How to concatenate dicts in python

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: dict3How to concatenate dicts in python = [value , dict1How to concatenate dicts in python] 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: dict3How to concatenate dicts in python = [value , dict1How to concatenate dicts in python] 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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Adding dictionaries together [duplicate]

I have two dictionaries and I’d like to be able to make them one: Something like this pseudo-Python would be nice:

dic0 = dic1 = ndic = dic0 + dic1 # ndic would equal

6 Answers 6

If you’re interested in creating a new dict without using intermediary storage: (this is faster, and in my opinion, cleaner than using dict.items() )

Or if you’re happy to use one of the existing dicts:

@BerryTsakala The problem with + is what happens in case of conflicts ? .update() is properly asymmetric.

@BerryTsakala you got your wish with python 3.9 🙂 But the operator is | , not + . docs.python.org/3/whatsnew/3.9.html

Here are quite a few ways to add dictionaries.

You can use Python3’s dictionary unpacking feature:

Note that in the case of duplicates, values from later arguments are used. This is also the case for the other examples listed here.

Or create a new dict by adding both items.

ndic = dict(tuple(dic0.items()) + tuple(dic1.items())) 

If modifying dic0 is NOT OK:

ndic = dic0.copy() ndic.update(dic1) 

If all the keys in one dict are ensured to be strings ( dic1 in this case, of course args can be swapped)

In some cases it may be handy to use dict comprehensions (Python 2.7 or newer),
Especially if you want to filter out or transform some keys/values at the same time.

>>> dic0 = >>> dic1 = >>> ndic = dict(list(dic0.items()) + list(dic1.items())) >>> ndic >>> 

Note that the equivalent syntax for this in Python 3.x is ndic = list(dict(dic0.items()) + list(dic1.items())) since .items() not longer returns a list, but a (iterable)view

@dimo414 Yes, my bad. I can’t change it now however. I use ChainMap from collections to achieve this functionality now via dict(ChainMap(dic1, dic0)) . However I have had to grab the source code from the chainmap pypi package for Python2.7. Notice how I switched the order of the dicts. In the Vijay’s example the rightmost keys’ values overwrite the leftmost while ChainMap gets it right and the leftmost keys’ values have precedence over the right.

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