Ayelavan Ayelavan - 24 days ago 30
Python Question

Anti-Join Pandas

I have two tables and I would like to append them so that only all the data in table A is retained and data from table B is only added if its key is unique (Key values are unique in table A and B however in some cases a Key will occur in both table A and B).

I think the way to do this will involve some sort of filtering join (anti-join) to get values in table B that do not occur in table A then append the two tables.

I am familiar with R and this is the code I would use to do this in R.

library("dplyr")

## Filtering join to remove values already in "TableA" from "TableB"
FilteredTableB <- anti_join(TableB,TableA, by = "Key")

## Append "FilteredTableB" to "TableA"
CombinedTable <- bind_rows(TableA,FilteredTableB)


How would I achieve this in python?

Answer

Consider the following dataframes

TableA = pd.DataFrame(np.random.rand(4, 3),
                      pd.Index(list('abcd'), name='Key'),
                      ['A', 'B', 'C']).reset_index()
TableB = pd.DataFrame(np.random.rand(4, 3),
                      pd.Index(list('aecf'), name='Key'),
                      ['A', 'B', 'C']).reset_index()

TableA

enter image description here


TableB

enter image description here

This is one way to do what you want

Method 1

# Identify what values are in TableB and not in TableA
key_diff = set(TableB.Key).difference(TableA.Key)
where_diff = TableB.Key.isin(key_diff)

# Slice TableB accordingly and append to TableA
TableA.append(TableB[where_diff], ignore_index=True)

enter image description here

Method 2

rows = []
for i, row in TableB.iterrows():
    if row.Key not in TableA.Key.values:
        rows.append(row)

pd.concat([TableA.T] + rows, axis=1).T

Timing

4 rows with 2 overlap

Method 1 is much quicker

enter image description here

10,000 rows 5,000 overlap

loops are bad

enter image description here

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