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Overview

Documentation for version: v1.0b2

Note

These docs refer to Version 1 of pydantic which is as-yet unreleased. v0.32 docs are available here.

Data validation and settings management using python type hinting.

Define how data should be in pure, canonical python; validate it with pydantic.

PEP 484 introduced type hinting into python 3.5; PEP 526 extended that with syntax for variable annotation in python 3.6.

pydantic uses those annotations to validate that untrusted data takes the form you want.

There's also support for an extension to dataclasses where the input data is validated.

Example

from datetime import datetime
from typing import List
from pydantic import BaseModel

class User(BaseModel):
    id: int
    name = 'John Doe'
    signup_ts: datetime = None
    friends: List[int] = []

external_data = {
    'id': '123',
    'signup_ts': '2019-06-01 12:22',
    'friends': [1, '2', 3.1415]
}
user = User(**external_data)
print(user.id)
#> 123
print(repr(user.signup_ts))
#> datetime.datetime(2019, 6, 1, 12, 22)
print(user.friends)
#> [1, 2, 3]
print(user.dict())
"""
{
    'id': 123,
    'signup_ts': datetime.datetime(2019, 6, 1, 12, 22),
    'friends': [1, 2, 3],
    'name': 'John Doe',
}
"""

(This script is complete, it should run "as is")

What's going on here:

• id is of type int; the annotation-only declaration tells pydantic that this field is required. Strings, bytes or floats will be coerced to ints if possible; otherwise an exception will be raised.
• name is inferred as a string from the provided default; because it has a default, it is not required.
• signup_ts is a datetime field which is not required (and takes the value None if it's not supplied). pydantic will process either a unix timestamp int (e.g. 1496498400) or a string representing the date & time.
• friends uses python's typing system, and requires a list of inputs. As with id, integer-like objects will be converted to integers.

If validation fails pydantic will raise an error with a breakdown of what was wrong:

from pydantic import ValidationError

try:
    User(signup_ts='broken', friends=[1, 2, 'not number'])
except ValidationError as e:
    print(e.json())

outputs:

[
  {
    "loc": [
      "id"
    ],
    "msg": "field required",
    "type": "value_error.missing"
  },
  {
    "loc": [
      "signup_ts"
    ],
    "msg": "invalid datetime format",
    "type": "value_error.datetime"
  },
  {
    "loc": [
      "friends",
      2
    ],
    "msg": "value is not a valid integer",
    "type": "type_error.integer"
  }
]

Rationale

So pydantic uses some cool new language features, but why should I actually go and use it?

no brainfuck

there's no new schema definition micro-language to learn. If you know python (and perhaps skim the type hinting docs) you know how to use pydantic.

plays nicely with your IDE/linter/brain

pydantic data structures are just instances of classes you define, so auto-completion, linting, mypy, IDEs (especially PyCharm), and your intuition should all work properly with your validated data.

dual use

pydantic's BaseSettings class allows pydantic to be used in both a "validate this request data" context and in a "load my system settings" context. The main differences are that system settings can be read from environment variables, and more complex objects like DSNs and python objects are often required.

fast

In benchmarks pydantic is faster than all other tested libraries.

validate complex structures

use of recursive pydantic models, typing's standard types (e.g. List, Tuple, Dict etc.) and validators allow complex data schemas to be clearly and easily defined, validated, and parsed.

extensible

pydantic allows custom data types to be defined or you can extend validation with methods on a model decorated with the validator decorator.

Using Pydantic

Hundreds of organisations and packages are using pydantic, including:

FastAPI

a high performance API framework, easy to learn, fast to code and ready for production, based on pydantic and Starlette.

Project Jupyter

developers of the Jupyter notebook are using pydantic for subprojects.

Microsoft

are using pydantic (via FastAPI) for numerous services, some of which are "getting integrated into the core Windows product and some Office products."

Amazon Web Services

are using pydantic in gluon-ts, an open-source probabilistic time series modeling library.

The NSA

are using pydantic in WALKOFF, an open-source automation framework.

Uber

are using pydantic in Ludwig, an an open-source TensorFlow wrapper.

Cuenca

are a Mexican neobank that uses pydantic for several internal tools (including API validation) and for open source projects like stpmex, which is used to process real-time, 24/7, inter-bank transfers in Mexico.

For a more comprehensive list of open-source projects using pydantic see the list of dependents on github.