pyochain ⛓️

Fluent method chaining for Python.

Inspired by Rust's Iterator, Result, Option, and DataFrame libraries like Polars, pyochain is a no-dependency library providing a set of classes with a fluent API, to work with iterations, collections, handle optional values, or manage errors.

For a quick overview of the core types and their relationships, see the core types overview page.

Key Features

• ⛓️ Declarative & fluent chaining — Replace for loops, None checks, and error handling with chainable methods.
• 🦥 Lazy-first, 🔒 explicit mutability — Iter[T] for lazy, efficient iterations; Seq and Set for immutable data; Vec and SetMut when you need to mutate.
• Memory efficient - Almost all methods from Iter[T] operate in streaming fashion, and Vec[T] provides in-place methods with more memory efficiency than standard list methods (e.g. x.extend_move(y) won't create intermediate allocations like x.extend(y) followed by y.clear()).
• 🎯 Result and Option types - Handle None and exceptions in a fluent, explicit way.
• 🔥 Blazingly fast — Core Option and Result types are written in Rust for minimal overhead, and iterators use almost always compiled C or Rust level code, from the builtins, itertools or custom Pyo3 implementations.
• 🛡️ 100% type-safe — Full generic support and autocompletion in your IDE.
• 📚 Accurate Documentation — Every method is documented and tested with runnable examples. Every code example in the website (or this README) is also tested, ensuring accuracy and reliability.
• 🔄 Interoperable — Seamlessly convert to/from types with various methods like .into() and .collect(), convert Iterables to Option or Result based on their truthiness, and more.
• 🐍 Mixins and ABC's — Extend your own classes with the mixins, Protocol and ABC's provided by the traits module.

Installation

uv add pyochain # or pip install pyochain

See the package page on Pypi

Quick Start

Iterations

>>> from pyochain import Iter, Seq
>>> # Lazy processing with Iter
>>> res: Seq[tuple[int, str]] = (
...     Iter.from_count(1)
...     .filter(lambda x: x % 2 != 0)
...     .map(lambda x: x**2)
...     .take(5)
...     .enumerate()
...     .map_star(lambda idx, value: (idx, str(value)))
...     .collect()
... )
>>> res
Seq((0, '1'), (1, '9'), (2, '25'), (3, '49'), (4, '81'))

For comparison, the above can be written in pure Python as the following (note that Pylance strict will complain because itertools.starmap has not the same overload exhaustiveness as pyochain's Iter.map_star):

>>> import itertools
>>>
>>> res: tuple[tuple[int, str], ...] = tuple(
...     itertools.islice(
...         itertools.starmap(
...             lambda idx, val: (idx, str(val)),
...             enumerate(
...                 map(lambda x: x**2, filter(lambda x: x % 2 != 0, itertools.count(1)))
...             ),
...         ),
...         5,
...     )
... )
>>>
>>> res
((0, '1'), (1, '9'), (2, '25'), (3, '49'), (4, '81'))

Result and Option Types

>>> from pyochain import Option, NONE, Some, Seq, Vec, Set, Ok, Err, Result
>>> from pyochain.traits import PyoIterable
>>>
>>> def divide(a: int, b: int) -> Option[float]:
...     return NONE if b == 0 else Some(a / b)
>>> divide(10, 2)
Some(5.0)
>>> # Provide a default value
>>> divide(10, 0).unwrap_or(-1.0)
-1.0
>>> # Convert between Collections -> Option -> Result
>>> data = Seq((1, 2, 3))
>>> # Convert Seq to Option
>>> data.then_some()
Some(Seq(1, 2, 3))
>>> # Accept any Pyochain Iterable
>>> def _process(data: PyoIterable[int]) -> str:
...     return data.iter().map(str).join(", ")
>>> # Process only if non-empty, convert Option to Result
>>> data.then(_process).ok_or("No values") 
Ok('1, 2, 3')
>>> Vec(()).then(_process).ok_or("No values")
Err('No values')
>>> # Create empty Set, convert to Result, then back to Option
>>> Set(()).then(_process).ok_or("No values").ok()
NONE
>>> def try_parse_int(s: str) -> Result[int, ValueError]:
...     try:
...         return Ok(int(s))
...     except ValueError as e:
...         return Err(e)
>>> # Type safe exhaustive handling with pattern matching
>>> def handle_result(res: Result[int, ValueError]) -> str:
...     match res:
...         case Ok(value):
...             return f"Parsed value: {value}"
...         case Err(error):
...             return f"Error parsing int!"
>>>
>>> try_parse_int("123").into(handle_result)
'Parsed value: 123'
>>> try_parse_int("abc").into(handle_result)
'Error parsing int!'

Documentation

For comprehensive guides and examples:

🔍 Types Overview — Roles, comparisons and visual relationships

📚 Full API Reference — Complete API documentation

Notice on Stability ⚠️

pyochain is currently in early development (< 1.0), and the API may undergo significant changes multiple times before reaching a stable 1.0 release.

Contributing

Want to contribute? Read our contributing guide

Credits

Most of the custom computation algorithms have been inspired by implementations from itertools, cytoolz and more-itertools.