Better type at Python

7 November 2015

Rick Mak

Oursky

Python 3.5 Release

https://docs.python.org/3.5/whatsnew/3.5.html
Released on September 13, 2015, 2 months ago

New library modules introduced
typing: PEP 484 – Type Hints

The syntax introduced long ago, Dec-2006
PEP 3107 - Function Annotations

Your view on type

Anyone used typing?
Anyone used function annotations?
Anyone thinks using it will have performance boost or punishment?

Get it working

Python 3.5

import typing

Python 3.2-3.4

pip install typing

Some code

import sys


class Person:
    def __init__(self, name, lang):
        self.name = name
        self.lang = lang


def greeting(man):
    print(hello(man.name))


def hello(name = 'nobody'):
    return 'Hello' + name


if __name__ == '__main__':
    p = Person(sys.argv[1], sys.argv[2])
    greeting(p)

Same code with type hints

import sys


class Person:
    def __init__(self, name: str, lang: str) -> None:
        self.name = name
        self.lang = lang


def greeting(man: Person) -> None:
    print(hello(man.name))


def hello(name: str = 'nobody') -> str:
    return 'Hello' + name


if __name__ == '__main__':
    p = Person(sys.argv[1], sys.argv[2])
    greeting(p)

More complex type hints

Dictionary

from typing import Dict


def inc(d: Dict[str, int]) -> Dict[str, int]:
    for k in d.keys():
        d[k] = d[k] + 1
    return d

print(inc({
    'a': 1,
    'b': 2,
    'c': 3
}))

More complex type hints

Sequence and List

from typing import Sequence, List, TypeVar

T = TypeVar('T') # Declare type variable

def pick_odd(s: Sequence[T]) -> List[T]:
    len_ = len(s)
    list_ = []  # type: List
    for i in range(0, len_):
        if i % 2 == 1:
            list_.append(s[i])
    return list_

print(pick_odd([0,1,2,3,4,5,6,7]))
print(pick_odd('abcdefg'))

Forward references

class Human:
    def __init__(self, father: 'Human' = None, mother: 'Human' = None) -> None:
        self.father = father
        self.mother = mother

    def make(self, couple: 'Human') -> 'Human':
        return Human(self, couple)


print(Human())

What are type hints good for?

Make IDE works better
Make developement of static analysis tools easier
Improve readability, for tools and for human
Serve as Documentation

Make IDE works better

Let see how PyCharm works

Make development of static analysis tools easier

mypy

JukkaL/mypy

pip install mypy-lang

Analyse a file without running it

(venv)$ mypy callee.py
callee.py, line 7: Argument 1 to "hello" has incompatible type "int"; expected "str"

Serve as (additional) documentation

Docstring for sphinx to gen autodoc

def hello(name='nobody'):
    """Say hello to a person

    :param name: string value
    :rtype: string value
    """
    return 'Hello' + name

Same information using func type annotation

def hello(name: str = 'nobody') -> str:
    '''Say hello to a person
    '''
    return 'Hello' + name

Python2 with pyi stub

An *.pyi file that only have interface

def greeting(name: str) -> None: ...
def hello(name: str) -> str: ...

Also useful for
- Package you can't change
- Legacy package you don't want to change

There is github repos hosting stub for stdlib
python/typeshed

What type hints are not

Not about code generation
Your code can still break after type checking
No runtime type checking, No performance overhead

How you can find out function annotation in your code

You can use inspect to get back the annotation.

import inspect
print(inspect.getfullargspec(inc).annotations)
'''
{
    'd': typing.Dict[str, int],
    'return': typing.Dict[str, int]
}
'''
print(inspect.getfullargspec(Human.make).annotations)
'''
{
    'return': 'Human',
    'couple': 'Human'
}
'''

i.e. You may abuse it to do type conversion or validation at runtime.

Reference

mypy
Guido van Rossum - Type Hints for Python 3.5
http://blog.pirx.ru/media/files/2015/type-hinting-talk/type-hinting.html

Thank you

Rick Mak

Oursky

rickmak@oursky.com

@rickmak

CC81327D