How to Use MappingProxyType to Create Immutable Dict Views in Python

Create a read-only, immutable view of a dictionary using MappingProxyType from the types module, while the original dict stays mutable.

Easy Python 3.3+ Aug 9, 2026 Dictionaries & sets 12 views 0 copies

mappingproxytype dict immutable read-only types

Python code

22 lines
Python 3.3+
Run it 
from types import MappingProxyType

config = {"debug": True, "port": 8080}

# Create an immutable read-only view of the dict
read_only_config = MappingProxyType(config)

print(f"Read-only value: {read_only_config['debug']}")
print(f"Dict is mapping: {isinstance(read_only_config, dict)}")

# Original dict can still be modified
config["port"] = 9090
print(f"View reflects changes: {read_only_config['port']}")

# Attempting to modify the immutable view raises TypeError
try:
    read_only_config["debug"] = False
except TypeError as e:
    print(f"Modification blocked: {e}")

# Show all items through the view
print(f"Items: {dict(read_only_config)}")

Output

stdout 
Read-only value: True
Dict is mapping: True
View reflects changes: 9090
Modification blocked: 'mappingproxy' object does not support item assignment
Items: {'debug': True, 'port': 9090}

How it works

MappingProxyType(config) wraps the original dict in a read-only mapping. The proxy is a live view — changes to the original dict appear through the proxy. Accessing items works exactly like a normal dict, but assigning proxy['key'] = value raises TypeError. Use this when you need to pass a dict to other code without letting it mutate your data. The proxy is not a copy; modifying the original dict still updates what the proxy sees.

Common mistakes

  • Confusing MappingProxyType with deepcopy — the proxy is a view, not a copy
  • Expecting the proxy to be an instance of dict — use isinstance(obj, Mapping) instead
  • Trying to update the proxy after creation, which always raises TypeError

Variations

  1. Use `Mapping` from collections.abc for type hints instead of dict
  2. Wrap a `ChainMap` or `defaultdict` in MappingProxyType for a read-only facade

Real-world use cases

  • Exposing application configuration to plugins without letting them overwrite settings
  • Returning a read-only snapshot of a cached dataset to API consumers
  • Passing a protected dictionary to a worker thread so it cannot corrupt shared state

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