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iron-monk

Business-constraint validation for people who hate data mutation.

A pure-Python validator with zero dependencies, zero coercion, and zero base classes. Just a decorator.

Get Started View on GitHub

Why iron-monk

Most validation libraries do too much. They coerce your data, force you to inherit from a base class, and stop at the first error so you debug payloads one field at a time.

iron-monk flips all three.

  • Zero coercion

    "123" is never silently rewritten to 123. If a string is the wrong format, that is an error.

  • Zero dependencies

    Pure Python. No compiled binaries, no install-time toolchain, no pydantic_core lurking in your container.

  • Aggregate, don't fail-fast

    Validation collects every error and reports them in a single ValidationError. No whack-a-mole.

  • Annotation is the schema

    Constraints live inside typing.Annotated. No BaseModel, no Schema class, no metaclass.

A 30-second tour

from typing import Annotated
from monk import monk, validate
from monk.constraints import Email, Interval

@monk
class User:
    email: Annotated[str, Email]
    age:   Annotated[int, Interval(ge=18)]

user = User(email="bad-email", age=12)
validate(user)
# ValidationError aggregating BOTH the email and age failures

Where to next

  1. Getting Started — installation, the validation lifecycle, error handling.
  2. Core Concepts — the four pillars and the philosophy behind them.
  3. Constraints — the full catalog of built-in rules.
  4. Advanced Usage — cross-field rules, custom constraints, settings.
  5. Integrations — Strawberry, Starlette, SQLAlchemy, Tortoise, tyro, beartype.