# Python Merge Two Dictionaries in One Expression

> Use the | merge operator in Python 3.9+ to combine dictionaries in a single expression, or the ** unpacking operator and dict.update() for older versions

**URL:** https://sentry.io/answers/python-merge-two-dicts/

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## The Problem

You'd like to merge two or more Python dictionaries in a single expression. For example, you have the following dictionaries storing some data about fictional characters:

```python
c1 = {
	'The Hound of the Baskervilles': 'Sherlock Holmes',
	'To Kill A Mockingbird': 'Scout Finch',
	'Heart of Darkness': 'Kurtz'
}

c2 = {
	'Lord of the Rings': 'Frodo Baggins',
	'The Count of Monte Cristo': 'Edmond Dantès'
}
```

How can you merge them into a single dictionary in a single line of code?

## The Solution

Depending on which version of Python you're using, you can choose one of the following methods to merge Python dictionaries.

### Merging Dictionaries Using the `|` Operator

Python 3.9 introduced the [merge operator(`|`)](https://docs.python.org/3.9/whatsnew/3.9.html#dictionary-merge-update-operators) in the `dict` class. Using the merge operator is the easiest way to merge dictionaries. The merge operator returns a new dictionary, leaving the original dictionaries unchanged.

```shell
>>> c = c1 | c2
>>> c
{'The Hound of the Baskervilles': 'Sherlock Holmes', 'To Kill A Mockingbird': 'Scout Finch', 'Heart of Darkness': 'Kurtz', 'Lord of the Rings': 'Frodo Baggins', 'The Count of Monte Cristo': 'Edmond Dantès'}
```

We can change the order of the dictionaries, like so:

```shell
>>> c = c2 | c1
>>> c
{'Lord of the Rings': 'Frodo Baggins', 'The Count of Monte Cristo': 'Edmond Dantès', 'The Hound of the Baskervilles': 'Sherlock Holmes', 'To Kill A Mockingbird': 'Scout Finch', 'Heart of Darkness': 'Kurtz'}
```

We can merge multiple dictionaries at once using this method.

### Merging Dictionaries Using the `|=` Operator

If we want to merge the dictionaries in-place, we can use the [update operator](https://docs.python.org/3.9/whatsnew/3.9.html#dictionary-merge-update-operators) (`|=`):

```shell
>>> c1 |= c2
>>> c1
{'The Hound of the Baskervilles': 'Sherlock Holmes', 'To Kill A Mockingbird': 'Scout Finch', 'Heart of Darkness': 'Kurtz', 'Lord of the Rings': 'Frodo Baggins', 'The Count of Monte Cristo': 'Edmond Dantès'}
>>> c2
{'Lord of the Rings': 'Frodo Baggins', 'The Count of Monte Cristo': 'Edmond Dantès'}
```

We can merge multiple dictionaries at once using this method.

Like the merge operator, the update operator will work with Python 3.9 and above only.

### Merging Dictionaries Using the `**` Operator

In Python 3.5 and above, we can also use the [unpacking operator (`**`)](https://docs.python.org/3/tutorial/controlflow.html#unpacking-argument-lists) to merge the dictionaries:

```shell
>>> c3 = {**c1, **c2}
>>> c3
{'The Hound of the Baskervilles': 'Sherlock Holmes', 'To Kill A Mockingbird': 'Scout Finch', 'Heart of Darkness': 'Kurtz', 'Lord of the Rings': 'Frodo Baggins', 'The Count of Monte Cristo': 'Edmond Dantès'}
```

Using the unpacking operator doesn't change the original dictionaries. If you want to merge the dictionaries in-place, you cannot use the unpacking operator. We can merge more than two dictionaries at once using this method.

### Merging Dictionaries Using the `dict.update()` Method

For Python versions below 3.5, we can use the `update()` method of the `dict` class to merge one dictionary into another:

```shell
>>> c1.update(c2)
>>> c1
{'The Hound of the Baskervilles': 'Sherlock Holmes', 'To Kill A Mockingbird': 'Scout Finch', 'Heart of Darkness': 'Kurtz', 'Lord of the Rings': 'Frodo Baggins', 'The Count of Monte Cristo': 'Edmond Dantès'}
```

Using this method, we can only merge the dictionaries in-place, i.e. one of the dictionaries will be modified. So if we need to keep the original dictionaries unchanged, we need to create a copy of one of the dictionaries and then merge the second one into this copy.

However, we can only merge two dictionaries at a time using this method.

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*Source: [sentry.io/answers/python-merge-two-dicts/](https://sentry.io/answers/python-merge-two-dicts/)*