dict Merge Operators

Python 3.9+ introduced the | and |= operators for merging dictionaries, providing a cleaner syntax than update() and providing merge semantics. This is part of PEP 584 and improves dict manipulation.

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The | Operator (Merge)

Basic Dict Merging

d1 = {'a': 1, 'b': 2}
d2 = {'c': 3, 'd': 4}

merged = d1 | d2
print(merged)

Output:

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

Right Side Overwrites

d1 = {'a': 1, 'b': 2}
d2 = {'b': 20, 'c': 3}

merged = d1 | d2
print(merged)

Output:

{'a': 1, 'b': 20, 'c': 3}

The |= Operator (In-Place Merge)

Updating a Dictionary

d = {'a': 1, 'b': 2}
d |= {'c': 3, 'd': 4}
print(d)

Output:

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

Overwriting Values

d = {'a': 1, 'b': 2}
d |= {'b': 20, 'c': 3}
print(d)

Output:

{'a': 1, 'b': 20, 'c': 3}

Practical Applications

Configuration Merging

defaults = {'host': 'localhost', 'port': 8000, 'debug': False}
custom = {'port': 5000, 'debug': True}

config = defaults | custom
print(config)

Output:

{'host': 'localhost', 'port': 5000, 'debug': True}

Layered Configuration

system = {'theme': 'dark', 'language': 'en'}
user = {'theme': 'light'}
session = {'theme': 'auto'}

final = system | user | session
print(final)

Output:

{'theme': 'auto', 'language': 'en'}

Comparison with update()

| Creates New Dict

d1 = {'a': 1, 'b': 2}
d2 = {'c': 3}

d3 = d1 | d2
print(f"d1: {d1}")
print(f"d3: {d3}")
print(f"d1 unchanged: {d1 is not d3}")

Output:

d1: {'a': 1, 'b': 2}
d3: {'a': 1, 'b': 2, 'c': 3}
d1 unchanged: True