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.

dict.update([other])

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.

Suppose we have two dictionaries i.e.

# Create first dictionary
dict1 = {  'Ritika': 5, 'Sam': 7, 'John' : 10 }

# Create second dictionary
dict2 = {'Aadi': 8,'Sam': 20,'Mark' : 11 }

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,
{'Ritika': 5, 'Sam': 20, 'John': 10, 'Aadi': 8, 'Mark': 11}

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.

dict1 = {  'Ritika': 5, 'Sam': 7, 'John' : 10 }

When we apply ** to this dictionary, it de-serializes the contents of dictionary to a collection of key/value pairs i.e.
**dict1

is equivalent to,
'Ritika': 5, 'Sam': 7, 'John' : 10

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 = {'Aadi': 8,'Sam': 20,'Mark' : 11 }

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,
Dictionary 3 :
{'Ritika': 5, 'Sam': 20, 'John': 10, 'Aadi': 8, 'Mark': 11}

How did it worked ?

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

dict3 = {'Ritika': 5, 'Sam': 7, 'John' : 10 , 'Aadi': 8,'Sam': 20,'Mark' : 11}

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 = {'Aadi': 8,'Sam': 20,'Mark' : 11 }

# Create second dictionary
dict3 = {'Mark': 18,'Rose': 22,'Wong' : 22 }

# Merge contents of dict3, dict2 and dict1 to dict4
dict4 = {**dict1, **dict2, **dict3}

print('Dictionary 3 :')
print(dict4)

Output:
Dictionary 4 :
{'Ritika': 5, 'Sam': 20, 'John': 10, 'Aadi': 8, 'Mark': 18, 'Rose': 22, 'Wong': 22}

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 = {'Aadi': 8,'Sam': 20,'Mark' : 11 }

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:
               dict3[key] = [value , dict1[key]]

   return dict3

# Merge dictionaries and add values of common keys in a list
dict3 = mergeDict(dict1, dict2)

print('Dictionary 3 :')
print(dict3)

Output:
{'Ritika': 5, 'Sam': [20, 7], 'John': 10, 'Aadi': 8, 'Mark': 11}

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 = {'Aadi': 8,'Sam': 20,'Mark' : 11 }

# Third Dictionary
dict3 = {'Mark': 18, 'Rose': 22, 'Wong': 22}

# Merge 3 dictionary and keep values of common keys in a list
finalDict = mergeDict(dict3, mergeDict(dict1, dict2))

print('Final Dictionary :')
print(finalDict)

Output:
Final Dictionary :
{'Mark': [11, 18], 'Rose': 22, 'Wong': 22, 'Ritika': 5, 'Sam': [20, 7], 'John': 10, 'Aadi': 8}

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:
               dict3[key] = [value , dict1[key]]

   return dict3

def main():


   # Create first dictionary
   dict1 = {  'Ritika': 5, 'Sam': 7, 'John' : 10 }

   # Create second dictionary
   dict2 = {'Aadi': 8,'Sam': 20,'Mark' : 11 }

   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 = {'Aadi': 8,'Sam': 20,'Mark' : 11 }

   # 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 = {'Mark': 18,'Rose': 22,'Wong' : 22 }

   # 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 = {'Mark': 18, 'Rose': 22, 'Wong': 22}

   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()

Output:
Dictionary 1 :
{'Ritika': 5, 'Sam': 7, 'John': 10}
Dictionary 2 :
{'Aadi': 8, 'Sam': 20, 'Mark': 11}
*** Merge two dictionaries using update() ***
Updated dictionary 1 :
{'Ritika': 5, 'Sam': 20, 'John': 10, 'Aadi': 8, 'Mark': 11}
*** Merge two dictionaries using ** trick ***
Dictionary 3 :
{'Ritika': 5, 'Sam': 20, 'John': 10, 'Aadi': 8, 'Mark': 11}
*** Merge 3 dictionaries using ** trick ***
Dictionary 4 :
{'Ritika': 5, 'Sam': 20, 'John': 10, 'Aadi': 8, 'Mark': 18, 'Rose': 22, 'Wong': 22}
*** Merge two dictionaries and add values of common keys ***
{'Ritika': 5, 'Sam': 7, 'John': 10}
{'Aadi': 8, 'Sam': 20, 'Mark': 11}
Dictionary 3 :
{'Ritika': 5, 'Sam': [20, 7], 'John': 10, 'Aadi': 8, 'Mark': 11}
{'Mark': 18, 'Rose': 22, 'Wong': 22}
Final Dictionary :
{'Mark': [11, 18], 'Rose': 22, 'Wong': 22, 'Ritika': 5, 'Sam': [20, 7], 'John': 10, 'Aadi': 8}

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