conditional-method

conditional-method is a zero-dependency, pure-C Python library that lets you select, at class build time, which of several identically-named method implementations survives on a class — based on a runtime, startup, or call-time condition.

import os

from conditional_method import cfg

ENV = os.environ.get("ENV", "development")


class Worker:
    @cfg(condition=ENV == "production")
    def work(self, *args, **kwargs):
        return "production"

    @cfg(condition=ENV == "development")
    def work(self, *args, **kwargs):
        return "development"

    @cfg(condition=ENV == "staging")
    def work(self, *args, **kwargs):
        return "staging"


worker = Worker()
print(worker.work())  # development (only ONE `work` survives on Worker)

Installation

pip install conditional-method

The whole implementation is a C extension (conditional_method._c) built against the Python Limited API (abi3): a single cp39-abi3 wheel covers CPython 3.9–3.14.

What's here

• @cfg (aliases @cm, @if_) — conditional method selection at class build time.
• @cfg_attr — conditionally apply a chain of decorators.
• debug / debug_enabled — opt-in C debug logging.

Why "well-formed" classes

Only the selected implementation remains in the class's attribute set, so the resulting class is well-formed: there is exactly one work attribute, and no runtime branching inside the method body.

See the benchmarks for measured performance.