pyochain 0.6.0.3

Method chaining for iterables and dictionaries in Python.

pyochain ⛓️

Functional-style method chaining for Python data structures.

pyochain brings a fluent, declarative API inspired by Rust's Iterator, Result, Option, and DataFrame libraries like Polars to your everyday Python iterables and dictionaries.

Manipulate data through composable chains of operations and manage errors and optional values safely, all while enjoying type-safe guarantees.

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.

Installation

uv add pyochain

API Reference 📖

The full API reference can be found at: https://outsquarecapital.github.io/pyochain/

Overview

Provides the following core classes and utilities:

• Iter[T]: A superset of Python collections.abc.Iterator, with chainable functional methods. Implement Iterator Protocol.
• Seq[T]: An immutable collection with chainable methods. Underlying data structure is a tuple. Can be converted to Iter with the .iter() method. Implement Sequence Protocol.
• Vec[T]: A mutable collection with chainable methods. Underlying data structure is a list. Can be converted to Iter with the .iter() method. Implement MutableSequence Protocol.
• Dict[K, V]: An immutable mapping with chainable methods. Underlying data structure is a dict.
• Option[T] | Some[T] | NONE: A type representing an optional value, similar to Rust's Option Enum. Analog to a superset of T | None.
• Result[T, E] | Ok[T] | Err[E]: A type representing either a success (Ok) or failure (Err), similar to Rust's Result Enum. Analog to a superset of T | Exception.

All classes have:.tap(), from_ and .into() methods (or equivalents, see Option and Result for tap) for:

• Easy conversion between types, whilst keeping the chain uninterrupted (into). E.g Seq[T].into() can take any function/object that expect a Sequence[T] as argument.
• Creating instances from various inputs (from_*). This allow flexible instantiation when needed, without runtime checks in the base init of the class.
• Inserting side-effects in the chain without breaking it (tap, inspect, for_each, etc...) for side-effect operations.

Philosophy

• Declarative over Imperative: Replace explicit for and while loops with sequences of high-level operations (map, filter, group, join...).
• Fluent Chaining: Each method transforms the data and returns a new wrapper instance, allowing for seamless chaining.
• Lazy and Eager: Iter operates lazily for efficiency on large or infinite sequences, while Seq and Vec represent materialized sequences for eager operations.
• Explicit mutability: Seq is the usual return type for most methods who materialize data, hence improving memory efficiency and safety, compared to using list everytime. Vec is provided when mutability is required.
• 100% Type-safe: Extensive use of generics and overloads ensures type safety and improves developer experience.
• Documentation-first: Each method is thoroughly documented with clear explanations, and usage examples. Before any commit is made, each docstring is automatically tested to ensure accuracy. This also allows for a convenient experience in IDEs, where developers can easily access documentation with a simple hover of the mouse.
• Functional and chained paradigm: Design encourages building complex data transformations by composing simple, reusable functions on known buildings blocks, rather than implementing customs classes each time.

Inspirations

• Rust's language and Rust stdlib: Emulate naming conventions (from_(), into()) and leverage concepts from Rust's powerful iterator traits (method chaining, lazy evaluation), Option and Result enums, to bring similar expressiveness to Python.
• Python iterators libraries: Libraries like rolling, cytoolz, and more-itertools provided ideas, inspiration, and implementations for many of the iterator methods.
• PyFunctional: Although not directly used (because I started writing pyochain before discovering it), also shares similar goals and ideas.

Core Components

Iter[T]

A superset of Iterator. All operations are lazy, consuming the underlying iterator only when needed.

This allows for efficient processing of large or even infinite sequences.

To create an Iter, you can:

• Take any iterable: pc.Iter(my_iterable)
• Wrap unpacked values: pc.Iter.from_(1, 2, 3)
• Use built-in constructors like pc.Iter.from_count() for infinite sequences.

Seq[T]

A wrapper for a Sequence (like a list or tuple), representing an eagerly evaluated collection of data. Seq is useful when you need to store results in memory, access elements by index, or reuse the data multiple times.

It shares many methods with Iter but performs operations immediately. You can switch between lazy and eager evaluation by using my_seq.iter() and my_iter.collect().

Dict[K, V]

A wrapper for a dict, providing a rich, chainable API for dictionary manipulation. It simplifies common tasks like filtering, mapping, and transforming dictionary keys and values.

Key features include:

• Immutability: Most methods return a new Dict instance, preventing unintended side effects.
• Nested Data Utilities: Easily work with complex, nested dictionaries using methods like pluck and flatten.
• Flexible Instantiation: Create a Dict from mappings, iterables of pairs, or even object attributes with Dict.from_object().

Result[T, E]

A type for functions that can fail, inspired by Rust's Result. It represents either a success (Ok[T]) containing a value or an error (Err[E]) containing an error. It forces you to handle potential failures explicitly, leading to more robust code.

Option[T]

A type for values that may be absent, inspired by Rust's Option. It represents either the presence of a value (Some[T]) or its absence (NONE). It provides a safe and expressive way to handle optional values without resorting to None checks everywhere.

Core Piping Methods

All wrappers provide a small set of core methods for chaining and passing values to your own functions.

This is covered in detail in the guide: see Guides → Chaining Methods.

Rich Lazy Iteration (Iter)

Leverage dozens of methods inspired by Rust's Iterator, itertools, cytoolz, and more-itertools.

>>> import pyochain as pc
>>>
>>> result = (
...    pc.Iter.from_count(1)  # Infinite iterator: 1, 2, 3, ...
...    .filter(lambda x: x % 2 != 0)  # Keep odd numbers
...    .map(lambda x: x * x)  # Square them
...    .take(5)  # Take the first 5
...    .collect()  # Materialize the result into a Seq
)
Seq(1, 9, 25, 49, 81)

Type-Safe Error Handling (Result and Option)

Write robust code by handling potential failures explicitly.

>>> import pyochain as pc
>>>
>>> def divide(a: int, b: int) -> pc.Result[float, str]:
...     if b == 0:
...         return pc.Err("Cannot divide by zero")
...     return pc.Ok(a / b)
>>>
>>> # --- With Result ---
>>> res1 = divide(10, 2)
Ok(5.0)
>>> res2 = divide(10, 0)
Err("Cannot divide by zero")
>>> # Safely unwrap or provide a default
>>> res2.unwrap_or(0.0)
0.0
>>> # Map over a successful result
>>> res1.map(lambda x: x * x)
Ok(25.0)
>>> # --- With Option ---
>>> def find_user(user_id: int) -> pc.Option[str]:
...     users = {1: "Alice", 2: "Bob"}
...     return pc.Some(users.get(user_id)) if user_id in users else pc.NONE
>>>
>>> find_user(1).map(str.upper).unwrap_or("Not Found")
"ALICE"
>>> find_user(3).unwrap_or("Not Found")
"Not Found"

Typing enforcement

Each method and class make extensive use of generics, type hints, and overloads (when necessary) to ensure type safety and improve developer experience.

Since there's much less need for intermediate variables, the developper don't have to annotate them as much, whilst still keeping a type-safe codebase.

Key Dependencies and credits

Most of the computations are done with implementations from the cytoolz, more-itertools, and rolling libraries.

An extensive use of the itertools stdlib module is also to be noted.

pyochain acts as a unifying API layer over these powerful tools.

https://github.com/pytoolz/cytoolz

https://github.com/more-itertools/more-itertools

https://github.com/ajcr/rolling

The stubs used for the developpement, made by the maintainer of pyochain, can be found here:

https://github.com/py-stubs/cytoolz-stubs