mapdispatch

A minimalist library to bring Clojure-style multimethods, method combinations, and tag-based polymorphism into Python — without using classes.

Features

• Single and multiple argument dispatch
• Hierarchical dispatch using derive and isa
• Extensible method registration with defmethod
• Compositional hooks with :before, :after, and :around
• Class-free, functional programming style

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Installation

Just clone or copy the mapdispatch/ folder into your project.

# if you still use `pip`:
pip install mapdispatch

# but we highly recommend to use `uv`:
uv add mapdispatch

# and after installation via uv you can run the examples by:
mapdispatch-zoo

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Quickstart

Define animals as dictionaries

simba = {"name": "Simba", "type": "lion"}
dumbo = {"name": "Dumbo", "type": "elephant"}

Create a hierarchy

from mapdispatch import Hierarchy, MultiMethod

h = Hierarchy()
h.derive("lion", "big-cat")
h.derive("big-cat", "animal")
h.derive("elephant", "animal")

Define a multimethod

feed = MultiMethod(lambda a: a["type"], hierarchy=h)

Register methods

def lion_feed(a):
    print(f"{a['name']} eats meat")

def elephant_feed(a):
    print(f"{a['name']} eats bananas")

feed.defmethod("lion", lion_feed)
feed.defmethod("elephant", elephant_feed)

Use it

feed(simba)   # Simba eats meat
feed(dumbo)   # Dumbo eats bananas

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Real Multiple Dispatch

You can dispatch on multiple arguments by making the dispatch function return a tuple:

interact = MultiMethod(lambda a, b: (a["type"], b["type"]), hierarchy=h)

def lion_vs_elephant(a, b):
    print(f"{a['name']} chases {b['name']}")

interact.defmethod(("lion", "elephant"), lion_vs_elephant)

interact(simba, dumbo)  # Simba chases Dumbo

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Method Combinations

:before – run logic before the main method

def open_cage(a):
    print(f"Opening cage for {a['name']}")

feed.defmethod("lion", open_cage, kind=":before")

:after – run logic after the main method

def cleanup(a):
    print(f"Cleaning up after {a['name']}")

feed.defmethod("lion", cleanup, kind=":after")

:around – wrap the primary method

def log_feed(inner_fn, a):
    print(f"[LOG] Start {a['name']}")
    result = inner_fn(a)
    print(f"[LOG] End {a['name']}")
    return result

feed.defmethod("lion", log_feed, kind=":around")

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Method Registration via Decorators

You can also define methods using decorators instead of manual registration.

Primary method:

@feed.defmethod("lion")
def lion_feed(a):
    print(f"{a['name']} eats meat")

With hooks:

@feed.defmethod("lion", kind=":before")
def open_cage(a):
    print(f"Opening cage for {a['name']}")

@feed.defmethod("lion", kind=":after")
def cleanup(a):
    print(f"Cleaning cage after {a['name']}")

@feed.defmethod("lion", kind=":around")
def logger(inner_fn, a):
    print("[LOG] Start")
    result = inner_fn(a)
    print("[LOG] End")
    return result

this is fully equivalent to manually registering via feed.defmethod(...) and is great for organizeing logic clearly and idiomatically.

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Why mapdispatch?

This library helps you:

• Write open, extensible, polymorphic code
• Avoid deep class hierarchies
• Dispatch based on dynamic rules
• Cleanly separate data from behavior

Inspired by:

• Clojure’s multimethods
• Common Lisp Object System (CLOS)

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Example Output

Opening cage for Simba
[LOG] Start Simba
Simba eats meat
[LOG] End Simba
Cleaning up after Simba

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Coming Soon

• Decorator-based syntax (@defmethod)
• Predicate-based dispatch
• Package on PyPI
• Integration with dataclasses or Pydantic (optional)

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Philosophy

“Data is just data. Behavior is just behavior. Dispatch is glue.”
– Not Rich Hickey, but close enough.

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