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JSON

Json Parsing

API Documentation

pydantic.main.BaseModel.model_validate_json pydantic.type_adapter.TypeAdapter.validate_json pydantic_core.from_json

Pydantic provides builtin JSON parsing, which helps achieve:

  • Significant performance improvements without the cost of using a 3rd party library
  • Support for custom errors
  • Support for strict specifications

Here's an example of Pydantic's builtin JSON parsing via the model_validate_json method, showcasing the support for strict specifications while parsing JSON data that doesn't match the model's type annotations:

from datetime import date
from typing import Tuple

from pydantic import BaseModel, ConfigDict, ValidationError


class Event(BaseModel):
    model_config = ConfigDict(strict=True)

    when: date
    where: Tuple[int, int]


json_data = '{"when": "1987-01-28", "where": [51, -1]}'
print(Event.model_validate_json(json_data))  # (1)!
#> when=datetime.date(1987, 1, 28) where=(51, -1)

try:
    Event.model_validate({'when': '1987-01-28', 'where': [51, -1]})  # (2)!
except ValidationError as e:
    print(e)
    """
    2 validation errors for Event
    when
      Input should be a valid date [type=date_type, input_value='1987-01-28', input_type=str]
    where
      Input should be a valid tuple [type=tuple_type, input_value=[51, -1], input_type=list]
    """
  1. JSON has no date or tuple types, but Pydantic knows that so allows strings and arrays as inputs respectively when parsing JSON directly.
  2. If you pass the same values to the model_validate method, Pydantic will raise a validation error because the strict configuration is enabled.
from datetime import date

from pydantic import BaseModel, ConfigDict, ValidationError


class Event(BaseModel):
    model_config = ConfigDict(strict=True)

    when: date
    where: tuple[int, int]


json_data = '{"when": "1987-01-28", "where": [51, -1]}'
print(Event.model_validate_json(json_data))  # (1)!
#> when=datetime.date(1987, 1, 28) where=(51, -1)

try:
    Event.model_validate({'when': '1987-01-28', 'where': [51, -1]})  # (2)!
except ValidationError as e:
    print(e)
    """
    2 validation errors for Event
    when
      Input should be a valid date [type=date_type, input_value='1987-01-28', input_type=str]
    where
      Input should be a valid tuple [type=tuple_type, input_value=[51, -1], input_type=list]
    """
  1. JSON has no date or tuple types, but Pydantic knows that so allows strings and arrays as inputs respectively when parsing JSON directly.
  2. If you pass the same values to the model_validate method, Pydantic will raise a validation error because the strict configuration is enabled.

In v2.5.0 and above, Pydantic uses jiter, a fast and iterable JSON parser, to parse JSON data. Using jiter compared to serde results in modest performance improvements that will get even better in the future.

The jiter JSON parser is almost entirely compatible with the serde JSON parser, with one noticeable enhancement being that jiter supports deserialization of inf and NaN values. In the future, jiter is intended to enable support validation errors to include the location in the original JSON input which contained the invalid value.

Partial JSON Parsing

Starting in v2.7.0, Pydantic's JSON parser offers support for partial JSON parsing, which is exposed via pydantic_core.from_json. Here's an example of this feature in action:

from pydantic_core import from_json

partial_json_data = '["aa", "bb", "c'  # (1)!

try:
    result = from_json(partial_json_data, allow_partial=False)
except ValueError as e:
    print(e)  # (2)!
    #> EOF while parsing a string at line 1 column 15

result = from_json(partial_json_data, allow_partial=True)
print(result)  # (3)!
#> ['aa', 'bb']
  1. The JSON list is incomplete - it's missing a closing "]
  2. When allow_partial is set to False (the default), a parsing error occurs.
  3. When allow_partial is set to True, part of the input is deserialized successfully.

This also works for deserializing partial dictionaries. For example:

from pydantic_core import from_json

partial_dog_json = '{"breed": "lab", "name": "fluffy", "friends": ["buddy", "spot", "rufus"], "age'
dog_dict = from_json(partial_dog_json, allow_partial=True)
print(dog_dict)
#> {'breed': 'lab', 'name': 'fluffy', 'friends': ['buddy', 'spot', 'rufus']}

Validating LLM Output

This feature is particularly beneficial for validating LLM outputs. We've written some blog posts about this topic, which you can find here.

In future versions of Pydantic, we expect to expand support for this feature through either Pydantic's other JSON validation functions (pydantic.main.BaseModel.model_validate_json and pydantic.type_adapter.TypeAdapter.validate_json) or model configuration. Stay tuned 🚀!

For now, you can use pydantic_core.from_json in combination with pydantic.main.BaseModel.model_validate to achieve the same result. Here's an example:

from pydantic_core import from_json

from pydantic import BaseModel


class Dog(BaseModel):
    breed: str
    name: str
    friends: list


partial_dog_json = '{"breed": "lab", "name": "fluffy", "friends": ["buddy", "spot", "rufus"], "age'
dog = Dog.model_validate(from_json(partial_dog_json, allow_partial=True))
print(repr(dog))
#> Dog(breed='lab', name='fluffy', friends=['buddy', 'spot', 'rufus'])

Tip

For partial JSON parsing to work reliably, all fields on the model should have default values.

Check out the following example for a more in-depth look at how to use default values with partial JSON parsing:

Using default values with partial JSON parsing

from typing import Any, Optional, Tuple

import pydantic_core
from typing_extensions import Annotated

from pydantic import BaseModel, ValidationError, WrapValidator


def default_on_error(v, handler) -> Any:
    """
    Raise a PydanticUseDefault exception if the value is missing.

    This is useful for avoiding errors from partial
    JSON preventing successful validation.
    """
    try:
        return handler(v)
    except ValidationError as exc:
        # there might be other types of errors resulting from partial JSON parsing
        # that you allow here, feel free to customize as needed
        if all(e['type'] == 'missing' for e in exc.errors()):
            raise pydantic_core.PydanticUseDefault()
        else:
            raise


class NestedModel(BaseModel):
    x: int
    y: str


class MyModel(BaseModel):
    foo: Optional[str] = None
    bar: Annotated[
        Optional[Tuple[str, int]], WrapValidator(default_on_error)
    ] = None
    nested: Annotated[
        Optional[NestedModel], WrapValidator(default_on_error)
    ] = None


m = MyModel.model_validate(
    pydantic_core.from_json('{"foo": "x", "bar": ["world",', allow_partial=True)
)
print(repr(m))
#> MyModel(foo='x', bar=None, nested=None)


m = MyModel.model_validate(
    pydantic_core.from_json(
        '{"foo": "x", "bar": ["world", 1], "nested": {"x":', allow_partial=True
    )
)
print(repr(m))
#> MyModel(foo='x', bar=('world', 1), nested=None)

Caching Strings

Starting in v2.7.0, Pydantic's JSON parser offers support for configuring how Python strings are cached during JSON parsing and validation (when Python strings are constructed from Rust strings during Python validation, e.g. after strip_whitespace=True). The cache_strings setting is exposed via both model config and pydantic_core.from_json.

The cache_strings setting can take any of the following values:

  • True or 'all' (the default): cache all strings
  • 'keys': cache only dictionary keys, this only applies when used with pydantic_core.from_json or when parsing JSON using Json
  • False or 'none': no caching

Using the string caching feature results in performance improvements, but increases memory usage slightly.

String Caching Details

  1. Strings are cached using a fully associative cache with a size of 16,384.
  2. Only strings where len(string) < 64 are cached.
  3. There is some overhead to looking up the cache, which is normally worth it to avoid constructing strings. However, if you know there will be very few repeated strings in your data, you might get a performance boost by disabling this setting with cache_strings=False.

JSON Serialization

API Documentation

pydantic.main.BaseModel.model_dump_json
pydantic.type_adapter.TypeAdapter.dump_json
pydantic_core.to_json

For more information on JSON serialization, see the Serialization Concepts page.