5 Best Ways to Merge Pandas DataFrames with Left Outer Join in Python

πŸ’‘ Problem Formulation: In data analysis, one often needs to combine different datasets with overlapping keys. A left outer join is useful for merging two DataFrames where you want to include all keys from the left DataFrame and only the matching keys from the right, with NaNs where data is missing. Suppose you have a sales DataFrame with all products and a reviews DataFrame with reviewed products, and you wish to merge them, keeping all products even if some do not have reviews.

Method 1: Using DataFrame.merge()

This primary method involves utilizing the merge() function from Pandas, which allows for SQL-like joining of DataFrames. It is specification rich, allowing for different types of join operations, including the left outer join. The left DataFrame is the one you call .merge() on, and you pass the right DataFrame as the first argument.

Here’s an example:

import pandas as pd

# Create two example DataFrames
df_left = pd.DataFrame({
    'A': ['A0', 'A1', 'A2'],
    'B': ['B0', 'B1', 'B2']
})
df_right = pd.DataFrame({
    'A': ['A1', 'A2', 'A3'],
    'C': ['C1', 'C2', 'C3']
})

# Perform a left outer join
result = df_left.merge(df_right, on='A', how='left')

The output of this code would be a DataFrame:

    A   B    C
0  A0  B0  NaN
1  A1  B1   C1
2  A2  B2   C2

This code snippet takes two DataFrames df_left and df_right and merges them on column ‘A’ using a left outer join. The resulting DataFrame result contains all rows from the left DataFrame and matching rows from the right, with NaN where there is no match.

Method 2: Using DataFrame.join()

The join() method is a convenient way to combine DataFrames when you want to join them on their indexes or a key column. By default, it performs a left join, and you can explicitly set this with the how parameter. It’s best used when the DataFrames have the same index or when joining on keys without merging columns.

Here’s an example:

import pandas as pd

# Create two example DataFrames
df_left = pd.DataFrame({
    'B': ['B0', 'B1', 'B2']
}, index=['A0', 'A1', 'A2'])
df_right = pd.DataFrame({
    'C': ['C0', 'C1', 'C2']
}, index=['A1', 'A2', 'A3'])

# Perform a left join on indexes
result = df_left.join(df_right, how='left')

The output of this code would be a DataFrame:

     B    C
A0  B0  NaN
A1  B1   C0
A2  B2   C1

This code snippet joins the DataFrames df_left and df_right using their indexes. It preserves the left DataFrame’s index, adding the column from the right DataFrame where the indexes match.

Method 3: Using concat() for Index-based Joining

The concat() function is typically used for concatenating along a particular axis. A less known feature is its ability to perform index-based outer joins. It’s useful for quickly joining multiple DataFrames vertically or horizontally while aligning on the index.

Here’s an example:

import pandas as pd

# Create two example DataFrames
df_left = pd.DataFrame({
    'B': ['B0', 'B1', 'B2']
}, index=['A0', 'A1', 'A2'])
df_right = pd.DataFrame({
    'C': ['C1', 'C2', 'C3']
}, index=['A1', 'A2', 'A3'])

# Use concat to perform an index-based left outer join
result = pd.concat([df_left, df_right], axis=1, join='outer')

The output of this code would be a DataFrame:

     B    C
A0  B0  NaN
A1  B1   C1
A2  B2   C2
A3  NaN  C3

In this example, pd.concat aligns the DataFrames based on their indexes across the given axis and fills with NaNs for any missing values, essentially performing an index-based left outer join. It is important to note that instead of focusing on left or right, it does a full outer join by default.

Method 4: Using DataFrame.merge() with Indicators

When merging DataFrames, sometimes it is useful to keep track of which side each row comes from. Pandas merge() can include an indicator that marks whether the row is from the left only, right only, or both. This is particularly helpful for debugging and data inspection after a merge.

Here’s an example:

import pandas as pd

# Create two example DataFrames
df_left = pd.DataFrame({
    'key': ['K0', 'K1', 'K2'],
    'A': ['A0', 'A1', 'A2']
})
df_right = pd.DataFrame({
    'key': ['K1', 'K2', 'K3'],
    'B': ['B1', 'B2', 'B3']
})

# Perform a left outer join with an indicator
result = df_left.merge(df_right, on='key', how='left', indicator=True)

The output of the code is a DataFrame with an additional column ‘_merge’ indicating the source of each row:

  key   A    B     _merge
0  K0  A0  NaN  left_only
1  K1  A1   B1       both
2  K2  A2   B2       both

This snippet demonstrates using the merge() function with an additional indicator parameter. This parameter adds a special column ‘_merge’ which clearly shows whether the data is merged from ‘left_only’, ‘right_only’, or from both DataFrames.

Bonus One-Liner Method 5: Using functools.reduce for Multiple DataFrames

For complex operations where multiple DataFrames are joined in a sequence, Python’s functools.reduce can be used to apply a merge operation cumulatively to an iterable of DataFrames. This is a compact, functional programming approach for successive merging.

Here’s an example:

import pandas as pd
from functools import reduce

# Create three example DataFrames
df1 = pd.DataFrame({'A': ['A0', 'A1'], 'B': ['B0', 'B1']})
df2 = pd.DataFrame({'A': ['A1', 'A2'], 'C': ['C1', 'C2']})
df3 = pd.DataFrame({'A': ['A2', 'A3'], 'D': ['D1', 'D2']})

# Perform a cumulative left outer join on multiple DataFrames
dfs = [df1, df2, df3]
result = reduce(lambda left, right: pd.merge(left, right, on='A', how='left'), dfs)

The output of this code will be:

    A   B    C    D
0  A0  B0  NaN  NaN
1  A1  B1   C1  NaN
2  A2  NaN  C2   D1

In this example, reduce() from the functools module applies the merge function cumulatively to the DataFrames in the list dfs. It sequentially performs left outer joins on the ‘A’ column of each DataFrame, resulting in a combined DataFrame.

Summary/Discussion

  • Method 1: Using DataFrame.merge(). Very flexible and the most common approach for merging DataFrames. Allows explicit specification of the join key and type of join.
  • Method 2: Using DataFrame.join(). More limited than merge but more succinct for joins on indexes or key columns without overlap. Defaults to a left join.
  • Method 3: Using concat() for Index-based Joining. A quick and straightforward approach for index-aligned DataFrames, with flexibility in handling axis.
  • Method 4: Using DataFrame.merge() with Indicators. Adds valuable debugging information about the origin of the merge result, especially in more complex data merges.
  • Method 5: Using functools.reduce for Multiple DataFrames. It simplifies multiple, sequential merge operations into a single line of code, though it may sacrifice some readability for those not familiar with functional programming concepts.