Pandas Drop Duplicate Rows - drop_duplicates() function

Pandas drop_duplicates() function removes duplicate rows from the DataFrame. Its syntax is:

drop_duplicates(self, subset=None, keep="first", inplace=False)

• subset: column label or sequence of labels to consider for identifying duplicate rows. By default, all the columns are used to find the duplicate rows.
• keep: allowed values are {‘first’, ‘last’, False}, default ‘first’. If ‘first’, duplicate rows except the first one is deleted. If ‘last’, duplicate rows except the last one is deleted. If False, all the duplicate rows are deleted.
• inplace: if True, the source DataFrame is changed and None is returned. By default, source DataFrame remains unchanged and a new DataFrame instance is returned.

Let’s look into some examples of dropping duplicate rows from a DataFrame object.

1. Drop Duplicate Rows Keeping the First One

This is the default behavior when no arguments are passed.

import pandas as pd

d1 = {'A': [1, 1, 1, 2], 'B': [2, 2, 2, 3], 'C': [3, 3, 4, 5]}

source_df = pd.DataFrame(d1)
print('Source DataFrame:\n', source_df)

# keep first duplicate row
result_df = source_df.drop_duplicates()
print('Result DataFrame:\n', result_df)

Output:

Source DataFrame:
    A  B  C
0  1  2  3
1  1  2  3
2  1  2  4
3  2  3  5
Result DataFrame:
    A  B  C
0  1  2  3
2  1  2  4
3  2  3  5

The source DataFrame rows 0 and 1 are duplicates. The first occurrence is kept and the rest of the duplicates are deleted.

2. Drop Duplicates and Keep Last Row

result_df = source_df.drop_duplicates(keep='last')
print('Result DataFrame:\n', result_df)

Output:

Result DataFrame:
    A  B  C
1  1  2  3
2  1  2  4
3  2  3  5

The index ‘0’ is deleted and the last duplicate row ‘1’ is kept in the output.

3. Delete All Duplicate Rows from DataFrame

result_df = source_df.drop_duplicates(keep=False)
print('Result DataFrame:\n', result_df)

Output:

Result DataFrame:
    A  B  C
2  1  2  4
3  2  3  5

Both the duplicate rows ‘0’ and ‘1’ are dropped from the result DataFrame.

4. Identify Duplicate Rows based on Specific Columns

import pandas as pd

d1 = {'A': [1, 1, 1, 2], 'B': [2, 2, 2, 3], 'C': [3, 3, 4, 5]}

source_df = pd.DataFrame(d1)
print('Source DataFrame:\n', source_df)

result_df = source_df.drop_duplicates(subset=['A', 'B'])
print('Result DataFrame:\n', result_df)

Output:

Source DataFrame:
    A  B  C
0  1  2  3
1  1  2  3
2  1  2  4
3  2  3  5
Result DataFrame:
    A  B  C
0  1  2  3
3  2  3  5

The columns ‘A’ and ‘B’ are used to identify duplicate rows. Hence, rows 0, 1, and 2 are duplicates. So, rows 1 and 2 are removed from the output.

5. Remove Duplicate Rows in place

source_df.drop_duplicates(inplace=True)
print(source_df)

Output:

   A  B  C
0  1  2  3
2  1  2  4
3  2  3  5

• Python Pandas Module Tutorial
• Pandas DataFrame drop_duplicates() API Doc