Python combine two dicts

How to Merge two or more Dictionaries in Python ?

In this article we will discuss different ways to merge two or more dictionaries. Also, handle scenarios where we need to keep the values of common keys instead of overwriting them.

Merge two dictionaries using dict.update()

In Python, the Dictionary class provides a function update() i.e.

It accepts an another dictionary or an Iterable object (collection of key value pairs) as argument. Then merges the contents of this passed dictionary or Iterable in the current dictionary.

Let’s use this update() function to merge two dictionaries.

Frequently Asked:

Suppose we have two dictionaries i.e.

# Create first dictionary dict1 = < 'Ritika': 5, 'Sam': 7, 'John' : 10 ># Create second dictionary dict2 =

Both dictionaries has a common key ‘Sam’ with different values. Now let’s merge the contents of dict2 in dict1 i.e.

# Merge contents of dict2 in dict1 dict1.update(dict2) print('Updated dictionary 1 :') print(dict1)

Now the content of dict1 is,

All the elements in dict2 are added to dict1. Keys which are common in both the dictionaries will contain the values as in dict2. Basically the dictionary we are passing in update() as argument will override the common key’s values. Therefore ‘Sam’ has value 20 now.

Another important point to notice is that, we didn’t got a new dictionary. The contents of dict1 changed and now apart from its existing contents it has the contents of dict2 too. What if we want to merged the contents of 2 or dictionaries to a new dictionary ? Let’s see how to do that.

Merge two or more Dictionaries using **kwargs

**kwargs

Using **kwargs we can send variable length key-value pairs to a function. When we apply ** to a dictionary, then it expands the contents in dictionary as a collection of key value pairs.
For example, if we have a dictionary i.e.

When we apply ** to this dictionary, it de-serializes the contents of dictionary to a collection of key/value pairs i.e.

So, let’s use **kwargs to merge two or more dictionaries.
Suppose we have two dictionaries i.e.

# Create first dictionary dict1 = < 'Ritika': 5, 'Sam': 7, 'John' : 10 ># Create second dictionary dict2 =

Now merge the contents of dict1 and dict2 to a new dictionary dict3 i.e.

# Merge contents of dict2 and dict1 to dict3 dict3 = <**dict1 , **dict2>print('Dictionary 3 :') print(dict3)

Content of the new dictionary is,

How did it worked ?

**dict1 & **dict2 expanded the contents of both the dictionaries to a collection of key value pairs i.e.

Therefore, a new dictionary is created that contains the data from both the dictionaries.

Both dict1 & dict2 had one common key ‘Sam’. In dict3 value for this common key ‘Sam’ is as in dict2 because we passed the **dict2 as second argument.

Merge three dictionaries

Similarly we can merge 3 dictionaries i.e.

# Create first dictionary dict1 = < 'Ritika': 5, 'Sam': 7, 'John' : 10 ># Create second dictionary dict2 = # Create second dictionary dict3 = # Merge contents of dict3, dict2 and dict1 to dict4 dict4 = <**dict1, **dict2, **dict3>print('Dictionary 3 :') print(dict4)

Till now we have seen that while merging dictionaries, values of common keys are getting overridden. What if we want to keep all the values ?

Merge two dictionaries and add values of common keys

Suppose we have two dictionaries with common key i.e.

# Create first dictionary dict1 = < 'Ritika': 5, 'Sam': 7, 'John' : 10 ># Create second dictionary dict2 =

Now we want to merge these dictionaries in way that it should keep all the values for common keys in a list i.e.

def mergeDict(dict1, dict2): ''' Merge dictionaries and keep values of common keys in list''' dict3 = <**dict1, **dict2>for key, value in dict3.items(): if key in dict1 and key in dict2: dict3Python combine two dicts = [value , dict1Python combine two dicts] return dict3 # Merge dictionaries and add values of common keys in a list dict3 = mergeDict(dict1, dict2) print('Dictionary 3 :') print(dict3)

Both the dictionaries had a common key ‘Sam’. In the merged dictionary dict3, both the values of ‘Sam’ from dict1 & dict2 are merged to a list.

We can use this function to merge 3 dictionaries and keep the all the values for common keys i.e.

# Create first dictionary dict1 = < 'Ritika': 5, 'Sam': 7, 'John' : 10 ># Create second dictionary dict2 = # Third Dictionary dict3 = # Merge 3 dictionary and keep values of common keys in a list finalDict = mergeDict(dict3, mergeDict(dict1, dict2)) print('Final Dictionary :') print(finalDict)

Python Dictionary Tutorial — Series:

1. What is a Dictionary in Python & why do we need it?
2. Creating Dictionaries in Python
3. Iterating over dictionaries
4. Check if a key exists in dictionary
5. Check if a value exists in dictionary
6. Get all the keys in Dictionary
7. Get all the Values in a Dictionary
8. Remove a key from Dictionary
9. Add key/value pairs in Dictionary
10. Find keys by value in Dictionary
11. Filter a dictionary by conditions
12. Print dictionary line by line
13. Convert a list to dictionary
14. Sort a Dictionary by key
15. Sort a dictionary by value in descending or ascending order
16. Dictionary: Shallow vs Deep Copy
17. Remove keys while Iterating
18. Get all keys with maximum value
19. Merge two or more dictionaries in python

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Complete example is as follows :

def mergeDict(dict1, dict2): ''' Merge dictionaries and keep values of common keys in list''' dict3 = <**dict1, **dict2>for key, value in dict3.items(): if key in dict1 and key in dict2: dict3Python combine two dicts = [value , dict1Python combine two dicts] return dict3 def main(): # Create first dictionary dict1 = < 'Ritika': 5, 'Sam': 7, 'John' : 10 ># Create second dictionary dict2 = print('Dictionary 1 :') print(dict1) print('Dictionary 2 :') print(dict2) print('*** Merge two dictionaries using update() ***') # Merge contents of dict2 in dict1 dict1.update(dict2) print('Updated dictionary 1 :') print(dict1) print('*** Merge two dictionaries using ** trick ***') # Create first dictionary dict1 = < 'Ritika': 5, 'Sam': 7, 'John' : 10 ># Create second dictionary dict2 = # Merge contents of dict2 and dict1 to dict3 dict3 = <**dict1 , **dict2>print('Dictionary 3 :') print(dict3) print('*** Merge 3 dictionaries using ** trick ***') # Create second dictionary dict3 = # Merge contents of dict3, dict2 and dict1 to dict4 dict4 = <**dict1, **dict2, **dict3>print('Dictionary 4 :') print(dict4) print('*** Merge two dictionaries and add values of common keys ***') # Create second dictionary # Merge contents of dict2 and dict1 to dict3 print(dict1) print(dict2) # Merge dictionaries and add values of common keys in a list dict3 = mergeDict(dict1, dict2) print('Dictionary 3 :') print(dict3) dict3 = print(dict3) # Merge 3 dictionary and keep values of common keys in a list finalDict = mergeDict(dict3, mergeDict(dict1, dict2)) print('Final Dictionary :') print(finalDict) if __name__ == '__main__': main()

Dictionary 1 : Dictionary 2 : *** Merge two dictionaries using update() *** Updated dictionary 1 : *** Merge two dictionaries using ** trick *** Dictionary 3 : *** Merge 3 dictionaries using ** trick *** Dictionary 4 : *** Merge two dictionaries and add values of common keys *** Dictionary 3 : Final Dictionary :

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How to Merge Dictionaries in Python

Ashutosh Krishna

In Python, a dictionary is a collection you use to store data in pairs. It is ordered and mutable, and it cannot store duplicate data.

We write a dictionary using curly brackets like this:

Sometimes, we need to merge two or more dictionaries to create a bigger dictionary. For example:

dict_one = < "id": 1, "name": "Ashutosh", "books": ["Python", "DSA"] >dict_two = < "college": "NSEC", "city": "Kolkata", "country": "India" >merged_dict =

In the merged_dict we have the key-value pairs of both dict_one and dict_two . This is what we wish to achieve programmatically.

There are various ways we can do this in Python:

1. Using a for loop
2. Using the dict.update() method
3. Using the ** operator
4. Using the | (Union) operator (for Python 3.9 and above)

Let’s explore each way one after another.

How to Merge Dictionaries in Python Using a For Loop

We can merge two or more dictionaries using for loop like this:

>>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"], . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> for key,value in dict_two.items(): . merged_dictPython combine two dicts = value . >>> merged_dict >>> for key,value in dict_three.items(): . merged_dictPython combine two dicts = value . >>> merged_dict

But the problem with this method is that we need to run so many loops to merge the dictionaries.

How to Merge Dictionaries in Python Using the dict.update() Method

If you explore the dict class, there are various methods inside it. One such method is the update() method which you can use to merge one dictionary into another.

>>> dict_one = < . "id": 1, . "name": "Ashutosh", . "books": ["Python", "DSA"] . >>>> dict_two = < . "college": "NSEC", . "city": "Kolkata", . "country": "India" . >>>> dict_one.update(dict_two) >>> dict_one

But the problem when we use the update() method is that it modifies one of the dictionaries. If we wish to create a third dictionary without modifying any of the other dictionaries, we cannot use this method.

Also, you can only use this method to merge two dictionaries at a time. If you wish to merge three dictionaries, you first need to merge the first two, and then merge the third one with the modified dictionary.

>>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"], . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> dict_one.update(dict_two) >>> dict_one >>> dict_one.update(dict_three) >>> dict_one

Let’s explore some other options.

How to Merge Dictionaries in Python Using the ** operator

You can use the double-asterisk (**) method to unpack or expand a dictionary like this:

>>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"] . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> merged_dict = <**dict_one, **dict_two, **dict_three>>>> merged_dict

Using the ** operator to merge the dictionaries doesn’t affect any of the dictionaries.

How to Merge Dictionaries in Python Using the | Operator

Starting with Python 3.9, we can use the Union ( | ) operator to merge two or more dictionaries.

>>> dict_one = < . "id": 1, . "name": "Ashutosh", . >>>> dict_two = < . "books": ["Python", "DSA"], . "college": "NSEC", . >>>> dict_three = < . "city": "Kolkata", . "country": "India" . >>>> merged_dict = dict_one | dict_two | dict_three >>> merged_dict

This is the most convenient method available for merging dictionaries in Python.

Conclusion

We have explored several different methods for merging dictionaries. If you have Python 3.9 or above, you should use the | operator. But if you use older versions of Python, you can still use the other methods discussed above.

Ashutosh Krishna

Application Developer at Thoughtworks India

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