df.merge()

df.merge(right, on=column, left_on=left_column, right_on=right_column, left_index=False, right_index=False)

Merges two DataFrames by specified columns or indexes.

Input:

right : DataFrame

DataFrame to merge df with.

on : label or list

Column or index names to join on.

left_on : label, list, or array-like

Column or index names to join on in df.

right_on : label, list, or array-like

Column or index names to join on in the right DataFrame.

left_index : boolean, default False

Use index from left DataFrame as join key(s).

right_index : boolean, default False

Use index from right DataFrame as join key(s).

Returns:

A new DataFrame with the two DataFrames merged on the specified input combinations.

Return Type:

DataFrame

Note:

• The number of columns merged on the left and right DataFrames must be the same.
• left_on and left_index cannot happen at the same time. Same as right_on and right_index.

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The diagrams below are two visualizations of how merge works albeit with different datasets from the main one we use here on bpd reference. For additional helpful visual guides, please visit the Diagrams site.

(Source)

(Source)

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pets_info = bpd.DataFrame().assign(
    Pet_ID=['dog_001', 'cat_001', 'cat_002', 'dog_002', 'dog_003', 'ham_001'],
    Breed=['Labrador', 'Sphinx', 'Siamese', 'Chihuahua', 'Labrador', 'Roborovski'],
    Owner=['Jason', 'Lauren', 'Cassidy', 'Bobby', 'Dhruv', 'Cassidy']
)
pets_info

Index	Pet_ID	Breed	Owner
0	dog_001	Labrador	Jason
1	cat_001	Sphinx	Lauren
2	cat_002	Siamese	Cassidy
3	dog_002	Chihuahua	Bobby
4	dog_003	Labrador	Dhruv
5	ham_001	Roborovski	Cassidy

pets.merge(pets_info, left_on='ID', right_on='Pet_ID')

Index	ID	Species	Color	Weight	Age	Is_Cat	Owner_Comment	Pet_ID	Breed	Owner
0	dog_001	dog	black	40	5	False	There are no bad dogs, only bad owners.	dog_001	Labrador	Jason
1	cat_001	cat	golden	1.5	0.2	True	My best birthday present ever!!!	cat_001	Sphinx	Lauren
2	cat_002	cat	black	15	9	True	****All you need is love and a cat.****	cat_002	Siamese	Cassidy
3	dog_002	dog	white	80	2	False	Love is a wet nose and a wagging tail.	dog_002	Chihuahua	Bobby
4	dog_003	dog	black	25	0.5	False	Be the person your dog thinks you are.	dog_003	Labrador	Dhruv
5	ham_001	hamster	black	1	3	False	No, thank you!	ham_001	Roborovski	Cassidy

pets_info = pets_info.set_index('Pet_ID')
pets.merge(pets_info, left_on='ID', right_index=True)

Index	ID	Species	Color	Weight	Age	Is_Cat	Owner_Comment	Breed	Owner
0	dog_001	dog	black	40	5	False	There are no bad dogs, only bad owners.	Labrador	Jason
1	cat_001	cat	golden	1.5	0.2	True	My best birthday present ever!!!	Sphinx	Lauren
2	cat_002	cat	black	15	9	True	****All you need is love and a cat.****	Siamese	Cassidy
3	dog_002	dog	white	80	2	False	Love is a wet nose and a wagging tail.	Chihuahua	Bobby
4	dog_003	dog	black	25	0.5	False	Be the person your dog thinks you are.	Labrador	Dhruv
5	ham_001	hamster	black	1	3	False	No, thank you!	Roborovski	Cassidy

pets_info2 = pets_info.assign(ID = pets_info.index)
pets.merge(pets_info2, on='ID')

Index	ID	Species	Color	Weight	Age	Is_Cat	Owner_Comment	Breed	Owner
0	dog_001	dog	black	40	5	False	There are no bad dogs, only bad owners.	Labrador	Jason
1	cat_001	cat	golden	1.5	0.2	True	My best birthday present ever!!!	Sphinx	Lauren
2	cat_002	cat	black	15	9	True	****All you need is love and a cat.****	Siamese	Cassidy
3	dog_002	dog	white	80	2	False	Love is a wet nose and a wagging tail.	Chihuahua	Bobby
4	dog_003	dog	black	25	0.5	False	Be the person your dog thinks you are.	Labrador	Dhruv
5	ham_001	hamster	black	1	3	False	No, thank you!	Roborovski	Cassidy

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