Deleting rows from Pandas dataframe based on groupby values

Question

I have a large Pandas dataframe (> 1 million rows) that I have retrieved from a SQL Server database. In a small number of cases, some of the records have duplicate entries. All cells are identical except for a single, text field. It looks as though the record has been entered into the database and then, at a later time, additional text has been added to the field and the record stored in the database as a separate entry. So basically, I want to keep only the record with the longest text string. A simplified version of the database can be created as follows:

tempDF = pd.DataFrame({ 'recordID': [1,2,3,3,4,5,6,6,6,7,7,8,9,10],
                        'text': ['abc', 'def', 'ghi', 'ghijkl', 'mto', 'per', 'st', 'stuvw', 'stuvwx', 'yz', 'yzab', 'cde', 'fgh', 'ijk']})

Which looks like this:

    recordID    text
0         21     abc
1         22     def
2         23     ghi
3         23  ghijkl
4         24     mno
5         25     pqr
6         26      st
7         26   stuvw
8         26  stuvwx
9         27      yz
10        27    yzab
11        28     cde
12        29     fgh
13        30     ijk

So far, I've identified the rows with duplicate recordID and calculated the length of the text field:

tempDF['dupl'] = tempDF.duplicated(subset = 'recordID',keep=False)
tempDF['texLen'] = tempDF['text'].str.len()
print(tempDF)

To produce:

    recordID    text   dupl  texLen
0         21     abc  False       3
1         22     def  False       3
2         23     ghi   True       3
3         23  ghijkl   True       6
4         24     mno  False       3
5         25     pqr  False       3
6         26      st   True       2
7         26   stuvw   True       5
8         26  stuvwx   True       6
9         27      yz   True       2
10        27    yzab   True       4
11        28     cde  False       3
12        29     fgh  False       3
13        30     ijk  False       3

I can groupby all the dupl==True records based on recordID using:

tempGrouped = tempDF[tempDF['dupl']==True].groupby('recordID')

And print off each group separately:

for name, group in tempGrouped:
    print('n',name)
    print(group)

23
   recordID    text  dupl  texLen
2        23     ghi  True       3
3        23  ghijkl  True       6

26
   recordID    text  dupl  texLen
6        26      st  True       2
7        26   stuvw  True       5
8        26  stuvwx  True       6

27
    recordID  text  dupl  texLen
9         27    yz  True       2
10        27  yzab  True       4

I want the final dataframe to consist of those records where dupl==False and, if dupl==True then only the replicate with the longest text field should be retained. So, the final dataframe should look like:

    recordID    text   dupl  texLen
0         21     abc  False       3
1         22     def  False       3
3         23  ghijkl   True       6
4         24     mno  False       3
5         25     pqr  False       3
8         26  stuvwx   True       6
10        27    yzab   True       4
11        28     cde  False       3
12        29     fgh  False       3
13        30     ijk  False       3

How can I delete from the original dataframe only those rows where recordID is duplicated and where texLen is less than the maximum?

Answer 1

You can try find indexes with max values by idxmax , concat with False values in dupl column and last sort_index :

idx = tempDF[tempDF['dupl']==True].groupby('recordID')['texLen'].idxmax()   

print tempDF.loc[idx]
    recordID    text  dupl  texLen
3         23  ghijkl  True       6
8         26  stuvwx  True       6
10        27    yzab  True       4

print pd.concat([tempDF[tempDF['dupl']==False], tempDF.loc[idx]]).sort_index(0)
    recordID    text   dupl  texLen
0         21     abc  False       3
1         22     def  False       3
3         23  ghijkl   True       6
4         24     mto  False       3
5         25     per  False       3
8         26  stuvwx   True       6
10        27    yzab   True       4
11        28     cde  False       3
12        29     fgh  False       3
13        30     ijk  False       3

The simplier solution use sort_values and first , because rows with False have unique recordID (are NOT duplicated):

df=tempDF.sort_values(by="texLen", ascending=False).groupby("recordID").first().reset_index()
print df   
   recordID    text   dupl  texLen
0        21     abc  False       3
1        22     def  False       3
2        23  ghijkl   True       6
3        24     mto  False       3
4        25     per  False       3
5        26  stuvwx   True       6
6        27    yzab   True       4
7        28     cde  False       3
8        29     fgh  False       3
9        30     ijk  False       3