Python etc / types.MappingProxyType

Published: 29 April 2021, 18:00

If any function can modify any passed argument, how to prevent a value from modification? Make it immutable! That means the object doesn’t have methods to modify it in place, only methods returning a new value. This is how numbers and str are immutable. While list has append method that modifies the object in place, str just doesn’t have anything like this, all modifications return a new str:

a = b = 'ab'
a is b  # True
b += 'cd'
a is b  # False

This is why every built-in collection has an immutable version:

  • Immutable list is tuple.
  • Immutable set is frozenset.
  • Immutable bytearray is bytes.
  • dict doesn’t have an immutable version but since Python 3.3 it has types.MappingProxyType wrapper that makes it immutable:
from types import MappingProxyType

orig = {1: 2}
immut = MappingProxyType(orig)

immut[3] = 4
# TypeError: 'mappingproxy' object does not support item assignment

And since it is just a proxy, not a new type, it reflects all the changes in the original mapping:

orig[3] = 4
# 4