An application applies environment overrides to default settings.
What You Need to Understand
There are several ways to combine mappings: dictionary unpacking and | create a new shallow mapping, while update() mutates an existing one. Conflicting keys receive the later value.
When to Use It
An application applies environment overrides to default settings.
Step-by-Step Python Example
defaults = {"theme": "light", "timeout": 30}
overrides = {"timeout": 60}
result = defaults | overrides
print(result)
print(defaults)How the Example Works
The new dictionary has timeout 60, but defaults still contains timeout 30.
Example Output
{'theme': 'light', 'timeout': 60}
{'theme': 'light', 'timeout': 30}
{'a': 3, 'b': 2}A Second Example
Compare this smaller variation with the main example, then experiment with your own values.
base = {"a": 1}
team = {"b": 2}
user = {"a": 3}
print(base | team | user)Important Behavior and Edge Cases
A shallow merge is not a recursive configuration merge: inner dictionaries can be replaced entirely. Choose explicit rules for nested settings.
Try It Yourself
Combine three settings layers in a predictable precedence order.
Common Beginner Mistakes
- A shallow merge is not a recursive configuration merge: inner dictionaries can be replaced entirely. Choose explicit rules for nested settings.
- Applying the example without a clearly defined policy for missing keys, matching identifiers, or validating external values.
Key Takeaways
- There are several ways to combine mappings: dictionary unpacking and
|create a new shallow mapping, whileupdate()mutates an existing one. - The new dictionary has timeout 60, but
defaultsstill contains timeout 30. - A shallow merge is not a recursive configuration merge: inner dictionaries can be replaced entirely.