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Runtype is a collection of run-time type utilities for Python.

It is:

🏃 Fast! Uses an internal typesystem for maximum performance.

🧠 Smart! Supports typing, constraints, auto-casting, and much more.

⚙️ Configurative! Write your own type system, and use it with dataclass and dispatch.


  • validation - Provides a smarter alternative to isinstance and issubclass, with support for the typing module, and type constraints.

  • dataclass - Adds run-time type validation to the built-in dataclass.

    • Improves dataclass ergonomics.
    • Supports automatic value casting, Pydantic-style. (Optional, off by default)
    • Supports types with constraints. (e.g. String(max_length=10))
    • Supports optional sampling for faster validation of big lists and dicts.
    • 40% faster than Pydantic (read here)
  • dispatch - Provides fast multiple-dispatch for functions and methods, via a decorator.

    • Inspired by Julia.
  • type utilities - Provides a set of classes to implement your own type-system.

    • Used by runtype itself, to emulate the Python type-system.


Read the docs here:


pip install runtype

No dependencies.

Requires Python 3.6 or up.

Build Status codecov


Validation (Isa & Subclass)

from typing import Dict, Mapping
from runtype import isa, issubclass

print( isa({'a': 1}, Dict[str, int]) )
#> True
print( isa({'a': 'b'}, Dict[str, int]) )
#> False

print( issubclass(Dict[str, int], Mapping[str, int]) )
#> True
print( issubclass(Dict[str, int], Mapping[int, str]) )
#> False


from typing import List
from datetime import datetime
from runtype import dataclass

@dataclass(check_types='cast')  # Cast values to the target type, when applicable
class Person:
    name: str
    birthday: datetime = None   # Optional
    interests: List[str] = []   # The list is copied for each instance

print( Person("Beetlejuice") )
#> Person(name='Beetlejuice', birthday=None, interests=[])
print( Person("Albert", "1955-04-18T00:00", ['physics']) )
#> Person(name='Albert', birthday=datetime.datetime(1955, 4, 18, 0, 0), interests=['physics'])
print( Person("Bad", interests=['a', 1]) )
# Traceback (most recent call last):
#   ...
# TypeError: [Person] Attribute 'interests' expected value of type list[str]. Instead got ['a', 1]

#     Failed on item: 1, expected type str

Multiple Dispatch

from runtype import Dispatch
dp = Dispatch()

def append(a: list, b):
    return a + [b]

def append(a: tuple, b):
    return a + (b,)

def append(a: str, b: str):
    return a + b

print( append([1, 2, 3], 4) )
#> [1, 2, 3, 4]
print( append((1, 2, 3), 4) )
#> (1, 2, 3, 4)
print( append('foo', 'bar') )
#> foobar
print( append('foo', 4)     )
# Traceback (most recent call last):
#    ...
# runtype.dispatch.DispatchError: Function 'append' not found for signature (<class 'str'>, <class 'int'>)


Runtype uses the MIT license.


If you like Runtype and want to show your appreciation, you can do so at my patreon page, or ko-fi page.