  2016-08-01         2         
1      M000833  2016-08-01         2
2      M000833  2016-08-02         2
3      M000833  2016-08-02         2
4      M000511  2016-08-05         1

What is wrong with this approach? Why does the column noofvisits results in NAs rather than the count values?

回答1:

Use transform to generate a Series with it's index aligned to the original df:

In[32]:
df['noofvisits'] = df.groupby(['A'])['date'].transform('nunique')
df

Out[32]: 
             A        date  noofvisits
index                                 
0      M000833  2016-08-01           2
1      M000833  2016-08-01           2
2      M000833  2016-08-02           2
3      M000833  2016-08-02           2
4      M000511  2016-08-05           1

The problem with direct assigning is that you're grouping on column 'A' so this becomes the index of the groupby aggregation, you then try to assign to your df but the indices don't agree hence the NaN column values.

Also even if the index values did agree the shape is different anyway:

In[33]:
df.groupby(['A'])['date'].nunique()

Out[33]: 
A
M000511    1
M000833    2
Name: date, dtype: int64