How to merge two dictionaries in Python?

In Python, you can merge two dictionaries in several ways, depending on your Python version and whether you want to modify the original dictionaries or create a new one. Here are the most common methods:

1. Using the | Operator (Python 3.9+)

The simplest and most modern approach:

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}
merged = dict1 | dict2  # New dict (original dicts unchanged)
print(merged)  # Output: {'a': 1, 'b': 3, 'c': 4}

• Overlapping keys: Values from dict2 override those in dict1.
• Non-destructive: Creates a new dictionary.

2. Using ** Unpacking (Python 3.5+)

Merge dictionaries in a single expression:

merged = {**dict1, **dict2}  # Same result as above

• Works for merging multiple dictionaries:

  merged = {**dict1, **dict2, **dict3}

3. Using update() (In-Place Modification)

Modify dict1 to include dict2‘s key-value pairs:

dict1.update(dict2)  # Updates dict1 in-place
print(dict1)  # Output: {'a': 1, 'b': 3, 'c': 4}

• Destructive: Overwrites dict1.
• To preserve originals, create a copy first:

  merged = dict1.copy()
  merged.update(dict2)

4. Using collections.ChainMap (Non-Destructive View)

Group dictionaries without merging (access keys as a single view):

from collections import ChainMap
merged_view = ChainMap(dict1, dict2)
print(merged_view['b'])  # Output: 2 (first occurrence)

• No new dict: Acts as a read-only view.
• Changes to dict1 or dict2 reflect in merged_view.

Key Differences

Method	Python Version	Creates New Dict?	Modifies Original?	Overwrites Keys?
| Operator	3.9+	✅ Yes	❌ No	✅ Yes (last)
** Unpacking	3.5+	✅ Yes	❌ No	✅ Yes (last)
update()	All	❌ No	✅ Yes	✅ Yes (last)
ChainMap	All	❌ No (view)	❌ No	❌ No (first)

Recommendation

• Python 3.9+: Use | for clarity.
• Python 3.5–3.8: Use ** unpacking.
• Legacy Code: Use update() with a copy (e.g., merged = dict1.copy(); merged.update(dict2)).

For handling large dictionaries or performance-critical code, test the methods to compare efficiency. Always ensure you’re aware of key-overwrite behavior!