patos

A flock of typed, zero-dependency Python design patterns. Each pattern is a single self-contained pato (a duck you adopt), fully typed, standard library only, and small enough to read in one sitting.

Install

pip install patos

Or copy a single duck. Open its docs page, copy the source from the Source section, and paste it into your own project. No dependency, no version pin, no tool, just one module you own.

The optional PostgreSQL toolkit installs SQLAlchemy, pgvector, and inflection only when a project asks for them.

pip install "patos[sql]"

from patos import sql

Patterns

Creational

pato	description	docs
`Singleton`	one instance per class, `__init__` runs once	docs
`FlyweightMeta`	a metaclass that interns instances by constructor args, build-once and immutable	docs

Dispatch & selection

pato	description	docs
`Registry`	self-registering class hierarchies via `__init_subclass__`, with try-each `dispatch`, `select(predicate)` and `first_available()`	docs
`Strategy`	a named family of interchangeable implementations, with `first_available()`	docs
`value_dispatch`	dispatch on a value the way `singledispatch` dispatches on a type	docs
`type_dispatch`	dispatch on the first argument's type, the open-type-ladder dual of `value_dispatch`	docs

Structural & lifecycle

pato	description	docs
`Decorator`	a transparent wrapper that forwards everything to the wrapped object but what it overrides	docs
`Pipeline`	a reversible stack of stages, applied forward and unwound in reverse around a core op	docs
`Lifecycle`	a typed state machine that permits only the transitions its table declares	docs
`DerivedCache`	a load-once cache of a derived value keyed by exactly what it depends on	docs

Command-line

pato	description	docs
`StrFlag`	an enum `Flag` whose members carry a literal string, OR-combinable and iterable	docs

Optional extensions

namespace	description	docs
`sql`	typed SQLModel columns, native PostgreSQL enums, pgvector cosine distance, JSONB reads, and set based SQL helpers	docs

Example

from patos import Singleton


class Settings(Singleton):
    def __init__(self) -> None:
        self.debug = False


a = Settings()
a.debug = True
b = Settings()      # same object, __init__ did not run again

assert a is b and b.debug

Documentation

Full documentation lives at phvv.me/patos.

Development

Install: pip install -e ".[dev]"
Lint: ruff check . && ruff format --check .
Typecheck: pyrefly check
Test: pytest
Docs: pip install -e ".[docs]" && mkdocs build
Build: python -m build

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