[anishpentyala]

Problem

Nested read-only mappings satisfy VariableValue's existing Mapping[str, ScalarVariableValue] annotation, but runtime detection only recognizes dict and MutableMapping. As a result, a MappingProxyType or a custom read-only Mapping is expanded as its quoted Python representation rather than as an associative value.

For example:

from types import MappingProxyType
from uritemplate import expand

expand("{?v*}", v=MappingProxyType({"a": "one", "b": "two"}))
# Expected: ?a=one&b=two

Change

Use collections.abc.Mapping in dict_test. The existing associative expansion paths only read the mapping, so they do not need the stronger mutable interface.

The regression tests cover standard MappingProxyType and a custom read-only Mapping across all eight operators, with and without explode. They also check sorted-key behavior, no mutation, quoted-key/value parity with dictionaries, and that scalar/list/list-of-pairs values are not reclassified.

This follows the outer-mapping typing work in #130, but fixes nested associative value detection at runtime. It does not change the existing empty-collection behavior discussed in #164 or attempt to fix unrelated prefix/escaping behavior.

Validation

• Existing baseline: 58 passed, 66 subtests passed.
• The 53 added cases against unchanged runtime: 48 failed, 5 controls passed.
• Full suite after the fix: 111 passed, 66 subtests passed on Python 3.10.12 and 3.13.17.
• Fresh clone of the public branch: exact reviewed source tree and 111 passed, 66 subtests passed on Python 3.13.17.
• Black, isort, flake8 and strict mypy over all eight source files passed locally.

Hosted checks remain separate from these local results.