Skip to main content

Algebraic types for Python (notably providing Sum Types, aka Tagged Unions)

Project description


sumtypes provides Algebraic Data Types for Python. The main benefit is the implementation of Sum Types (aka Tagged Unions), which Python doesn’t have any native representation for. Product Types are just objects with multiple attributes.

Documentation is at

This module uses the attrs library to provide features like attribute validation and defaults.


Decorate your classes to make them a sum type:

import attr
from sumtypes import sumtype, constructor, match

class MyType(object):
    # constructors specify names for their arguments
    MyConstructor = constructor('x')
    AnotherConstructor = constructor('x', 'y')

    # You can also make use of any feature of the attrs
    # package by using attr.ib in constructors
    ThirdConstructor = constructor(

(attrs package, and attr.ib documentation)

Then construct them by calling the constructors:

v = MyType.MyConstructor(1)
v2 = MyType.AnotherConstructor('foo', 2)

You can get the values from the tagged objects:

assert v.x == 1
assert v2.x == 'foo'
assert v2.y == 2

You check the constructor used:

assert type(v) is MyType.MyConstructor

And, like Scala case classes, the constructor type is a subclass of the main type:

assert isinstance(v, MyType)

And the tagged objects support equality:

assert v == MyType.MyConstructor(1)
assert v != MyType.MyConstructor(2)

Simple pattern matching is also supported. To write a function over all the cases of a sum type:

class get_number(object):
    def MyConstructor(x): return x
    def AnotherConstructor(x, y): return y
    def ThirdConstructor(one, two): return one + two

assert get_number(v) == 1
assert get_number(v2) == 2

match ensures that all cases are handled. If you really want to write a ‘partial function’ (i.e. one that doesn’t cover all cases), use match_partial.

See Also

Over the past few years, the ecosystem of libraries to help with functional programming in Python has exploded. Here are some libraries I recommend:

  • effect - a library for isolating side-effects

  • pyrsistent - persistent (optimized immutable) data structures in Python

  • toolz - a general library of pure FP functions

  • - a Scala-inspired set of tools, including a weird lambda syntax, option type, and monads

Project details

Download files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

Source Distribution

sumtypes-0.1a6.tar.gz (5.3 kB view hashes)

Uploaded source

Built Distribution

sumtypes-0.1a6-py2.py3-none-any.whl (5.8 kB view hashes)

Uploaded py2 py3

Supported by

AWS AWS Cloud computing and Security Sponsor Datadog Datadog Monitoring Fastly Fastly CDN Google Google Download Analytics Microsoft Microsoft PSF Sponsor Pingdom Pingdom Monitoring Sentry Sentry Error logging StatusPage StatusPage Status page