• PEP-484
• Designed on top of PEP-3107 (Function Annotations)
• Created at September 2014
• Became part of standard library in Python 3.5

• Still in provisional status
• PEP-526: Syntax for Variable Annotations
• Bunch of updates: ContextManager, Collection, ClassVar
• TYPE_CHECKING constant
• NamedTuple improvements

def hello(name: str) -> str:
    content: str
    if name == 'world':
        content = 'WORLD'
    else:
        content = name
    return f'Hello, {content}'

class Article(object):

    name: str
    content: str

    def __init__(self, name: str, content: str) -> None:
        self.name = name
        self.content = content

from typing import List, Optional, NamedTuple


class Author(NamedTuple):

    full_name: str


class Article(NamedTuple):

    author: Author

    name: str
    content: str

    tags: Optional[List[str]] = None

    @property
    def url(self) -> str:
        return f'/talks/{slugify(self.name)}'

author = Author('Igor Davydenko')
kyiv_py_19 = Article(author, 'Kyiv.py#19', ...)
kyiv_py_21 = Article(
    author=author,
    content=...,
    name='Kyiv.py#21',
    tags=['mypy', 'type', 'hinting'])

assert kyiv_py_21._fields == ('author', 'name', 'content', 'tags')
assert kyiv_py_21._asdict() == {...}

assert kyiv_py_21.author.full_name == 'Igor Davydenko'
assert kyiv_py_21.url == '/talks/kyivpy-21'

kyiv_py_22 = kyiv_py_21._replace(
    name='Kyiv.py#22',
    tags=['graphql', 'rules', 'maybe'])

    files: DefaultDict[str, List[DocumentFile]] = defaultdict(list)
    for row in (db_files or []):
        files[row.meta_id].append(
            DocumentFile(
                **rowproxy_to_dict(row, rename_fields=RP_ID_TYPE_MAPPING)))

    meta = tuple(
        DocumentMeta(  # type: ignore
            **rowproxy_to_dict(row, rename_fields=RP_ID_TYPE_MAPPING),
            files=tuple(files.get(row.id) or []))
        for row in db_meta or [])

add : Int -> Int -> Int
add x y =
    x + y

type alias TweetId =
    Int


type alias Tweet =
    { id : TweetId
    , text : String
    , url : String
    , createdAt : String
    }

tweetDecoder : Decode.Decoder Tweet
tweetDecoder =
    decode Tweet
        |> required "id" Decode.int
        |> required "text" Decode.string
        |> required "url" Decode.string
        |> custom (Decode.at [ "created_at" ] Decode.string)


tweetsDecoder : Decode.Decoder (List Tweet)
tweetsDecoder =
    Decode.list tweetDecoder


fetchTweets : Cmd Msg
fetchTweets =
    Http.get "http://vusaifootball.com/api/tweets" tweetsDecoder
        |> RemoteData.sendRequest
        |> Cmd.map Msgs.OnFetchTweets

fn add(x: i32, y: i32) -> i32 {
    x + y
}

struct DebugWriter {
    sha: Writer<Blake2b512>,
    data: Opt<Vec<u8>>,
}

Code from tailhook/vagga

impl Digest {
    pub fn new(debug: bool, raw_debug: bool) -> Digest {
        Digest {
            sha: DebugWriter {
                sha: Writer::new(Blake2b512::new()),
                data: if raw_debug { Opt::Out(Vec::new()) } else { Opt::Sink },
            },
            debug: if debug { Opt::Out(String::new()) } else { Opt::Sink },
        }
    }
}

Code from tailhook/vagga

def add(x: int, y: int) -> int:
    return x + y

module Main exposing (..)

import Html exposing (Html, text)


add : Int -> Int -> Int
add x y =
    x + y


main : Html msg
main =
    let
        value =
            add "1" "2"
    in
        text (toString value)

fn add(x: i32, y: i32) -> i32 {
    x + y
}

fn main() {
    println!("{}", add("x", "y"));
}

def add(x: int, y: int) -> int:
    return x + y


if __name__ == '__main__':
    print(add("x", "y"))

import enforce


@enforce.runtime_validation
def add(x: int, y: int) -> int:
    return x + y


if __name__ == '__main__':
    print(add("x", "y"))

In [1]: def add(x: int, y: int) -> int:
   ...:     return x + y
   ...:

In [2]: import enforce

In [3]: @enforce.runtime_validation
   ...: def enforce_add(x: int, y: int) -> int:
   ...:     return x + y
   ...:

In [4]: %timeit add(1, 2)
132 ns ± 1.78 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

In [5]: %timeit enforce_add(1, 2)
245 µs ± 14.4 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

In [6]: %prun add(1, 2)
         4 function calls in 0.000 seconds

   Ordered by: internal time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 {built-in method builtins.exec}
        1    0.000    0.000    0.000    0.000 <ipython-input-1-1e63accb4849>:1(add)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}

In [7]: %prun enforce_add(1, 2)
         641 function calls (599 primitive calls) in 0.003 seconds

   Ordered by: internal time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        3    0.000    0.000    0.001    0.000 types.py:139(is_type_of_type)
        1    0.000    0.000    0.002    0.002 enforcers.py:57(validate_inputs)
        1    0.000    0.000    0.000    0.000 inspect.py:2829(_bind)
       39    0.000    0.000    0.001    0.000 typing.py:1177(__instancecheck__)
        1    0.000    0.000    0.003    0.003 decorators.py:78(universal)
      9/3    0.000    0.000    0.002    0.001 utils.py:5(visit)
       39    0.000    0.000    0.000    0.000 typing.py:1166(__subclasscheck__)
        3    0.000    0.000    0.001    0.000 nodes.py:220(validate_data)
        3    0.000    0.000    0.000    0.000 settings.py:50(covariant)
        9    0.000    0.000    0.001    0.000 nodes.py:48(validate)
       39    0.000    0.000    0.000    0.000 abc.py:194(__subclasscheck__)
        1    0.000    0.000    0.000    0.000 validator.py:35(reset)
    99/75    0.000    0.000    0.001    0.000 {built-in method builtins.isinstance}
        6    0.000    0.000    0.000    0.000 types.py:189(<genexpr>)
       51    0.000    0.000    0.000    0.000 _weakrefset.py:70(__contains__)
        1    0.000    0.000    0.003    0.003 <string>:1(<module>)
        3    0.000    0.000    0.000    0.000 nodes.py:198(reset)
       39    0.000    0.000    0.000    0.000 {built-in method builtins.issubclass}
       12    0.000    0.000    0.000    0.000 types.py:260(sort_and_flat_type)

• Latest version: 0.521
• Installed as pip install mypy
• Supports Python 3.6 features by default
• Extended Callable Types
• Improved error messages
• A lot of new features, really…

[mypy]
check_untyped_defs = True
disallow_any = generics
disallow_untyped_calls = True
disallow_untyped_defs = True
follow_imports = silent
ignore_missing_imports = True
strict_optional = True
warn_redundant_casts = True
warn_unused_ignores = True

• python/typeshed
• Collection of library stubs for Python, with static types
• Stubs for standart library & third party libraries
• Third party libraries:
  • dateutil
  • jwt
  • lxml
  • requests
  • click / itsdangerous / jinja2 / werkzeug
• #1105

• Third party code is not annotated in most cases
• You need to ignore missed annotations
• Third party code == Any

• One does not simply
• use type annotations without # type: ignore
• Our codebase: cloc 28K, 33 type ignores
• Will overview occurences later

• 3 developers without any static typing background
• 28K lines of code as of now
• Started covering with typing annotations when project already run in production
• Extermely happy with result!

• Enable mypy in your editor / IDE
• Run mypy on pre-commit / pre-push hook
• Upgrade to latest mypy

1. Enable mypy check on CI (with or after flake8)
2. Cover smallest package
3. Include package config in mypy.ini
4. Repeat before all packages covered

• Better codebase understanding
• Faster refactoring
• Easier code reviews
• Less dicts
• More NamedTuples & Enums

from typing import ...

from aiohttp import web
from aiopg.sa.result import RowProxy


AnyAwaitable = Awaitable[Any]
AnyDict = Dict[Any, Any]
AnyList = List[Any]
AnyMapping = Mapping[Any, Any]
DataDict = Dict[str, Any]
IntDict = Dict[int, str]
IntList = List[int]
StrDict = Dict[str, str]
StrIterable = Iterable[str]
StrList = List[str]
StrMapping = Mapping[str, str]

UserHandler = Callable[[web.Request, RowProxy], Awaitable[web.Response]]

RequestOrView = Union[web.Request, web.View]
ResponseDecorator = Callable[..., web.Response]

• pip install mypy-extensions
• Don't forget to include to requirements.txt instead of requirements-dev.txt
• TypedDict
• Arg, VarArg, NamedArg, DefaultArg, DefaultNamedArg
• NoReturn

from mypy_extensions import TypedDict


class Article(TypedDict):

    name: str
    content: str


article = Article({'name': 'Kyiv.py#21', 'content': ...})

• Missed stubs for new features in stdlib
• Incomplete stubs for third party libraries (see, lxml)

data = {'name': 'Kyiv.py#21'}
Article(**data, content=...)

• error: Incompatible types in assignment (expression has type "str", target has type "int")
• error: Module 'lib.helpers' has no attribute 'ensure_list'
• error: Missing return statement
• error: Assertion is always true, perhaps remove parentheses?
• error: Cannot assign to a method

• Any
• Any
• Any
• Tests are not annotated
• Bad type annotations leads to issues in runtime

• samuelcolvin/pydantic

class Model(BaseModel):
    cos_function: PyObject = None
    path_to_something: Path = None

    short_str: constr(min_length=2, max_length=10) = None
    regex_str: constr(regex='apple (pie|tart|sandwich)') = None

    big_int: conint(gt=1000, lt=1024) = None
    pos_int: PositiveInt = None
    neg_int: NegativeInt = None

    email_address: EmailStr = None
    email_and_name: NameEmail = None

• Tables declared as sa.Table
• Queries made via sa.sql.Select
• Data fetched to aiopg.sa.result.RowProxy
• RowProxy sometimes translated to NamedTuple
• Code duplication
• Bad maintainability
• It will fail at runtime in case of typo anyway

• SQLAlchemy removed from typeshed
• aio-libs not covered enough
• I want easier way for providing extra stubs