Python Merge Two Dictionaries into Single

Python Merge two dictionaries into a single is a common operation in Python when working with dictionaries. There are multiple ways to achieve this, and in this article, we will explore some of these approaches. For example, using the update() method, using the unpacking operator, and using dictionary comprehension.

Quick Examples of Merge Two Dictionaries into a Single

Here are some quick examples that will give you a high-level overview of how to merge two dictionaries into a single dictionary in Python:


# Quick Examples

# Method 1: Use the update() method
dict1.update(dict2)

# Method 2: Use the {**dict1, **dict2} syntax
dict3 = {**dict1, **dict2}

# Method 3: Use the ChainMap class
import collections
chain = collections.ChainMap(dict1, dict2)

1. Use update() to Merge Two Dictionaries

The update() method allows you to update the values of an existing dictionary with the values of another dictionary. This is one of the easiest and most straightforward ways to merge two dictionaries in Python. The syntax of the update() function is dict.update(other_dict). It is a method of the dictionary object, and it takes a dictionary as an argument. For more examples on dict, refer to Python Dictionary with examples.


# Create two dictionaries
states_1 = {'CA': 'California', 'NY': 'New York'}
states_2 = {'CA': 'California', 'DC': 'Washigton DC'}

# Use update() method to merge dictionaries
states_1.update(states_2)
print(states_1) 

# Output: 
# {'CA': 'California', 'NY': 'New York', 'DC': 'Washigton DC'}

In the above example to merge these dictionaries, we use the update() method on the states_1 dictionary, passing in the states_2 dictionary as an argument.

2. Merge Two Dictionaries using Unpacking Approach

This syntax is known as the dictionary unpacking operator and it allows you to merge two dictionaries by “unpacking” them into a new dictionary. It allows you to “unpack” the key-value pairs from a dictionary and assign them to variables.


# Use unpacking
merged_states={**states_1, **states_2}
# Print resulting dictionary
print(merged_states) 

# Output: 
# {'CA': 'California', 'NY': 'New York', 'DC': 'Washigton DC'}

3. Merge Dictionaries Using Comprehension

A dictionary comprehension is a concise way to create a list by iterating over an iterable, such as a dictionary. so You can also merge multiple dictionaries in Python by using a dictionary comprehension.


# Use comprehension
merged_states= {k: v for d in (states_1, states_2) for k, v in d.items()}
print(merged_states) 

# Output: 
# {'CA': 'California', 'NY': 'New York', 'DC': 'Washigton DC'}

4. Handle Conflicts when Merging Dictionaries

When merging dictionaries in Python, you may encounter a key conflict if two or more dictionaries contain a key with the same name. In this case, you need to decide how to handle the conflict and determine which value to use for the key.

Here are 4 different cases in that you can encounter conflict. Each one has its own solution.

4.1 Use first dictionary value

If you want to use the value from the first dictionary in case of a key conflict, you can use the update() method to merge the dictionaries. The update() method will overwrite the value of the key in the first dictionary with the value from the second dictionary.

Just make sure to call it in the right order. If you want to have a value of the first dict then you should call update() function on the second.


# Get value of first dict
states_2.update(states_1)
print(states_2)

4.2 Use second dictionary value

In this case, we can either use the same update function in the reverse order or we can use the unpacking method. For the sake of this topic we will use the unpacking method to use the value of the second dict while merging.


# Get value of second dict
merged_stats = {**states_1, **states_2}
print(states_2)

4.3 Use a custom function to determine value

This allows you to specify your own logic for handling key conflicts. If you want to use a custom function to determine the value in case of a key conflict, you can use a for loop to iterate over the dictionaries and apply the function to each key-value pair.


# Use a for loop and a custom function to merge dictionaries
def merge_dicts(dict1, dict2):
  result = {}
  for d in (dict1, dict2):
    for k, v in d.items():
      if k in result:
        result[k] = custom_function(result[k], v)
      else:
        result[k] = v
  return result

def custom_function(v1, v2):
  # Add your own logic here to determine the value
  return v1 + ' ' + v2

dict3 = merge_dicts(states_1, states_2)
print(dict3)  
# {'CA': 'California California', 'NY': 'New York', 'DC': 'Washigton DC'}

4.4 Raise an exception while merging dictionaries

This method can be useful if you want to ensure that the dictionaries you are merging do not contain any key conflicts. If you want to raise an exception in case of a key conflict, you can use a try-except block to catch the exception and handle it.


# Use a try-except block to handle key conflicts
try:
  dict3 = {**dict1, **dict2}
except KeyError as e:
  print(f'Key conflict: {e}')

5. Use Collections Module to Merge Multiple Dictionaries

This can be useful when you want to merge multiple dictionaries in Python and have them take precedence in a specific order. The ChainMap class from the collections module allows you to create a single view of multiple dictionaries.

It behaves like a regular dictionary, but internally it maintains a list of dictionaries and searches through them in the order they were added when looking up keys.


from collections import ChainMap

dict3 = {'Ruby': 'OO', 'Perl': 'LL'}

# Create a ChainMap using the dictionaries
merged_dict = ChainMap(dict1, dict2, dict3)

6. Merge Dictionaries using for Loop

When using a for loop to merge dictionaries, you can iterate over the dictionaries and add the key-value pairs to a new dictionary, allowing you to combine multiple dictionaries into a single dictionary. It allows you to iterate over the dictionaries and add the key-value pairs to a new dictionary in a loop.


# Initialize an empty dictionary 
merged_dict = {}

# Iterate over the dictionaries
for d in (dict1, dict2):
  for k, v in d.items():
    merged_dict[k] = v

print(merged_dict)  
# {'Python': 'HL', 'C++': 'LL', 'Java': 'OO', 'C#': 'OO'}

7. Summary and Conclusion

In this article, we covered a variety of techniques for merging dictionaries in Python. We learned how to use the built-in update() method and the concise {**dict1, **dict2} syntax to merge two dictionaries. If you have any further questions or comments, please feel free to leave them in the comments section below.

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