Python frozendict vs dict: Differences and Use Cases

A program loads editable settings, then exposes a safe snapshot.

What You Need to Understand

Both dict and frozendict map hashable keys to values. dict allows adding, replacing and deleting entries; frozendict offers a read-only mapping structure without mutation methods such as update() or pop().

When to Use It

A program loads editable settings, then exposes a safe snapshot.

Step-by-Step Python Example

editable = {"theme": "light"}
immutable = frozendict(editable)
editable["theme"] = "dark"
print(editable["theme"])
print(immutable["theme"])

How the Example Works

Constructing the frozen mapping makes a shallow copy of entries, so changing the outer editable dictionary does not update the snapshot.

A Second Example

Compare this smaller variation with the main example, then experiment with your own values.

settings = frozendict(a=1)
print(settings | {"b": 2})

Important Behavior and Edge Cases

Unlike dict, frozendict is not a dict subclass. Use collections.abc.Mapping when accepting any read-only mapping interface.

This example uses a Python feature. It must be run with Python or newer; earlier interpreters may reject the syntax or lack the built-in type.

Try It Yourself

Try immutable | {"page_size": 10} and inspect the returned type.

Common Beginner Mistakes

Unlike dict, frozendict is not a dict subclass. Use collections.abc.Mapping when accepting any read-only mapping interface.
Treating missing dictionary keys, replacement precedence, and shallow copies as if they had identical behavior in every context.

Key Takeaways

Both dict and frozendict map hashable keys to values.
Constructing the frozen mapping makes a shallow copy of entries, so changing the outer editable dictionary does not update the snapshot.
Unlike dict, frozendict is not a dict subclass.