pystructlight 0.1.3

A lightweight, immutable struct library with type enforcement.

# pystructlight

**A lightweight, memory‑efficient, and type‑safe immutable data structure for Python.**

`pystructlight` lets you define schemas for your data objects and ensures that every instance adheres to those types at runtime – all while keeping a tiny memory footprint thanks to `__slots__`.

## Features

- **Immutable** – instances cannot be modified after creation
- **Runtime type enforcement** – validates that all field values match your schema
- **Memory efficient** – uses `__slots__` to avoid per‑instance `__dict__`
- **Flexible field definitions** – support for default values, custom validators, and type coercion
- **Rich type support** – `Union`, `Optional`, `Literal`, `Any`, generic containers (`list[T]`, `dict[K, V]`, etc.)
- **Convenient methods** – `evolve()`, `matches()`, `to_dict()`, `from_dict()`, `is_instance()`
- **Positional or keyword instantiation** – choose the style you prefer

## Installation

```bash
pip install pystructlight

Quick Start

1. Define a Struct

Pass a name and a dictionary mapping field names to their expected types (or to a FieldDef for advanced options).

from pystructlight import Struct, field

# Simple type‑only schema
Book = Struct("Book", {
    "title": str,
    "author": str,
    "pages": int,
    "is_hardcover": bool,
})

# Schema with defaults, validator, and coercion
Person = Struct("Person", {
    "name": str,
    "age": field(int, default=0, validator=lambda x: x >= 0),
    "email": str,
    "tags": field(list[str], default=list),
})

2. Create Instances

Use keyword arguments…

book1 = Book.new(
    title="The Great Gatsby",
    author="F. Scott Fitzgerald",
    pages=180,
    is_hardcover=False
)

…or positional arguments (in the order the fields were defined):

book2 = Book.new("1984", "George Orwell", 328, True)

3. Access and Use

print(book1.title)                     # "The Great Gatsby"
print(book1.pages)                     # 180

# Immutability
try:
    book1.pages = 200
except AttributeError:
    print("Cannot modify immutable struct!")

# Runtime type checking
try:
    Book.new("The Hobbit", "J.R.R. Tolkien", "many", True)
except TypeError as e:
    print(e)   # Field 'pages': expected int, got str

# Convert to dict
data = book1.to_dict()
# {'title': 'The Great Gatsby', 'author': 'F. Scott Fitzgerald', 'pages': 180, 'is_hardcover': False}

# Create from dict
book3 = Book.from_dict({"title": "Moby Dick", "author": "Herman Melville", "pages": 585, "is_hardcover": False})

Advanced Field Definitions

Use the field() helper to add defaults, validators, or type coercion.

Parameter	Description
type	Expected type (e.g. str, int, list[str], Union[int, str], Any)
default	Default value. Can be a constant or a callable (e.g. list, dict).
validator	Callable that receives the value and returns True, a string error, or False.
coerce	If True, attempts to convert the input to the expected type (e.g. int("42")).

from pystructlight import Struct, field

Config = Struct("Config", {
    "timeout": field(float, default=5.0, coerce=True),
    "retries": field(int, default=3, validator=lambda x: 0 <= x <= 10),
    "tags": field(list[str], default=list),
})

c = Config.new(timeout="2.5", retries=5, tags=["prod", "api"])
print(c.timeout)   # 2.5 (coerced from string)
print(c.retries)   # 5
print(c.tags)      # ['prod', 'api']

Methods on Struct Instances

Every instance provides the following methods:

Method	Description
to_dict()	Returns a dict mapping field names to values.
evolve(**changes)	Returns a new instance with updated fields (original unchanged).
matches(**kwargs)	Returns True if all given fields equal the specified values.
__iter__()	Iterates over field values (in definition order).
__len__()	Number of fields.
__repr__()	Human‑readable representation.

book = Book.new(title="Brave New World", author="Aldous Huxley", pages=268, is_hardcover=False)

# Evolve
new_book = book.evolve(pages=300, is_hardcover=True)
print(new_book.pages)      # 300
print(book.pages)          # 268 (unchanged)

# Matching
if book.matches(title="Brave New World"):
    print("It's the same book!")

# Iteration
for value in book:
    print(value)

Methods on the Struct Type

The object returned by Struct() (a StructType) provides:

Method	Description
new(*args, **kwargs)	Creates a new instance.
from_dict(data: dict)	Creates an instance from a dictionary (calls new(**data)).
is_instance(obj)	Returns True if obj is an instance of this exact struct type.

Book.is_instance(book)   # True
Book.is_instance("foo")  # False

Runtime Type Checking

pystructlight validates values against your schema when you create the instance.

Supported types include:

• Built‑in types: int, float, str, bool, bytes, None
• Type unions: Union[int, str], int | str (Python 3.10+)
• Optional[T] (same as Union[T, None])
• Literal["red", "green"]
• Any (no checking)
• Generic containers: list[T], tuple[T], set[T], frozenset[T], dict[K, V]
• Any user‑defined class

Example:

from typing import Union, Literal, Any

Data = Struct("Data", {
    "id": Union[int, str],
    "status": Literal["active", "inactive"],
    "payload": Any,
})

d = Data.new(id=42, status="active", payload={"anything": 123})  # OK
d2 = Data.new(id="abc", status="inactive", payload=None)         # OK

# This fails:
# Data.new(id=3.14, status="active", payload=None)   # id must be int or str

Memory Efficiency

Instances do not have a __dict__. All fields are stored in a compact tuple, and access is routed through __slots__. The overhead is roughly the same as a namedtuple, but with full runtime type checking.

import sys

Book = Struct("Book", {"title": str, "pages": int})
b = Book.new("The Pearl", 96)
print(sys.getsizeof(b))   # typically 56 bytes (plus the tuple, but overall very small)

API Reference

Struct(name: str, schema: dict) -> StructType

Creates a new struct type.

• name – the name of the struct (used in repr and error messages)
• schema – dictionary where each key is a field name and each value is either a type or a FieldDef (created by field()).

field(type, default=_MISSING, validator=None, coerce=False) -> FieldDef

Returns a field definition for use in a schema.

• type – the expected type (e.g. str, list[int]).
• default – optional default value (if callable, it is called each time a default is needed).
• validator – optional callable (value) -> bool | str | None. Return False or a string to raise an error.
• coerce – if True, tries to convert the input to the given type using typ(value).