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Validation Decorator

API Documentation


The @validate_call decorator allows the arguments passed to a function to be parsed and validated using the function's annotations before the function is called.

While under the hood this uses the same approach of model creation and initialisation (see Validators for more details), it provides an extremely easy way to apply validation to your code with minimal boilerplate.

Example of usage:

from pydantic import ValidationError, validate_call

def repeat(s: str, count: int, *, separator: bytes = b'') -> bytes:
    b = s.encode()
    return separator.join(b for _ in range(count))

a = repeat('hello', 3)
#> b'hellohellohello'

b = repeat('x', '4', separator=b' ')
#> b'x x x x'

    c = repeat('hello', 'wrong')
except ValidationError as exc:
    1 validation error for repeat
      Input should be a valid integer, unable to parse string as an integer [type=int_parsing, input_value='wrong', input_type=str]

Argument types

Argument types are inferred from type annotations on the function, arguments without a type decorator are considered as Any. All types listed in types can be validated, including Pydantic models and custom types. As with the rest of Pydantic, types can be coerced by the decorator before they're passed to the actual function:

# TODO replace find_file with something that isn't affected the filesystem
import os
from pathlib import Path
from typing import Optional, Pattern

from pydantic import DirectoryPath, validate_call

def find_file(path: DirectoryPath, regex: Pattern, max=None) -> Optional[Path]:
    for i, f in enumerate(path.glob('**/*')):
        if max and i > max:
        if f.is_file() and regex.fullmatch(str(f.relative_to(path))):
            return f

# note: this_dir is a string here
this_dir = os.path.dirname(__file__)

print(find_file(this_dir, '^validation.*'))
print(find_file(this_dir, '^foobar.*', max=3))

A few notes:

  • Though they're passed as strings, path and regex are converted to a Path object and regex respectively by the decorator.
  • max has no type annotation, so will be considered as Any by the decorator.

Type coercion like this can be extremely helpful, but also confusing or not desired. See Coercion and strictness for a discussion of @validate_call's limitations in this regard.

Function signatures

The @validate_call decorator is designed to work with functions using all possible parameter configurations and all possible combinations of these:

  • Positional or keyword arguments with or without defaults.
  • Variable positional arguments defined via * (often *args).
  • Variable keyword arguments defined via ** (often **kwargs).
  • Keyword-only arguments: arguments after *,.
  • Positional-only arguments: arguments before , / (new in Python 3.8).

To demonstrate all the above parameter types:

from pydantic import validate_call

def pos_or_kw(a: int, b: int = 2) -> str:
    return f'a={a} b={b}'

#> a=1 b=2
#> a=1 b=2
print(pos_or_kw(1, 3))
#> a=1 b=3
print(pos_or_kw(a=1, b=3))
#> a=1 b=3

def kw_only(*, a: int, b: int = 2) -> str:
    return f'a={a} b={b}'

#> a=1 b=2
print(kw_only(a=1, b=3))
#> a=1 b=3

def pos_only(a: int, b: int = 2, /) -> str:  # python 3.8 only
    return f'a={a} b={b}'

#> a=1 b=2
print(pos_only(1, 2))
#> a=1 b=2

def var_args(*args: int) -> str:
    return str(args)

#> (1,)
print(var_args(1, 2))
#> (1, 2)
print(var_args(1, 2, 3))
#> (1, 2, 3)

def var_kwargs(**kwargs: int) -> str:
    return str(kwargs)

#> {'a': 1}
print(var_kwargs(a=1, b=2))
#> {'a': 1, 'b': 2}

def armageddon(
    a: int,
    /,  # python 3.8 only
    b: int,
    *c: int,
    d: int,
    e: int = None,
    **f: int,
) -> str:
    return f'a={a} b={b} c={c} d={d} e={e} f={f}'

print(armageddon(1, 2, d=3))
#> a=1 b=2 c=() d=3 e=None f={}
print(armageddon(1, 2, 3, 4, 5, 6, d=8, e=9, f=10, spam=11))
#> a=1 b=2 c=(3, 4, 5, 6) d=8 e=9 f={'f': 10, 'spam': 11}

Using Field to describe function arguments

Field can also be used with @validate_call to provide extra information about the field and validations. In general it should be used in a type hint with Annotated, unless default_factory is specified, in which case it should be used as the default value of the field:

from datetime import datetime

from typing_extensions import Annotated

from pydantic import Field, ValidationError, validate_call

def how_many(num: Annotated[int, Field(gt=10)]):
    return num

except ValidationError as e:
    1 validation error for how_many
      Input should be greater than 10 [type=greater_than, input_value=1, input_type=int]

def when(dt: datetime = Field(
    return dt

#> <class 'datetime.datetime'>

The alias can be used with the decorator as normal.

from typing_extensions import Annotated

from pydantic import Field, validate_call

def how_many(num: Annotated[int, Field(gt=10, alias='number')]):
    return num


Usage with mypy

The validate_call decorator should work "out of the box" with mypy since it's defined to return a function with the same signature as the function it decorates. The only limitation is that since we trick mypy into thinking the function returned by the decorator is the same as the function being decorated; access to the raw function or other attributes will require type: ignore.

Raw function

The raw function which was decorated is accessible, this is useful if in some scenarios you trust your input arguments and want to call the function in the most performant way (see notes on performance below):

from pydantic import validate_call

def repeat(s: str, count: int, *, separator: bytes = b'') -> bytes:
    b = s.encode()
    return separator.join(b for _ in range(count))

a = repeat('hello', 3)
#> b'hellohellohello'

b = repeat.raw_function('good bye', 2, separator=b', ')
#> b'good bye, good bye'

Async functions

@validate_call can also be used on async functions:

class Connection:
    async def execute(self, sql, *args):
        return '[email protected]'

conn = Connection()
# ignore-above
import asyncio

from pydantic import PositiveInt, ValidationError, validate_call

async def get_user_email(user_id: PositiveInt):
    # `conn` is some fictional connection to a database
    email = await conn.execute('select email from users where id=$1', user_id)
    if email is None:
        raise RuntimeError('user not found')
        return email

async def main():
    email = await get_user_email(123)
    #> [email protected]
        await get_user_email(-4)
    except ValidationError as exc:
                'type': 'greater_than',
                'loc': (0,),
                'msg': 'Input should be greater than 0',
                'input': -4,
                'ctx': {'gt': 0},
                'url': '',
# requires: `conn.execute()` that will return `'[email protected]'`

Custom config

The model behind @validate_call can be customised using a config setting, which is equivalent to setting the ConfigDict sub-class in normal models.

Configuration is set using the config keyword argument to the decorator, it may be either a config class or a dict of properties which are converted to a class later.

from pydantic import ValidationError, validate_call

class Foobar:
    def __init__(self, v: str):
        self.v = v

    def __add__(self, other: 'Foobar') -> str:
        return f'{self} + {other}'

    def __str__(self) -> str:
        return f'Foobar({self.v})'

def add_foobars(a: Foobar, b: Foobar):
    return a + b

c = add_foobars(Foobar('a'), Foobar('b'))
#> Foobar(a) + Foobar(b)

    add_foobars(1, 2)
except ValidationError as e:
    2 validation errors for add_foobars
      Input should be an instance of Foobar [type=is_instance_of, input_value=1, input_type=int]
      Input should be an instance of Foobar [type=is_instance_of, input_value=2, input_type=int]


Validation exception

Currently upon validation failure, a standard Pydantic ValidationError is raised. See model error handling for details.

This is helpful since its str() method provides useful details of the error which occurred and methods like .errors() and .json() can be useful when exposing the errors to end users. However, ValidationError inherits from ValueError not TypeError, which may be unexpected since Python would raise a TypeError upon invalid or missing arguments. This may be addressed in future by either allowing a custom error or raising a different exception by default, or both.

Coercion and strictness

Pydantic currently leans on the side of trying to coerce types rather than raise an error if a type is wrong, see model data conversion and @validate_call is no different.


We've made a big effort to make Pydantic as performant as possible and argument inspect and model creation is only performed once when the function is defined, however there will still be a performance impact to using the @validate_call decorator compared to calling the raw function.

In many situations this will have little or no noticeable effect, however be aware that @validate_call is not an equivalent or alternative to function definitions in strongly typed languages; it never will be.

Return value

The return value of the function may optionally be validated against its return type annotation.