[Tut] Python How to Join a List of Dictionaries into a Single One?

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Python How to Join a List of Dictionaries into a Single One?



Problem: Say, you’ve got a list of dictionaries:

Code:

[{'a': 0}, {'b': 1}, {'c': 2}, {'d': 3}, {'e': 4, 'a': 4}]

Notice how the first and the last dictionaries carry the same key

Code:

'a'

.

How do you merge all those dictionaries into a single dictionary to obtain the following one?

Code:

{'a': 4, 'b': 1, 'c': 2, 'd': 3, 'e': 4}

Notice how the value of the duplicate key

Code:

'a'

is the value of the last and not the first dict in the list of dicts.

To merge multiple dictionaries, the most Pythonic way is to use dictionary comprehension

Code:

{k:v for x in l for k,v in x.items()}

to first iterate over all dictionaries in the list

Code:

l

and then iterate over all (key, value) pairs in each dictionary.

Let’s explore all the available options in the remaining article:

https://trinket.io/embed/python/0e07682a4a

Exercise: Run the code—which method generates a different output than all the other methods?

Method 1: Dictionary Comprehension With Nested Context

You can use dictionary comprehension

Code:

{k:v for x in l for k,v in x.items()}

to first iterate over all dictionaries in the list

Code:

l

and then iterate over all (key, value) pairs in each dictionary.

• Create a new dictionary using the
  Code:

  {...}
  notation.
  
  
• Go over all dictionaries in the list of dictionaries
  Code:

  l
  by using the outer loop
  Code:

  for x in l
  .
  
  
• Go over all (key, value) pairs in the current dictionary
  Code:

  x
  by using the
  Code:

  x.items()
  method that returns an iterable of (key, value) pairs.
  
  
• Fill the new dictionary with (key, value) pairs
  Code:

  k:v
  using the general dictionary comprehension syntax
  Code:

  {k:v for ...}
  .
  

Code:

l = [{'a': 0}, {'b': 1}, {'c': 2}, {'d': 3}, {'e': 4, 'a': 4}] d = {k:v for x in l for k,v in x.items()} print(d) # {'a': 4, 'b': 1, 'c': 2, 'd': 3, 'e': 4}

This is the most Pythonic way to merge multiple dictionaries into a single one and it works for an arbitrary number of dictionaries.

Method 2: Simple Nested Loop

Use a simple nested loop to add each (key, value) pair separately to a newly created dictionary:

• Create a new, empty dictionary.
  
  
• Go over each dictionary in the list of dictionaries.
  
  
• Go over each (key, value) pair in the current dictionary.
  
  
• Add the (key, value) pair to the new dictionary—possibly overwriting “older” keys with the current one.
  

Code:

l = [{'a': 0}, {'b': 1}, {'c': 2}, {'d': 3}, {'e': 4, 'a': 4}] d = {} for dictionary in l: for k, v in dictionary.items(): d[k] = v print(d) {'a': 4, 'b': 1, 'c': 2, 'd': 3, 'e': 4}

You can visualize the execution flow of this code here:

https://pythontutor.com/iframe-embed.htm...nces=false

This is the easiest to read for many beginner coders—but it’s much less concise and less Pythonic.

Method 3: Use the update() Method

The

Code:

dict.update(x)

method updates the dictionary on which it is called with a bunch of new (key, value) pairs given in the dictionary argument

Code:

x

. The method to merge multiple dictionaries is simple:

• Create a new, empty dictionary.
  
  
• Go over each dictionary in the list of dictionaries.
  
  
• Update the new dictionary with the values in the current dictionary.
  

Code:

l = [{'a': 0}, {'b': 1}, {'c': 2}, {'d': 3}, {'e': 4, 'a': 4}] d = {} for dictionary in l: d.update(dictionary) print(d) # {'a': 4, 'b': 1, 'c': 2, 'd': 3, 'e': 4}

This is a very readable and efficient way and it’s shorter than method 2.

Method 4: Dictionary Unpacking

When applied to a dictionary

Code:

d

, the double asterisk operator

Code:

**d

unpacks all elements in

Code:

d

into the outer dictionary. You can only use this “dictionary unpacking” method within an environment (in our case a dictionary) that’s capable of handling the (key, value) pairs.

Side note: Sometimes it’s also used for keyword arguments.

Code:

l = [{'a': 0}, {'b': 1}, {'c': 2}, {'d': 3}, {'e': 4, 'a': 4}] d = {**l[0], **l[1], **l[2], **l[3], **l[4]} print(d) # {'a': 4, 'b': 1, 'c': 2, 'd': 3, 'e': 4}

This is a concise, efficient, and Pythonic way to merge multiple dictionaries. However, it’s not optimal because you must manually type each unpacking operation. If the dictionary has 100 elements, this wouldn’t be feasible.

Note: you cannot use dictionary unpacking in dictionary comprehension to alleviate this problem—Python will throw a SyntaxError!


Method 5: Use ChainMap With Unpacking

If you’re not happy with either of those methods, you can also use the

Code:

ChainMap

data structure from the collectionslibrary.

Code:

l = [{'a': 0}, {'b': 1}, {'c': 2}, {'d': 3}, {'e': 4, 'a': 4}] from collections import ChainMap d = dict(ChainMap(*l)) print(d) # {'e': 4, 'a': 0, 'd': 3, 'c': 2, 'b': 1}

However, this does not exactly meet our specifications: the fifth dictionary in our collection does not overwrite the (key, value) pairs of the first dictionary. Other than that, it’s a fast way to merge multiple dictionaries and I wanted to include it here for comprehensibility.

Where to Go From Here?

Enough theory, let’s get some practice!

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