How to Merge Multiple Dictionaries with Python

In this tutorial, we'll explore how to create a dict, or dictionary, from one or more existing dictionaries in Python.

As usual in programming, there are several ways to achieve this.

I recently started to practice my writing in English. I apologize in advanced for any errors. :)

Prerequisites

• Basic understanding of Python

Initial Approach

To begin, let's assume we have the following dictionaries:

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

dict2 = {
    'b': 3,
    'c': 4,
}

For example, let's create a new dictionary called dictx using the values from dict1 and dict2 mentioned above. A common method to do this is by using the update method.

dictx = {}

dictx.update(dict1)
dictx.update(dict2)

So, dictx will be:

print(dictx)
# output: {'a': 1,'b': 3,'c': 4}

This method works well, but we have to call the update method for each dictionary we want to merge into dictx. Wouldn't it be interesting if we could pass all the dictionaries needed right when we create dictx?

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Create a dictionary from multiple dictionaries

Python 3 introduced a very useful way to do this by using the ** operator.

dictx = {
 **dict1,
 **dict2,
}

print(dictx)
# output: {'a': 1,'b': 3,'c': 4}

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A real copy of Dictionaries

When using the method described above, we need to keep a few things in mind. Only the first-level values will be copied into the new dictionary. For example, let's change the key 'a' present in both dictionaries and see if they have the same value:

dict1['a'] = 10
dictx['a'] = 11

print(dict1['a'])
# output: 10

print(dictx['a'])
# output: 11

We change the value of key 'a' in both dictionaries. However, this changes if one of the values in dict1 is a data structure, like a list, another dict, or a complex object. For example:

dict3 = {
    'a': 1,
    'b': 2,
    'c': {
        'd': 5,
    },
}

We have a dict object under the key 'c'. Now, let's create a new dict from it:

dictx = {
**dict3,
}

As in the previous example, we might think all the elements of dict3 are copied, but this isn't completely accurate. What actually happens is a shallow copy of dict3's values is made, meaning only the first-level values are duplicated. Let's see what happens when we change the value of the dictionary under the key 'c'.

dictx['c']['d'] = 11

print(dictx['c']['d'])
# output: 11

print(dict3['c']['d'])
# output: 11 (previous value was 5)

For the key 'c', it holds a reference to another data structure (a dict in this case). When we change any value in dict3['c'], it affects all dictionaries that were initialized with dict3. In other words, we need to be careful when creating a dictionary from others if they have complex values like list, dict, or other objects, as the attributes of these objects will not be duplicated.

To solve this issue, we can use the built-in dictionary method copy. Now, when we create dictx:

dict3 = {
    'a': 1,
    'b': 2,
    'c': {
        'd': 5,
    },
}

dictx = dict3.copy()

The copy method creates a shallow copy of each element in dict3, which helps solve our problem.

Here's another example:

dictx['c']['d'] = 11

print(dictx['c']['d'])
# output: 11

print(dict3['c']['d'])
# outuput: 5 (value has not been changed)

The values in dictx and dict3 are not equal because 'd' in dict3['c'] and dictx['c'] are references to different objects.

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Conclusion

This article aims to simply demonstrate how to create dictionaries using some features the language offers, along with the pros and cons of each approach.

That's it.

Thanks for reading!