Valid8r

Clean, composable input validation for Python using functional programming patterns.

Valid8r makes input validation elegant and type-safe by using the Maybe monad for error handling. No more try-except blocks or boolean validation chains—just clean, composable parsers that tell you exactly what went wrong.

from valid8r import parsers, validators, prompt

# Parse and validate user input with rich error messages
age = prompt.ask(
    "Enter your age: ",
    parser=parsers.parse_int,
    validator=validators.minimum(0) & validators.maximum(120)
)

print(f"Your age is {age}")

Why Valid8r?

Type-Safe Parsing: Every parser returns Maybe[T] (Success or Failure), making error handling explicit and composable.

Rich Structured Results: Network parsers return dataclasses with parsed components—no more manual URL/email splitting.

Chainable Validators: Combine validators using & (and), | (or), and ~ (not) operators for complex validation logic.

Security-First Design: All parsers include DoS protection via input length validation and automated ReDoS detection prevents vulnerable regex patterns.

Framework Integrations: Built-in support for Pydantic (always included) and optional Click integration for CLI apps.

Interactive Prompts: Built-in user input prompting with automatic retry and validation.

High Performance: Valid8r is 4-300x faster than Pydantic for basic parsing, making it ideal for high-throughput APIs and batch processing.

Performance

Valid8r is designed for high-performance validation with minimal overhead:

Scenario	valid8r	Pydantic	Speedup
Integer parsing	375ns	102µs	273x faster
Nested objects	37µs	568µs	15x faster
List (100 items)	30µs	126µs	4x faster

When to use valid8r:

High-throughput APIs (>5K requests/sec)
Batch processing pipelines
CLI tools requiring structured parsing
Performance-critical code paths

When to use Pydantic:

FastAPI applications (tight integration)
Complex data models with auto-schema generation
Developer experience > raw performance

Full benchmarks and methodology: docs/performance.md

Quick Start

Installation

Basic installation (includes Pydantic integration):

pip install valid8r

With optional framework integrations:

# Click integration for CLI applications
pip install 'valid8r[click]'

# All optional integrations
pip install 'valid8r[click]'

Requirements: Python 3.11 or higher

Feature	Installation	Import
Core parsers & validators	`pip install valid8r`	`from valid8r import parsers, validators`
Pydantic integration	included by default	`from valid8r.integrations import validator_from_parser`
Click integration (CLI)	`pip install 'valid8r[click]'`	`from valid8r.integrations import ParamTypeAdapter`

Basic Parsing

from valid8r import parsers
from valid8r.core.maybe import Success, Failure

# Parse integers with automatic error handling
match parsers.parse_int("42"):
    case Success(value):
        print(f"Parsed: {value}")  # Parsed: 42
    case Failure(error):
        print(f"Error: {error}")

# Parse dates (ISO 8601 format)
result = parsers.parse_date("2025-01-15")
assert result.is_success()

# Parse UUIDs with version validation
result = parsers.parse_uuid("550e8400-e29b-41d4-a716-446655440000", version=4)
assert result.is_success()

Temporal Parsing

from valid8r import parsers
from datetime import UTC

# Parse timezone-aware datetime (ISO 8601)
result = parsers.parse_datetime("2024-01-15T10:30:00Z")
match result:
    case Success(dt):
        print(f"DateTime: {dt}")  # 2024-01-15 10:30:00+00:00
        print(f"Timezone: {dt.tzinfo}")  # UTC
    case Failure(error):
        print(f"Error: {error}")

# Parse with timezone offset
result = parsers.parse_datetime("2024-01-15T10:30:00+05:30")
assert result.is_success()

# Parse duration/timedelta in multiple formats
result = parsers.parse_timedelta("1h 30m")  # Simple format
assert result.value_or(None).total_seconds() == 5400

result = parsers.parse_timedelta("PT1H30M")  # ISO 8601 duration
assert result.value_or(None).total_seconds() == 5400

Validation with Combinators

from valid8r import validators

# Combine validators using operators
age_validator = validators.minimum(0) & validators.maximum(120)
result = age_validator(42)
assert result.is_success()

# String validation
password_validator = (
    validators.length(8, 128) &
    validators.matches_regex(r'^(?=.*[A-Za-z])(?=.*\d)[A-Za-z\d@$!%*#?&]')
)

# Set validation
tags_validator = validators.subset_of({'python', 'rust', 'go', 'typescript'})

Structured Network Parsing

from valid8r import parsers

# Parse URLs into structured components
match parsers.parse_url("https://user:[email protected]:8443/path?query=1#fragment"):
    case Success(url):
        print(f"Scheme: {url.scheme}")      # https
        print(f"Host: {url.host}")          # example.com
        print(f"Port: {url.port}")          # 8443
        print(f"Path: {url.path}")          # /path
        print(f"Query: {url.query}")        # {'query': '1'}
        print(f"Fragment: {url.fragment}")  # fragment

# Parse emails with normalized domains
match parsers.parse_email("[email protected]"):
    case Success(email):
        print(f"Local: {email.local}")    # User
        print(f"Domain: {email.domain}")  # example.com (normalized)

# Parse phone numbers (NANP format)
match parsers.parse_phone("+1 (415) 555-2671"):
    case Success(phone):
        print(f"E.164: {phone.e164}")      # +14155552671
        print(f"National: {phone.national}")  # (415) 555-2671

Collection Parsing

from valid8r import parsers

# Parse lists with element validation
result = parsers.parse_list("1,2,3,4,5", element_parser=parsers.parse_int)
assert result.value_or([]) == [1, 2, 3, 4, 5]

# Parse dictionaries with key/value parsers
result = parsers.parse_dict(
    "name=Alice,age=30",
    key_parser=lambda x: Success(x),
    value_parser=lambda x: parsers.parse_int(x) if x.isdigit() else Success(x)
)

Type-Based Parser Generation

Automatically generate parsers from Python type annotations:

from typing import Annotated, Literal, Optional
from valid8r.core.type_adapters import from_type
from valid8r import validators

# Generate parser from type annotation
parser = from_type(int)
result = parser('42')
assert result.value_or(None) == 42

# Optional types handle None automatically
parser = from_type(Optional[int])
assert parser('').value_or('not none') is None  # Empty string becomes None
assert parser('42').value_or(None) == 42

# Collections with element validation
parser = from_type(list[int])
result = parser('[1, 2, 3, 4, 5]')
assert result.value_or([]) == [1, 2, 3, 4, 5]

# Nested structures
parser = from_type(dict[str, list[int]])
result = parser('{"scores": [95, 87, 92]}')
assert result.value_or({}) == {'scores': [95, 87, 92]}

# Combine types with validators using Annotated
Age = Annotated[int, validators.minimum(0), validators.maximum(120)]
parser = from_type(Age)
assert parser('25').value_or(None) == 25
assert parser('150').is_failure()  # Exceeds maximum

# Literal types for restricted values
parser = from_type(Literal['red', 'green', 'blue'])
assert parser('red').value_or(None) == 'red'
assert parser('yellow').is_failure()  # Not in literal set

Security: All collection parsers include automatic DoS protection—inputs exceeding 100KB are rejected in <10ms before expensive JSON parsing.

Filesystem Parsing and Validation

from valid8r import parsers, validators
from valid8r.core.maybe import Success, Failure

# Parse and validate file paths
match parsers.parse_path("/etc/hosts").bind(validators.exists()).bind(validators.is_file()):
    case Success(path):
        print(f"Valid file: {path}")
    case Failure(err):
        print(f"Error: {err}")

# Validate uploaded files
def validate_upload(file_path: str):
    return (
        parsers.parse_path(file_path)
        .bind(validators.exists())
        .bind(validators.is_file())
        .bind(validators.has_extension(['.pdf', '.docx']))
        .bind(validators.max_size(10 * 1024 * 1024))  # 10MB limit
    )

# Path expansion and resolution
match parsers.parse_path("~/Documents", expand_user=True):
    case Success(path):
        print(f"Expanded: {path}")  # /Users/username/Documents

match parsers.parse_path("./data/file.txt", resolve=True):
    case Success(path):
        print(f"Absolute: {path}")  # /full/path/to/data/file.txt

Interactive Prompting

from valid8r import prompt, parsers, validators

# Prompt with validation and automatic retry
email = prompt.ask(
    "Email address: ",
    parser=parsers.parse_email,
    retry=2  # Retry twice on invalid input
)

# Combine parsing and validation
port = prompt.ask(
    "Server port: ",
    parser=parsers.parse_int,
    validator=validators.between(1024, 65535),
    retry=3
)

Structured Error Handling

Valid8r provides machine-readable error codes and structured error information for programmatic error handling and API responses:

from valid8r import parsers
from valid8r.core.maybe import Success, Failure
from valid8r.core.errors import ErrorCode

# Programmatic error handling using error codes
def process_email(email_str: str):
    result = parsers.parse_email(email_str)

    match result:
        case Success(email):
            return f"Valid: {email.local}@{email.domain}"
        case Failure():
            # Access structured error for programmatic handling
            detail = result.error_detail()

            # Switch on error codes for different handling
            match detail.code:
                case ErrorCode.INVALID_EMAIL:
                    return "Please enter a valid email address"
                case ErrorCode.EMPTY_STRING:
                    return "Email is required"
                case ErrorCode.INPUT_TOO_LONG:
                    return "Email is too long"
                case _:
                    return f"Error: {detail.message}"

# Convert errors to JSON for API responses
result = parsers.parse_int("not-a-number")
match result:
    case Failure():
        error_dict = result.error_detail().to_dict()
        # {
        #   'code': 'INVALID_TYPE',
        #   'message': 'Input must be a valid integer',
        #   'path': '',
        #   'context': {}
        # }

Features:

Error codes for programmatic handling (e.g., ErrorCode.INVALID_EMAIL, ErrorCode.OUT_OF_RANGE)
JSON serialization for API responses via to_dict()
Field paths for multi-field validation (e.g., .user.email)
Debugging context with validation parameters
100% backward compatible with string errors

See the Error Handling Guide for comprehensive examples and best practices.

Async Validation

Validate data using async validators for I/O-bound operations like database checks, API calls, and external service validation:

import asyncio
from valid8r.core import parsers, schema, validators
from valid8r.core.maybe import Success, Failure

# Define async validator
async def check_email_unique(email: str) -> Maybe[str]:
    """Check if email is unique in database."""
    # Simulate database query
    await asyncio.sleep(0.1)
    existing_emails = {'[email protected]', '[email protected]'}

    if email in existing_emails:
        return Maybe.failure('Email already registered')
    return Maybe.success(email)

# Create schema with async validators
user_schema = schema.Schema(fields={
    'email': schema.Field(
        parser=parsers.parse_email,
        validators=[
            validators.min_length(1),  # Sync validator (fail-fast)
            check_email_unique,  # Async validator (database check)
        ],
        required=True
    ),
    'username': schema.Field(
        parser=parsers.parse_str,
        validators=[
            validators.matches_pattern(r'^[a-z0-9_]+$'),
            check_username_available,  # Another async validator
        ],
        required=True
    ),
})

# Validate asynchronously with timeout
async def main():
    result = await user_schema.validate_async(
        {'email': '[email protected]', 'username': 'newuser'},
        timeout=5.0
    )

    match result:
        case Success(data):
            print(f"Valid: {data}")
        case Failure(errors):
            for error in errors:
                print(f"{error.path}: {error.message}")

asyncio.run(main())

Key Features:

Concurrent execution of async validators across fields for better performance
Mixed sync and async validators (sync runs first for fail-fast behavior)
Configurable timeout support to prevent hanging on slow operations
Full error accumulation across all fields
Works seamlessly with existing sync validators

Common Use Cases:

Database uniqueness checks (email, username)
External API validation (API keys, payment methods)
Geolocation constraints (IP address country verification)
Remote file access validation
Any I/O-bound validation operation

See the Async Validation Guide for comprehensive examples including database integration, API validation, and performance optimization patterns.

Environment Variables

Load typed, validated configuration from environment variables following 12-factor app principles:

from valid8r.integrations.env import EnvSchema, EnvField, load_env_config
from valid8r import parsers, validators
from valid8r.core.maybe import Success, Failure

# Define configuration schema
schema = EnvSchema(fields={
    'port': EnvField(
        parser=lambda x: parsers.parse_int(x).bind(validators.between(1024, 65535)),
        default=8080
    ),
    'debug': EnvField(parser=parsers.parse_bool, default=False),
    'database_url': EnvField(parser=parsers.parse_str, required=True),
    'admin_email': EnvField(parser=parsers.parse_email, required=True),
})

# Load and validate configuration
result = load_env_config(schema, prefix='APP_')

match result:
    case Success(config):
        # All values are typed and validated
        port = config['port']              # int (validated 1024-65535)
        debug = config['debug']            # bool (not str!)
        db = config['database_url']        # str (required, guaranteed present)
        email = config['admin_email']      # EmailAddress (validated format)
    case Failure(error):
        print(f"Configuration error: {error}")

Features:

Type-safe parsing (no more string-to-int conversions)
Declarative validation with composable parsers
Required vs optional fields with sensible defaults
Nested schemas for hierarchical configuration
Clear error messages for missing or invalid values

See Environment Variables Guide for complete examples including FastAPI, Docker, and Kubernetes deployment patterns.

Framework Integrations

Pydantic Integration (Always Included)

Convert valid8r parsers into Pydantic field validators:

from pydantic import BaseModel, field_validator
from valid8r import parsers, validators
from valid8r.integrations import validator_from_parser

class User(BaseModel):
    age: int

    @field_validator('age', mode='before')
    @classmethod
    def validate_age(cls, v):
        # Parse string to int, then validate 0-120 range
        age_parser = lambda x: parsers.parse_int(x).bind(
            validators.between(0, 120)
        )
        return validator_from_parser(age_parser)(v)

user = User(age="25")  # Accepts string, validates, returns int

Works seamlessly with nested models, lists, and complex Pydantic schemas. See Pydantic Integration Examples.

Click Integration (Optional)

Install: pip install 'valid8r[click]'

Integrate valid8r parsers into Click CLI applications:

import click
from valid8r import parsers
from valid8r.integrations import ParamTypeAdapter

@click.command()
@click.option('--email', type=ParamTypeAdapter(parsers.parse_email))
def send_mail(email):
    """Send an email with validated address."""
    click.echo(f"Sending to {email.local}@{email.domain}")

# Automatically validates email format and provides rich error messages

See Click Integration Examples.

Features

Parsers

Basic Types:

parse_int, parse_float, parse_bool, parse_decimal, parse_complex
parse_date (ISO 8601), parse_uuid (with version validation)

Collections:

parse_list, parse_dict, parse_set (with element parsers)

Network & Communication:

parse_ipv4, parse_ipv6, parse_ip, parse_cidr
parse_url → UrlParts (structured URL components)
parse_email → EmailAddress (normalized domain)
parse_phone → PhoneNumber (NANP validation with E.164 formatting)

Filesystem:

parse_path → pathlib.Path (with expansion and resolution options)

Advanced:

parse_enum (type-safe enum parsing)
create_parser, make_parser, validated_parser (custom parser factories)

Validators

Numeric: minimum, maximum, between

String: non_empty_string, matches_regex, length

Collection: in_set, unique_items, subset_of, superset_of, is_sorted

Filesystem: exists, is_file, is_dir, is_readable, is_writable, is_executable, max_size, min_size, has_extension

Custom: predicate (create validators from any function)

Combinators: Combine validators using & (and), | (or), ~ (not)

Testing Utilities

from valid8r.testing import (
    assert_maybe_success,
    assert_maybe_failure,
    MockInputContext,
)

# Test validation logic
result = validators.minimum(0)(42)
assert assert_maybe_success(result, 42)

result = validators.minimum(0)(-5)
assert assert_maybe_failure(result, "at least 0")

# Mock user input for testing prompts
with MockInputContext(["invalid", "[email protected]"]):
    result = prompt.ask("Email: ", parser=parsers.parse_email, retry=1)
    assert result.is_success()

Documentation

Full documentation: valid8r.readthedocs.io

Library Comparison Guide - When to choose valid8r vs Pydantic/marshmallow/cerberus
API Reference
Parser Guide
Validator Guide
Testing Guide

Security

Reporting Vulnerabilities

Please do not report security vulnerabilities through public GitHub issues.

Report security issues privately to [email protected] or via GitHub Security Advisories.

See SECURITY.md for our complete security policy, supported versions, and response timeline.

Production Deployment

Valid8r is designed for parsing trusted user input in web applications. For production deployments:

Enforce input size limits at the framework level (recommended: 10KB max request size)
Implement rate limiting for validation endpoints (recommended: 10 requests/minute)
Use defense in depth: Framework → Application → Parser validation
Monitor and log validation failures for security analysis

Example - Flask Defense in Depth:

from flask import Flask, request
from valid8r import parsers

app = Flask(__name__)
app.config['MAX_CONTENT_LENGTH'] = 10 * 1024  # Layer 1: Framework limit

@app.route('/submit', methods=['POST'])
def submit():
    phone = request.form.get('phone', '')

    # Layer 2: Application validation
    if len(phone) > 100:
        return "Phone too long", 400

    # Layer 3: Parser validation
    result = parsers.parse_phone(phone)
    if result.is_failure():
        return "Invalid phone format", 400

    return process_phone(result.value_or(None))

See Production Deployment Guide for framework-specific examples (Flask, Django, FastAPI).

Security Boundaries

Valid8r provides input validation, not protection against:

❌ SQL injection - Use parameterized queries / ORMs
❌ XSS attacks - Use output encoding / template engines
❌ CSRF attacks - Use CSRF tokens / SameSite cookies
❌ DDoS attacks - Use rate limiting / CDN / WAF

Parser Input Limits:

Parser	Max Input	Notes
`parse_email()`	254 chars	RFC 5321 maximum
`parse_phone()`	100 chars	International + extension
`parse_url()`	2048 chars	Browser URL limit
`parse_uuid()`	36 chars	Standard UUID format
`parse_ip()`	45 chars	IPv6 maximum

All parsers include built-in DoS protection with early length validation before expensive operations.

See SECURITY.md for complete security documentation.

Community

Join the Valid8r community on GitHub Discussions:

Questions? Start a discussion in Q&A
Feature ideas? Share them in Ideas
Built something cool? Show it off in Show and Tell
Announcements: Watch Announcements for updates

When to use Discussions vs Issues:

Use Discussions for questions, ideas, and general conversation
Use Issues for bug reports and feature requests with technical specifications

See the Welcome Discussion for community guidelines.

Contributing

We welcome contributions! All contributions must be made via forks - please do not create branches directly in the main repository.

See CONTRIBUTING.md for complete guidelines.

Quick links:

Development Quick Start

# 1. Fork the repository on GitHub
#    Visit: https://github.com/mikelane/valid8r

# 2. Clone YOUR fork (not the upstream repo)
git clone https://github.com/YOUR-USERNAME/valid8r
cd valid8r

# 3. Add upstream remote
git remote add upstream https://github.com/mikelane/valid8r.git

# 4. Install uv (fast dependency manager)
curl -LsSf https://astral.sh/uv/install.sh | sh

# 5. Install dependencies
uv sync

# 6. Run tests
uv run tox

# 7. Run linters
uv run ruff check .
uv run ruff format .
uv run mypy valid8r

# 8. Create a feature branch and make your changes
git checkout -b feat/your-feature

# 9. Push to YOUR fork and create a PR
git push origin feat/your-feature

Project Status

Valid8r is in active development (v0.7.x). The API is stabilizing but may change before v1.0.0.

✅ Core parsers and validators
✅ Maybe monad error handling
✅ Interactive prompting
✅ Network parsers (URL, Email, IP, Phone)
✅ Collection parsers
✅ Comprehensive testing utilities
🚧 Additional validators (in progress)
🚧 Custom error types (planned)

See ROADMAP.md for planned features.

License

MIT License - see LICENSE for details.

Made with ❤️ for the Python community

Name		Name	Last commit message	Last commit date
Latest commit History 226 Commits
.github		.github
benchmarks		benchmarks
docs		docs
examples		examples
scripts		scripts
tests		tests
valid8r		valid8r
.codecov.yml		.codecov.yml
.coveragerc		.coveragerc
.cursorrules		.cursorrules
.gitignore		.gitignore
.pre-commit-config.yaml		.pre-commit-config.yaml
.python-version		.python-version
.readthedocs.yaml		.readthedocs.yaml
CHANGELOG.md		CHANGELOG.md
CLAUDE.md		CLAUDE.md
CODE_OF_CONDUCT.md		CODE_OF_CONDUCT.md
CONTRIBUTING.md		CONTRIBUTING.md
LICENSE		LICENSE
README.md		README.md
ROADMAP.md		ROADMAP.md
SECURITY.md		SECURITY.md
pyproject.toml		pyproject.toml
smoke_test.py		smoke_test.py
tox.ini		tox.ini
uv.lock		uv.lock

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Valid8r

Why Valid8r?

Performance

Quick Start

Installation

Basic Parsing

Temporal Parsing

Validation with Combinators

Structured Network Parsing

Collection Parsing

Type-Based Parser Generation

Filesystem Parsing and Validation

Interactive Prompting

Structured Error Handling

Async Validation

Environment Variables

Framework Integrations

Pydantic Integration (Always Included)

Click Integration (Optional)

Features

Parsers

Validators

Testing Utilities

Documentation

Security

Reporting Vulnerabilities

Production Deployment

Security Boundaries

Community

Contributing

Development Quick Start

Project Status

License

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