The World of Python Programming

Merging Python Dictionaries: Complete Guide with Code

Data Types & Structures
目次

1. Introduction

Python’s dict is a convenient data structure for managing data as key-value pairs. There are many situations where you need to combine dictionaries—for example, when merging multiple configuration files or consolidating different datasets. This article explains in detail various ways to merge dictionaries in Python. Aimed at beginners through intermediate users, it covers basic techniques, the method introduced in Python 3.9, and approaches useful in special cases. With code examples, we’ll explore the characteristics of each method and when to use them.

2. Basic Methods for Merging Dictionaries

Python provides several ways to merge dictionaries. First, we’ll explain the basic methods.

2.1 Using the update() Method

Features

update() method is the most basic way to merge one dictionary into another. This operation is destructive (it modifies the original dictionary), so be careful if you need to preserve the original dictionary’s contents.

Code example

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

dict1.update(dict2)
print(dict1)  # {'a': 1, 'b': 3, 'c': 4}

Explanation

In the code above, dict2 is merged into dict1. If keys are duplicated, values from the dictionary merged later (in this case dict2) will overwrite them.

Use cases

  • When it’s acceptable to modify the original dictionary.
  • When you need a simple and efficient approach.

2.2 Using the unpacking operator (**)

Features

Since Python 3.5, you can use the unpacking operator (**) to merge dictionaries. This method is non-destructive (it does not modify the originals) and creates a new dictionary.

Code example

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

combined_dict = {**dict1, **dict2}
print(combined_dict)  # {'a': 1, 'b': 3, 'c': 4}

Explanation

This method expands multiple dictionaries to create a new one. If keys overlap, the values from the dictionary specified later (dict2) take precedence.

Use cases

  • When you need a new dictionary without modifying the originals.
  • When readability is important.

2.3 Using the merge operator (|)

Features

Starting with Python 3.9, the | operator was introduced, allowing concise dictionary merging. This method is also non-destructive and creates a new dictionary.

Code example

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

combined_dict = dict1 | dict2
print(combined_dict)  # {'a': 1, 'b': 3, 'c': 4}

Explanation

This approach lets you merge dictionaries in an intuitive way. It improves code readability and is easy for Python beginners to understand.

Use cases

  • When using Python 3.9 or later.
  • When you need simple, easy-to-understand syntax.
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3. Merging dictionaries in special cases

Having covered the basic methods for merging dictionaries, this section explains special cases and applied techniques. It covers methods useful in specific situations and scenarios that require more advanced operations.

3.1 Using the dict() constructor

Features

This method uses the dict() constructor to merge multiple dictionaries. It is useful when you want to directly specify additional values for a dictionary or create a new dictionary that includes unpacked dictionaries.

Code example

dict1 = {'a': 1, 'b': 2}
dict2 = {'c': 3}

combined_dict = dict(dict1, **dict2)
print(combined_dict)  # {'a': 1, 'b': 2, 'c': 3}

Explanation

  • dict1 serves as the base, and dict2 is unpacked on top of it to perform the merge.
  • Note that because it uses the ** operator, the keys of dict2 must be strings.

Use cases

  • When you want to add new keys and values in addition to basic dictionary merging.
  • Requires Python version 3.5 or later.

3.2 Using collections.ChainMap

Features

The ChainMap class in the Python standard library’s collections module is useful for temporarily combining multiple dictionaries for manipulation. With this method, the dictionaries are not merged and the original dictionaries are preserved.

Code example

from collections import ChainMap

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

combined = ChainMap(dict1, dict2)
print(combined['b'])  # 2
print(combined['c'])  # 4

Explanation

  • ChainMap treats multiple dictionaries as a single virtual dictionary.
  • Key lookups are performed preferentially from the first dictionary specified. In this example, for 'b', the value from dict1 (2) is chosen.

Use cases

  • When you need to manipulate dictionaries dynamically without actually merging them.
  • When dealing with large amounts of data and you want to prioritize memory efficiency.

4. Precautions for Merging Dictionaries

There are several points to be aware of when merging dictionaries. Understanding these can help prevent unexpected errors and data inconsistencies.

4.1 Behavior of Duplicate Keys

When merging dictionaries, if duplicate keys exist, the value from the dictionary specified later takes precedence.

Example

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

combined = {**dict1, **dict2}
print(combined)  # {'a': 1, 'b': 3, 'c': 4}
In this example, for the key 'b', the value dict2 (3) is chosen. It is important to understand this behavior.

4.2 Performance Differences

The performance may vary depending on the method used to merge dictionaries. Pay special attention when working with large volumes of data.

Performance Comparison (Overview)

  • update() method is efficient but modifies the original dictionary, making it unsuitable if you need to keep a backup.
  • Unpack operator (**) is non-destructive and convenient, but may increase memory usage when handling large amounts of data.
  • ChainMap does not actually merge the dictionaries, so it’s memory-efficient.
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5. Comparison of Methods and Selection Guide

Compares various methods for merging dictionaries and provides guidance for selecting the appropriate approach.

5.1 Comparison Table of Methods

MethodDestructive/Non-destructiveVersionDuplicate Key BehaviorPerformance
update()DestructiveAll versionsLater dictionary takes precedenceFast
Unpacking operator (**)Non-destructive3.5 and laterLater dictionary takes precedenceModerate
Merge operator (|)Non-destructive3.9 and laterLater dictionary takes precedenceModerate
ChainMapNon-destructiveAll versionsFirst dictionary takes precedenceHigh memory efficiency

5.2 Selection Guide

  • For Python 3.9 and later:
  • We recommend the | operator for its simplicity and readability.
  • For Python 3.5–3.8:
  • It’s recommended to use the unpacking operator (**).
  • If handling large datasets or prioritizing memory efficiency:
  • It’s best to use ChainMap.
  • If destructive operations are acceptable:
  • The update() method is efficient.

6. Frequently Asked Questions (FAQ)

This section addresses common questions readers have about merging dictionaries in Python, and explains causes of errors and how to handle special cases.

6.1 Why do errors occur when merging dictionaries?

Problem example

Running the following code produces an error.
dict1 = {'a': 1, 'b': 2}
dict2 = {('c',): 3}

combined_dict = {**dict1, **dict2}

Cause

When using the unpacking operator (**), keys must be strings; otherwise an error will occur. In the example above, the key ('c',) in dict2 is a tuple, which causes the error.

Solution

If the keys are not strings, use the update() method or manually merge the dictionaries using a loop.
dict1 = {'a': 1, 'b': 2}
dict2 = {('c',): 3}

dict1.update(dict2)  # Works correctly
print(dict1)  # {'a': 1, 'b': 2, ('c',): 3}

6.2 What are the alternatives if you want to use the | operator on Python versions earlier than 3.9?

Answer

On Python versions earlier than 3.9, the | operator cannot be used, but you can achieve the same result using the unpacking operator (**).

Example

You can use the following code on Python 3.8 and earlier.
dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

combined_dict = {**dict1, **dict2}
print(combined_dict)  # {'a': 1, 'b': 3, 'c': 4}

6.3 How do you merge nested dictionaries?

Problem

When merging nested dictionaries, the usual methods may not work as expected.
dict1 = {'a': {'x': 1}}
dict2 = {'a': {'y': 2}}

combined_dict = {**dict1, **dict2}
print(combined_dict)  # {'a': {'y': 2}}
In the example above, the 'a' key is overwritten by dict2, and the value dict1 is lost.

Solution

To merge nested dictionaries you need a recursive approach.
def merge_dicts(d1, d2):
    result = d1.copy()
    for key, value in d2.items():
        if key in result and isinstance(result[key], dict) and isinstance(value, dict):
            result[key] = merge_dicts(result[key], value)
        else:
            result[key] = value
    return result

dict1 = {'a': {'x': 1}}
dict2 = {'a': {'y': 2}}

combined_dict = merge_dicts(dict1, dict2)
print(combined_dict)  # {'a': {'x': 1, 'y': 2}}

6.4 Can you merge multiple dictionaries at once using the merge operator?

Answer

In Python 3.9 and later, when merging multiple dictionaries at once, you need to use the merge operator consecutively.

Example

dict1 = {'a': 1}
dict2 = {'b': 2}
dict3 = {'c': 3}

combined_dict = dict1 | dict2 | dict3
print(combined_dict)  # {'a': 1, 'b': 2, 'c': 3}
You can also achieve the same result using the unpacking operator.
combined_dict = {**dict1, **dict2, **dict3}
print(combined_dict)  # {'a': 1, 'b': 2, 'c': 3}

6.5 What’s the best way to merge dictionaries when working with large amounts of data?

Answer

When working with large datasets, using ChainMap can merge efficiently. ChainMap does not actually combine the dictionaries, so it can reduce memory usage.

Example

from collections import ChainMap

dict1 = {'a': 1}
dict2 = {'b': 2}

combined = ChainMap(dict1, dict2)
print(combined['a'])  # 1
print(combined['b'])  # 2

7. Summary

This article explained various ways to merge dictionaries in Python, covering everything from basic to the latest techniques with concrete examples that are easy for beginners to understand.

Key points

  • update() method is a simple and efficient way to modify the original dictionary.
  • Unpacking operator (**) is non-destructive and useful for creating a new dictionary.
  • Merge operator (|) is available in Python 3.9 and later, and makes code simpler and more readable.
  • ChainMap is useful when dealing with large amounts of data efficiently and can save memory.
Each method has its own characteristics, so it’s important to choose the best approach according to your goals and circumstances. For example, when processing large amounts of data or watching out for duplicate dictionary keys, ChainMap or the unpacking operator can be effective. When merging dictionaries in actual projects going forward, be sure to apply the methods learned in this article. Trying them out in real code is the most effective way to deepen your understanding. Also, check out other articles on data manipulation in Python.
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