sumtypes

0.1a6 verified Mon Apr 27 auth: no python deprecated

Provides algebraic data types (sum types / tagged unions) for Python, with pattern matching support, inspired by Rust and Haskell. Current version is 0.1a6 (alpha); no release cadence established.

pip install sumtypes

Common errors

error NameError: name 'Case' is not defined ↓

cause Forgot to import Case from sumtypes.

fix

Add from sumtypes import Case to your imports.

error TypeError: Can't instantiate abstract class Status without an implementation ↓

cause Trying to call Status() directly instead of a variant (e.g., Status.Active()).

fix

Use Status.Active() to create an instance.

error AttributeError: 'Status' object has no attribute 'value' ↓

cause Accessing .value on a variant that doesn't carry data (e.g., an empty Case).

fix

Only access .value on variants with parameters; use isinstance to determine variant first.

Warnings

deprecated Library is in alpha (0.1a6) and appears unmaintained; last updated in 2015. Consider alternatives like 'union' or 'adt' for production use. ↓

fix Migrate to 'adt' or 'union' library for actively maintained sum types.

gotcha Avoid Python 3.10+ pattern matching (match/case) as it conflicts with sumtypes.Case naming. Use isinstance instead. ↓

fix Always check variant type via isinstance, not structural pattern matching.

gotcha sumtypes.Enum is not iterable and cannot be used like stdlib Enum. Do not call list() on it. ↓

fix If you need iteration, maintain a separate list of variants.

Imports

SumType

wrong

import sumtypes.SumType

correct

from sumtypes import SumType

SumType is a class, not a submodule.

Enum
wrong
```
from enum import Enum
```
correct
```
from sumtypes import Enum
```
This is sumtypes.Enum, not stdlib enum.
Case
```
from sumtypes import Case
```
Case is used to define sum type variants.

Quickstart

Define a sum type with three variants; pattern match using isinstance.

from sumtypes import SumType, Case, Enum

class Status(SumType):
    Active = Case()
    Inactive = Case()
    Pending = Case(str)

statuses = [Status.Active(), Status.Inactive(), Status.Pending("waiting")]
for s in statuses:
    if isinstance(s, Status.Active):
        print("active")
    elif isinstance(s, Status.Inactive):
        print("inactive")
    else:
        print(f"pending: {s.value}")