A boolean flag satisfies an Integer request: bool is a subclass of int

Summary

A boolean flag satisfies an Integer request. Evaluating a boolean flag through get_integer_details returns True with reason STATIC and no error code, where the specification requires the code default and TYPE_MISMATCH.

The application sees a plausible value — True behaves as 1 in arithmetic — and no indication that anything went wrong. That is the worst failure mode a feature flag has.

Cause

_typecheck_flag_value in openfeature/client.py:

type_map: TypeMap = {
    FlagType.BOOLEAN: bool,
    FlagType.STRING: str,
    FlagType.OBJECT: (dict, list),
    FlagType.FLOAT: float,
    FlagType.INTEGER: int,
}
py_type = type_map.get(flag_type)
...
if not isinstance(value, py_type):
    return TypeMismatchError(f"Expected type {py_type} but got {type(value)}")

bool is a subclass of int in Python, so isinstance(True, int) is True and the check passes:

>>> isinstance(True, int)
True

The reverse direction is fine — isinstance(1, float) is False, so an integer requested as a float is correctly a mismatch — which is why this is easy to miss.

Reproducing

from openfeature import api
from openfeature.provider.in_memory_provider import InMemoryFlag, InMemoryProvider

api.set_provider(InMemoryProvider({
    "boolean-flag": InMemoryFlag(default_variant="on", variants={"on": True, "off": False}),
}))

details = api.get_client().get_integer_details("boolean-flag", 1)
print(details.value, details.reason, details.error_code)
# True Reason.STATIC None
# expected: 1 Reason.ERROR ErrorCode.TYPE_MISMATCH

Suggested fix

Reject bool explicitly when an integer or float was requested:

if flag_type in (FlagType.INTEGER, FlagType.FLOAT) and isinstance(value, bool):
    return TypeMismatchError(f"Expected type {py_type} but got {type(value)}")

Worth checking the same place for FlagType.OBJECT: (dict, list) accepts a list, which is correct per the spec's structure type, but is worth a test either way.

How it surfaced

Building the Python implementation of the cross-language provider conformance suite (spec#417). The suite runs an identical type-mismatch matrix against every provider in every language; this row passes everywhere else and fails only in Python, because only Python has bool as a subclass of int.

That is a reasonable advertisement for having more than one language implementation: a Java or Go suite could never have caught this.

The suite currently marks the case xfail(strict=True) with a pointer to this issue, so it stays visible and un-hides itself automatically once this is fixed.