jsonschema-rs 0.35.0

A high-performance JSON Schema validator for Python

jsonschema-rs

A high-performance JSON Schema validator for Python.

import jsonschema_rs

schema = {"maxLength": 5}
instance = "foo"

# One-off validation
try:
    jsonschema_rs.validate(schema, "incorrect")
except jsonschema_rs.ValidationError as exc:
    assert str(exc) == '''"incorrect" is longer than 5 characters

Failed validating "maxLength" in schema

On instance:
    "incorrect"'''

# Build & reuse (faster)
validator = jsonschema_rs.validator_for(schema)

# Iterate over errors
for error in validator.iter_errors(instance):
    print(f"Error: {error}")
    print(f"Location: {error.instance_path}")

# Boolean result
assert validator.is_valid(instance)

⚠️ Upgrading from older versions? Check our Migration Guide for key changes.

Highlights

• 📚 Full support for popular JSON Schema drafts
• 🌐 Remote reference fetching (network/file)
• 🔧 Custom format validators
• ✨ Meta-schema validation for schema documents

Supported drafts

The following drafts are supported:

• 
• 
• 
• 
• 

You can check the current status on the Bowtie Report.

Installation

To install jsonschema-rs via pip run the following command:

pip install jsonschema-rs

Usage

If you have a schema as a JSON string, then you could pass it to validator_for to avoid parsing on the Python side:

import jsonschema_rs

validator = jsonschema_rs.validator_for('{"minimum": 42}')
...

You can use draft-specific validators for different JSON Schema versions:

import jsonschema_rs

# Automatic draft detection
validator = jsonschema_rs.validator_for({"minimum": 42})

# Draft-specific validators
validator = jsonschema_rs.Draft7Validator({"minimum": 42})
validator = jsonschema_rs.Draft201909Validator({"minimum": 42})
validator = jsonschema_rs.Draft202012Validator({"minimum": 42})

JSON Schema allows for format validation through the format keyword. While jsonschema-rs provides built-in validators for standard formats, you can also define custom format validators for domain-specific string formats.

To implement a custom format validator:

1. Define a function that takes a str and returns a bool.
2. Pass it with the formats argument.
3. Ensure validate_formats is set appropriately (especially for Draft 2019-09 and 2020-12).

import jsonschema_rs

def is_currency(value):
    # The input value is always a string
    return len(value) == 3 and value.isascii()


validator = jsonschema_rs.validator_for(
    {"type": "string", "format": "currency"}, 
    formats={"currency": is_currency},
    validate_formats=True  # Important for Draft 2019-09 and 2020-12
)
validator.is_valid("USD")  # True
validator.is_valid("invalid")  # False

Additional configuration options are available for fine-tuning the validation process:

• validate_formats: Override the draft-specific default behavior for format validation.
• ignore_unknown_formats: Control whether unrecognized formats should be reported as errors.
• base_uri - a base URI for all relative $ref in the schema.

Example usage of these options:

import jsonschema_rs

validator = jsonschema_rs.Draft202012Validator(
    {"type": "string", "format": "date"},
    validate_formats=True,
    ignore_unknown_formats=False
)

# This will validate the "date" format
validator.is_valid("2023-05-17")  # True
validator.is_valid("not a date")  # False

# With ignore_unknown_formats=False, using an unknown format will raise an error
invalid_schema = {"type": "string", "format": "unknown"}
try:
    jsonschema_rs.Draft202012Validator(
        invalid_schema, validate_formats=True, ignore_unknown_formats=False
    )
except jsonschema_rs.ValidationError as exc:
    assert str(exc) == '''Unknown format: 'unknown'. Adjust configuration to ignore unrecognized formats

Failed validating "format" in schema

On instance:
    "unknown"'''

Arbitrary-Precision Numbers

The Python bindings always include the arbitrary-precision support from the Rust validator, so numeric values are exposed to Python using the most accurate type available:

• Integers, regardless of size, are returned as regular int objects.
• Floating-point literals that fit into IEEE-754 become Python floats.
• Floating-point literals that don't fit in float (for example 1e10000 or extremely precise decimals) fall back to decimal.Decimal using their original JSON string representation.

This means ValidationError.kind attributes may contain Decimal instances for very large numbers. Import Decimal from the standard library if you need to compare against or serialize those values exactly:

from decimal import Decimal
from jsonschema_rs import ValidationError, validator_for

validator = validator_for('{"const": 1e10000}')
try:
    validator.validate(0)
except ValidationError as exc:
    assert exc.kind.expected_value == Decimal("1e10000")

# Extremely large exponents (beyond ~10^1_000_000) are clamped internally to keep parsing
# predictable, matching the Rust implementation's guardrails.

Meta-Schema Validation

JSON Schema documents can be validated against their meta-schemas to ensure they are valid schemas. jsonschema-rs provides this functionality through the meta module:

import jsonschema_rs

# Valid schema
schema = {
    "type": "object",
    "properties": {
        "name": {"type": "string"},
        "age": {"type": "integer", "minimum": 0}
    },
    "required": ["name"]
}

# Validate schema (draft is auto-detected)
assert jsonschema_rs.meta.is_valid(schema)
jsonschema_rs.meta.validate(schema)  # No error raised

# Invalid schema
invalid_schema = {
    "minimum": "not_a_number"  # "minimum" must be a number
}

try:
    jsonschema_rs.meta.validate(invalid_schema)
except jsonschema_rs.ValidationError as exc:
    assert 'is not of type "number"' in str(exc)

Regular Expression Configuration

When validating schemas with regex patterns (in pattern or patternProperties), you can configure the underlying regex engine:

import jsonschema_rs
from jsonschema_rs import FancyRegexOptions, RegexOptions

# Default fancy-regex engine with backtracking limits
# (supports advanced features but needs protection against DoS)
validator = jsonschema_rs.validator_for(
    {"type": "string", "pattern": "^(a+)+$"},
    pattern_options=FancyRegexOptions(backtrack_limit=10_000)
)

# Standard regex engine for guaranteed linear-time matching
# (prevents regex DoS attacks but supports fewer features)
validator = jsonschema_rs.validator_for(
    {"type": "string", "pattern": "^a+$"},
    pattern_options=RegexOptions()
)

# Both engines support memory usage configuration
validator = jsonschema_rs.validator_for(
    {"type": "string", "pattern": "^a+$"},
    pattern_options=RegexOptions(
        size_limit=1024 * 1024,   # Maximum compiled pattern size
        dfa_size_limit=10240      # Maximum DFA cache size
    )
)

The available options:

• FancyRegexOptions: Default engine with lookaround and backreferences support

  • backtrack_limit: Maximum backtracking steps
  • size_limit: Maximum compiled regex size in bytes
  • dfa_size_limit: Maximum DFA cache size in bytes

• RegexOptions: Safer engine with linear-time guarantee

  • size_limit: Maximum compiled regex size in bytes
  • dfa_size_limit: Maximum DFA cache size in bytes

This configuration is crucial when working with untrusted schemas where attackers might craft malicious regex patterns.

External References

By default, jsonschema-rs resolves HTTP references and file references from the local file system. You can implement a custom retriever to handle external references. Here's an example that uses a static map of schemas:

import jsonschema_rs

def retrieve(uri: str):
    schemas = {
        "https://example.com/person.json": {
            "type": "object",
            "properties": {
                "name": {"type": "string"},
                "age": {"type": "integer"}
            },
            "required": ["name", "age"]
        }
    }
    if uri not in schemas:
        raise KeyError(f"Schema not found: {uri}")
    return schemas[uri]

schema = {
    "$ref": "https://example.com/person.json"
}

validator = jsonschema_rs.validator_for(schema, retriever=retrieve)

# This is valid
validator.is_valid({
    "name": "Alice",
    "age": 30
})

# This is invalid (missing "age")
validator.is_valid({
    "name": "Bob"
})  # False

Schema Registry

For applications that frequently use the same schemas, you can create a registry to store and reference them efficiently:

import jsonschema_rs

# Create a registry with schemas
registry = jsonschema_rs.Registry([
    ("https://example.com/address.json", {
        "type": "object",
        "properties": {
            "street": {"type": "string"},
            "city": {"type": "string"}
        }
    }),
    ("https://example.com/person.json", {
        "type": "object",
        "properties": {
            "name": {"type": "string"},
            "address": {"$ref": "https://example.com/address.json"}
        }
    })
])

# Use the registry with any validator
validator = jsonschema_rs.validator_for(
    {"$ref": "https://example.com/person.json"},
    registry=registry
)

# Validate instances
assert validator.is_valid({
    "name": "John",
    "address": {"street": "Main St", "city": "Boston"}
})

The registry can be configured with a draft version and a retriever for external references:

import jsonschema_rs

registry = jsonschema_rs.Registry(
    resources=[
        (
            "https://example.com/address.json",
            {}
        )
    ],  # Your schemas
    draft=jsonschema_rs.Draft202012,  # Optional
    retriever=lambda uri: {}  # Optional
)

Error Handling

jsonschema-rs provides detailed validation errors through the ValidationError class, which includes both basic error information and specific details about what caused the validation to fail:

import jsonschema_rs

schema = {"type": "string", "maxLength": 5}

try:
    jsonschema_rs.validate(schema, "too long")
except jsonschema_rs.ValidationError as error:
    # Basic error information
    print(error.message)       # '"too long" is longer than 5 characters'
    print(error.instance_path) # Location in the instance that failed
    print(error.schema_path)   # Location in the schema that failed

    # Detailed error information via `kind`
    if isinstance(error.kind, jsonschema_rs.ValidationErrorKind.MaxLength):
        assert error.kind.limit == 5
        print(f"Exceeded maximum length of {error.kind.limit}")

For a complete list of all error kinds and their attributes, see the type definitions file

Error Message Masking

When working with sensitive data, you might want to hide actual values from error messages. You can mask instance values in error messages by providing a placeholder:

import jsonschema_rs

schema = {
    "type": "object",
    "properties": {
        "password": {"type": "string", "minLength": 8},
        "api_key": {"type": "string", "pattern": "^[A-Z0-9]{32}$"}
    }
}

# Use default masking (replaces values with "[REDACTED]")
validator = jsonschema_rs.validator_for(schema, mask="[REDACTED]")

try:
    validator.validate({
        "password": "123",
        "api_key": "secret_key_123"
    })
except jsonschema_rs.ValidationError as exc:
    assert str(exc) == '''[REDACTED] does not match "^[A-Z0-9]{32}$"

Failed validating "pattern" in schema["properties"]["api_key"]

On instance["api_key"]:
    [REDACTED]'''

Performance

jsonschema-rs is designed for high performance, outperforming other Python JSON Schema validators in most scenarios:

• Up to 60-390x faster than jsonschema for complex schemas and large instances
• Generally 3-7x faster than fastjsonschema on CPython

For detailed benchmarks, see our full performance comparison.

Python support

jsonschema-rs supports CPython 3.8 through 3.14.

Acknowledgements

This library draws API design inspiration from the Python jsonschema package. We're grateful to the Python jsonschema maintainers and contributors for their pioneering work in JSON Schema validation.

Support

If you have questions, need help, or want to suggest improvements, please use GitHub Discussions.

Sponsorship

If you find jsonschema-rs useful, please consider sponsoring its development.

Contributing

We welcome contributions! Here's how you can help:

• Share your use cases
• Implement missing keywords
• Fix failing test cases from the JSON Schema test suite

See CONTRIBUTING.md for more details.

License

Licensed under MIT License.